Metric Tubing Size Chart: OD, ID & Wall Thickness

Metric tubing is widely used in hydraulic, pneumatic, instrumentation, automotive, laboratory, and industrial process systems. Unlike pipe, which is often specified using a nominal pipe size, tubing is generally identified by its actual outside diameter, or OD, together with the wall thickness.

A typical metric tube size may be written as 12 × 1.0 mm, where:

• 12 mm is the outside diameter
• 1.0 mm is the wall thickness

The inside diameter can then be determined from the outside diameter and wall thickness.

For engineers, technicians, and maintenance personnel, understanding metric tubing dimensions is important when selecting tubing, fittings, valves, clamps, and other system components. Even a small difference in outside diameter can prevent a tube from fitting correctly into a compression fitting or hydraulic connection.

A Metric Tubing Size Chart provides a quick reference for comparing common tube outside diameters, wall thicknesses, and inside diameters. It can also help when converting between metric and imperial tubing or identifying an unknown tube in an existing installation.

This article covers common metric tube sizes, dimension terminology, stainless steel and hydraulic tubing sizes, standards, metric-to-inch conversions, and practical methods for selecting and measuring tubing.

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1. What Is Metric Tubing?

Metric tubing is tubing whose dimensions are specified in millimeters rather than inches. The most important dimensions are normally the outside diameter (OD) and wall thickness.

For example:

12 × 1.5 mm tube

means:

• Outside diameter = 12 mm
• Wall thickness = 1.5 mm
• Inside diameter = 9 mm

The inside diameter is calculated as:

ID = OD – (2 × Wall Thickness)

For this example:

ID = 12 – (2 × 1.5) = 9 mm

Metric Tubing vs. Metric Pipe

Tubing and pipe are often confused, but they are generally specified differently.

Metric tubing is normally identified by its actual outside diameter.

Examples include:

• 6 mm tubing
• 8 mm tubing
• 10 mm tubing
• 12 mm tubing
• 16 mm tubing
• 25 mm tubing

Pipe, on the other hand, is often identified by a nominal size. The nominal size does not necessarily correspond exactly to either the outside or inside diameter.

This difference becomes particularly important when selecting fittings.

A fitting designed for 12 mm tubing is intended to grip a tube with an actual outside diameter of approximately 12 mm. It should not automatically be assumed to fit a pipe that is described as having a nominal size of 12 mm.

Common Metric Tubing Materials

Metric tubing is available in many materials depending on pressure, temperature, corrosion resistance, cleanliness, and application requirements.

Common materials include:

Stainless steel

Grades such as 304, 304L, 316, and 316L are widely used in instrumentation, chemical processing, semiconductor, pharmaceutical, food, and high-purity applications.

Carbon steel

Carbon steel metric tubing is commonly used in hydraulic systems, especially with DIN-style compression fittings.

Copper

Copper tubing is frequently used in refrigeration, air conditioning, gas, water, and some instrumentation applications.

Aluminum

Aluminum tubing is selected where low weight and corrosion resistance are important.

Plastic tubing

Materials such as nylon, polyurethane, polyethylene, PTFE, and PFA are commonly used in pneumatic, chemical, laboratory, and semiconductor systems.

Common Applications

Metric tubing can be found in systems such as:

• Hydraulic power units
• Pneumatic control systems
• Instrumentation systems
• Analyzer and sampling systems
• Chemical processing equipment
• Semiconductor equipment
• Automotive systems
• Refrigeration systems
• Laboratory equipment
• Process skids
• Machine tools

The required tube size depends not only on the required flow rate but also on operating pressure, material strength, temperature, fitting type, and applicable standards.

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2. Metric Tubing Dimensions and Terminology

Understanding tubing terminology is essential when reading a metric tubing size chart. The three main dimensions are outside diameter, inside diameter, and wall thickness.

Outside Diameter – OD

The Outside Diameter (OD) is the distance across the outside of the tube.

For metric tubing, OD is normally given directly in millimeters.

Typical metric tubing outside diameters include:

The outside diameter is especially important because most tube fittings are selected according to tube OD.

For example, a 12 mm compression fitting is normally designed for tubing with a 12 mm outside diameter.

Inside Diameter – ID

The Inside Diameter (ID) is the diameter of the open flow passage inside the tube.

Unlike OD, the ID changes depending on wall thickness.

For example, consider two tubes with the same 12 mm OD:

Tube Size	OD	Wall Thickness	ID
12 × 1.0 mm	12 mm	1.0 mm	10 mm
12 × 1.5 mm	12 mm	1.5 mm	9 mm
12 × 2.0 mm	12 mm	2.0 mm	8 mm

A thicker tube wall reduces the inside diameter.

This can affect:

• Flow capacity
• Fluid velocity
• Pressure drop
• Tube weight
• Pressure capability

Wall Thickness

Wall thickness is the radial thickness of material between the outside and inside surfaces of the tube.

Common metric tubing wall thicknesses include:

• 0.5 mm
• 0.7 mm
• 0.8 mm
• 1.0 mm
• 1.2 mm
• 1.5 mm
• 2.0 mm
• 2.5 mm
• 3.0 mm

In general, increasing wall thickness increases the mechanical strength and potential pressure capability of the tube, although actual allowable working pressure must always be verified using the applicable material specification, temperature, design factor, and tubing standard.

Relationship Between OD, ID, and Wall Thickness

The basic relationship is:

ID = OD – (2 × Wall Thickness)

For example, for 16 × 2 mm tubing:

ID = 16 – (2 × 2)

ID = 12 mm

Another example is 10 × 1 mm tubing:

ID = 10 – (2 × 1)

ID = 8 mm

This simple relationship makes it possible to calculate the approximate inside diameter when OD and wall thickness are known.

How Metric Tube Sizes Are Written

Metric tubing is commonly written in the following format:

OD × Wall Thickness

Examples:

• 6 × 1 mm
• 8 × 1 mm
• 10 × 1.5 mm
• 12 × 1.5 mm
• 16 × 2 mm
• 25 × 2.5 mm

For example:

16 × 1.5 mm

means:

• OD = 16 mm
• Wall thickness = 1.5 mm
• ID = 13 mm

It is important not to interpret the second number as the inside diameter.

Nominal vs. Actual Dimensions

Most metric instrumentation and hydraulic tubing is specified using actual outside diameter dimensions.

However, manufacturing tolerances still apply.

A tube labeled 12 mm OD may not measure exactly 12.000 mm at every location. The allowable dimensional variation depends on the relevant tubing specification or manufacturing standard.

These tolerances are particularly important when using:

• Compression tube fittings
• Ferrule fittings
• High-pressure fittings
• Orbital welding systems
• Precision bending equipment

For reliable assembly, the tube OD, wall thickness, hardness, ovality, and surface condition should all comply with the fitting manufacturer’s requirements.

3. Complete Metric Tubing Size Chart

A metric tubing size chart helps identify common combinations of tube outside diameter, wall thickness, and inside diameter. Because tubing is generally specified by OD × wall thickness, tubes with the same outside diameter can have different inside diameters.

The inside diameter can be calculated using:

ID = OD – (2 × Wall Thickness)

The following chart shows common metric tube sizes used in instrumentation, hydraulic, pneumatic, and industrial applications.

Tube OD	Wall Thickness	Inside Diameter
3 mm	0.5 mm	2.0 mm
4 mm	0.5 mm	3.0 mm
4 mm	1.0 mm	2.0 mm
6 mm	0.5 mm	5.0 mm
6 mm	1.0 mm	4.0 mm
6 mm	1.5 mm	3.0 mm
8 mm	0.5 mm	7.0 mm
8 mm	1.0 mm	6.0 mm
8 mm	1.5 mm	5.0 mm
10 mm	1.0 mm	8.0 mm
10 mm	1.5 mm	7.0 mm
10 mm	2.0 mm	6.0 mm
12 mm	1.0 mm	10.0 mm
12 mm	1.5 mm	9.0 mm
12 mm	2.0 mm	8.0 mm
14 mm	1.0 mm	12.0 mm
14 mm	1.5 mm	11.0 mm
14 mm	2.0 mm	10.0 mm
15 mm	1.0 mm	13.0 mm
15 mm	1.5 mm	12.0 mm
15 mm	2.0 mm	11.0 mm
16 mm	1.0 mm	14.0 mm
16 mm	1.5 mm	13.0 mm
16 mm	2.0 mm	12.0 mm
16 mm	2.5 mm	11.0 mm
18 mm	1.0 mm	16.0 mm
18 mm	1.5 mm	15.0 mm
18 mm	2.0 mm	14.0 mm
18 mm	2.5 mm	13.0 mm
20 mm	1.0 mm	18.0 mm
20 mm	1.5 mm	17.0 mm
20 mm	2.0 mm	16.0 mm
20 mm	2.5 mm	15.0 mm
20 mm	3.0 mm	14.0 mm
22 mm	1.0 mm	20.0 mm
22 mm	1.5 mm	19.0 mm
22 mm	2.0 mm	18.0 mm
22 mm	2.5 mm	17.0 mm
25 mm	1.0 mm	23.0 mm
25 mm	1.5 mm	22.0 mm
25 mm	2.0 mm	21.0 mm
25 mm	2.5 mm	20.0 mm
25 mm	3.0 mm	19.0 mm
28 mm	1.0 mm	26.0 mm
28 mm	1.5 mm	25.0 mm
28 mm	2.0 mm	24.0 mm
28 mm	2.5 mm	23.0 mm
28 mm	3.0 mm	22.0 mm
30 mm	1.5 mm	27.0 mm
30 mm	2.0 mm	26.0 mm
30 mm	2.5 mm	25.0 mm
30 mm	3.0 mm	24.0 mm
32 mm	1.5 mm	29.0 mm
32 mm	2.0 mm	28.0 mm
32 mm	2.5 mm	27.0 mm
32 mm	3.0 mm	26.0 mm
38 mm	1.5 mm	35.0 mm
38 mm	2.0 mm	34.0 mm
38 mm	2.5 mm	33.0 mm
38 mm	3.0 mm	32.0 mm
42 mm	1.5 mm	39.0 mm
42 mm	2.0 mm	38.0 mm
42 mm	2.5 mm	37.0 mm
42 mm	3.0 mm	36.0 mm
50 mm	1.5 mm	47.0 mm
50 mm	2.0 mm	46.0 mm
50 mm	2.5 mm	45.0 mm
50 mm	3.0 mm	44.0 mm

These dimensions should be treated as a practical reference rather than a universal manufacturing standard. Actual available combinations vary by material, manufacturer, tubing specification, pressure class, and regional standard.

How to Use the Chart

Suppose a tube is specified as:

12 × 1.5 mm

From the chart:

• OD = 12 mm
• Wall thickness = 1.5 mm
• ID = 9 mm

For another tube:

25 × 2.5 mm

the dimensions are:

• OD = 25 mm
• Wall thickness = 2.5 mm
• ID = 20 mm

Two tubes can therefore have the same outside diameter but significantly different flow areas depending on their wall thickness.

Why Wall Thickness Matters

Wall thickness affects several important characteristics of tubing, including:

• Pressure capability
• Internal flow area
• Fluid velocity
• Pressure drop
• Mechanical strength
• Weight
• Bendability

A thicker wall usually allows the tube to withstand greater mechanical stress, but it also reduces the inside diameter and therefore the available flow area.

For pressure systems, tube selection should not be based on dimensions alone. The tube material, hardness, temperature, corrosion allowance, applicable design code, and fitting manufacturer’s recommendations must also be considered.

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4. Metric Stainless Steel Tubing Size Chart

Metric stainless steel tubing is widely used in instrumentation, process control, semiconductor, chemical, pharmaceutical, food processing, laboratory, and general industrial applications.

Common stainless steel grades include:

• 304
• 304L
• 316
• 316L

Among these, 316 and 316L stainless steel are particularly common in instrumentation and corrosive process environments because of their improved corrosion resistance.

Common Metric Stainless Steel Tube Sizes

The following chart shows representative sizes commonly encountered in stainless steel tubing systems.

Tube OD	Common Wall Thicknesses
3 mm	0.5 mm
4 mm	0.5, 1.0 mm
6 mm	0.5, 1.0, 1.5 mm
8 mm	0.5, 1.0, 1.5 mm
10 mm	1.0, 1.5, 2.0 mm
12 mm	1.0, 1.5, 2.0 mm
14 mm	1.0, 1.5, 2.0 mm
15 mm	1.0, 1.5, 2.0 mm
16 mm	1.0, 1.5, 2.0 mm
18 mm	1.0, 1.5, 2.0 mm
20 mm	1.0, 1.5, 2.0 mm
22 mm	1.0, 1.5, 2.0 mm
25 mm	1.0, 1.5, 2.0, 2.5 mm
28 mm	1.0, 1.5, 2.0 mm
30 mm	1.5, 2.0, 2.5 mm
32 mm	1.5, 2.0, 2.5 mm
38 mm	1.5, 2.0, 2.5 mm
42 mm	1.5, 2.0, 3.0 mm
50 mm	1.5, 2.0, 3.0 mm

Actual availability depends on the tubing specification and manufacturer.

Instrumentation Stainless Steel Tubing

In instrumentation systems, smaller tube sizes are especially common.

Typical sizes include:

• 6 × 1 mm
• 8 × 1 mm
• 10 × 1 mm
• 10 × 1.5 mm
• 12 × 1 mm
• 12 × 1.5 mm
• 16 × 1.5 mm

These sizes may be used for:

• Instrument impulse lines
• Pressure transmitters
• Analyzer systems
• Sampling systems
• Chemical injection
• Calibration lines
• Utility gas systems
• Pneumatic control systems

Tube OD is particularly important in these systems because compression fittings are normally selected according to the actual tube outside diameter.

Seamless vs. Welded Stainless Steel Tubing

Metric stainless steel tubing may be manufactured as either seamless or welded tubing.

Seamless tubing is produced without a longitudinal weld seam and is frequently selected for demanding pressure and instrumentation applications.

Welded tubing is produced from formed strip or sheet that is welded along the tube length.

Both types can be suitable for industrial service when manufactured and tested according to the appropriate specification.

The selection between seamless and welded tubing depends on:

• Design pressure
• Temperature
• Material specification
• Corrosion requirements
• Surface finish
• Cleanliness requirements
• Customer specification
• Applicable code

Stainless Steel Tubing Surface Condition

Surface finish can also be important, especially in high-purity and semiconductor systems.

Depending on the application, tubing may be supplied with:

• Standard mill finish
• Bright annealed finish
• Mechanical polish
• Electropolished surface

High-purity applications may also impose strict requirements for internal roughness, cleaning, packaging, and particle contamination.

Tubing Hardness and Fitting Compatibility

For mechanical tube fittings, tubing hardness is an important consideration.

The tube must be sufficiently ductile for the fitting ferrules to grip and seal correctly, but it must also satisfy the mechanical requirements of the system.

Tubing should therefore be selected according to the fitting manufacturer’s recommendations for:

• Material
• Hardness
• OD tolerance
• Wall thickness
• Surface condition
• Ovality

Using tubing with the correct nominal OD but unsuitable hardness or surface condition can result in poor fitting performance.

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5. Metric Hydraulic Tubing Size Chart

Metric hydraulic tubing is commonly used in mobile equipment, industrial machinery, machine tools, hydraulic power units, presses, material handling systems, and manufacturing equipment.

Unlike flexible hydraulic hose, rigid hydraulic tubing provides a compact, durable method for routing hydraulic fluid between pumps, valves, actuators, manifolds, and other components.

Carbon steel tubing is particularly common in hydraulic applications, although stainless steel may be used where corrosion resistance is required.

Common Metric Hydraulic Tube Sizes

Typical hydraulic tube outside diameters include:

Tube OD	Typical Wall Thickness Range
4 mm	1.0 mm
6 mm	1.0–1.5 mm
8 mm	1.0–2.0 mm
10 mm	1.0–2.0 mm
12 mm	1.0–2.5 mm
14 mm	1.5–2.5 mm
15 mm	1.5–2.5 mm
16 mm	1.5–3.0 mm
18 mm	1.5–3.0 mm
20 mm	2.0–3.0 mm
22 mm	2.0–3.0 mm
25 mm	2.0–4.0 mm
28 mm	2.0–4.0 mm
30 mm	2.0–4.0 mm
32 mm	2.0–5.0 mm
35 mm	2.0–5.0 mm
38 mm	2.0–5.0 mm
42 mm	2.0–5.0 mm
50 mm	2.5–5.0 mm

These are representative combinations only. The actual tube series must be checked against the applicable hydraulic fitting and tube standard.

Metric Hydraulic Tubing and DIN 2353 Fittings

Metric hydraulic tubing is strongly associated with DIN 2353-style compression fittings and related standards such as ISO 8434-1.

These systems commonly use metric tube outside diameters and may be divided into different fitting series depending on pressure and application.

Common classifications include:

• Light series
• Heavy series
• Application-specific higher-pressure configurations

The fitting series determines which tube OD and wall thickness combinations are suitable.

For example, a 12 mm tube may be available in several wall thicknesses, but not every 12 mm tube is necessarily appropriate for every fitting series or working pressure.

Example Hydraulic Tube Sizes

Some practical examples include:

10 × 1.5 mm

• OD = 10 mm
• Wall = 1.5 mm
• ID = 7 mm

12 × 2 mm

• OD = 12 mm
• Wall = 2 mm
• ID = 8 mm

16 × 2 mm

• OD = 16 mm
• Wall = 2 mm
• ID = 12 mm

25 × 3 mm

• OD = 25 mm
• Wall = 3 mm
• ID = 19 mm

Increasing wall thickness generally increases tube strength but decreases the internal flow area.

Selecting Hydraulic Tube Diameter

Tube diameter should be selected according to the required flow rate and acceptable fluid velocity.

If the inside diameter is too small, the system may experience:

• Excessive fluid velocity
• Higher pressure drop
• Increased heat generation
• Noise
• Reduced system efficiency

Larger tubing reduces fluid velocity and pressure loss but increases cost, weight, and installation space.

Different hydraulic lines may also require different recommended velocities.

For example:

• Suction lines generally use relatively low velocity
• Return lines use moderate velocity
• Pressure lines can operate at higher velocity

The final tube size should therefore be based on both hydraulic flow requirements and mechanical pressure requirements.

Wall Thickness and Pressure Rating

Hydraulic tube pressure capability is influenced by:

• Outside diameter
• Wall thickness
• Tube material
• Material strength
• Temperature
• Manufacturing specification
• Tube condition
• Design factor
• Fitting pressure rating

A common mistake is assuming that all tubes with the same outside diameter have the same pressure rating.

For example, a:

12 × 1 mm tube

and a:

12 × 2 mm tube

have the same OD but very different wall thicknesses and mechanical characteristics.

The allowable working pressure should always be confirmed using the applicable tubing standard and fitting manufacturer’s technical data.

Tube Bending Considerations

Hydraulic tubing is frequently bent during installation.

When selecting a tube size, engineers should also consider:

• Minimum bend radius
• Wall thinning on the outside of the bend
• Flattening or ovality
• Tube material
• Tube bender compatibility
• Space available for installation

Thicker-wall tubing generally resists collapse better during bending, but it may require greater bending force.

Proper tube preparation and bending are important for maintaining full flow area and reliable fitting connections.

6. Metric vs. Imperial Tubing Size Conversion Chart

Metric and imperial tubing systems use different dimensional conventions. Metric tubing is specified in millimeters, while imperial tubing is generally specified in inches.

Although some metric sizes are close to common inch sizes, they are usually not exact equivalents.

For example:

• 6 mm is close to 1/4 in
• 10 mm is close to 3/8 in
• 12 mm is close to 1/2 in
• 16 mm is close to 5/8 in
• 25 mm is close to 1 in

However, the actual outside diameters are different.

Common Metric to Imperial Tubing Conversion Chart

Metric Tube OD	Inches, Exact Conversion	Closest Common Imperial Tube Size
3 mm	0.118 in	1/8 in
4 mm	0.157 in	5/32 in
6 mm	0.236 in	1/4 in
8 mm	0.315 in	5/16 in
10 mm	0.394 in	3/8 in
12 mm	0.472 in	1/2 in
14 mm	0.551 in	9/16 in
15 mm	0.591 in	5/8 in
16 mm	0.630 in	5/8 in
18 mm	0.709 in	3/4 in
20 mm	0.787 in	3/4 in
22 mm	0.866 in	7/8 in
25 mm	0.984 in	1 in
28 mm	1.102 in	1 1/8 in
30 mm	1.181 in	1 3/16 in
32 mm	1.260 in	1 1/4 in
38 mm	1.496 in	1 1/2 in
42 mm	1.654 in	1 5/8 in
50 mm	1.969 in	2 in

The exact conversion is based on:

1 inch = 25.4 mm

To convert millimeters to inches:

Inches = Millimeters ÷ 25.4

For example:

12 mm ÷ 25.4 = 0.472 in

This is significantly smaller than a true 1/2 in tube, which has an outside diameter of:

0.500 in = 12.70 mm

Why Metric and Imperial Tubing Are Not Interchangeable

The difference may look small, but it can be critical when using precision tube fittings.

For example:

• 12 mm tube OD = 12.00 mm
• 1/2 in tube OD = 12.70 mm

The difference is:

12.70 – 12.00 = 0.70 mm

That difference is large enough to prevent correct fitting assembly.

Similarly:

• 6 mm tube = 6.00 mm
• 1/4 in tube = 6.35 mm

A metric fitting designed for 6 mm tubing should not normally be used with 1/4 in tubing, and an imperial 1/4 in fitting should not automatically be used with 6 mm tubing.

Common Metric and Inch Comparisons

Metric Size	Metric OD	Imperial Size	Imperial OD	Difference
6 mm	6.00 mm	1/4 in	6.35 mm	0.35 mm
8 mm	8.00 mm	5/16 in	7.94 mm	0.06 mm
10 mm	10.00 mm	3/8 in	9.53 mm	0.47 mm
12 mm	12.00 mm	1/2 in	12.70 mm	0.70 mm
16 mm	16.00 mm	5/8 in	15.88 mm	0.12 mm
20 mm	20.00 mm	3/4 in	19.05 mm	0.95 mm
25 mm	25.00 mm	1 in	25.40 mm	0.40 mm

Some combinations, such as 8 mm and 5/16 in, are dimensionally very close. However, fitting compatibility should still be confirmed with the manufacturer rather than assumed.

Converting Wall Thickness

Wall thickness can also be converted between millimeters and inches.

Common examples include:

Metric Wall	Approx. Inches
0.5 mm	0.020 in
0.8 mm	0.031 in
1.0 mm	0.039 in
1.2 mm	0.047 in
1.5 mm	0.059 in
2.0 mm	0.079 in
2.5 mm	0.098 in
3.0 mm	0.118 in

These conversions are useful when reviewing drawings, specifications, or tubing catalogs that use different unit systems.

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7. Metric Tubing Standards

Metric tubing dimensions, materials, tolerances, mechanical properties, and testing requirements may be governed by different international standards.

There is no single standard that defines every type of metric tubing. The correct standard depends on the tube material and intended service.

Common standards associated with metric tubing include DIN, EN, ISO, and ASTM specifications.

EN 10305 Precision Steel Tubes

The EN 10305 series covers precision steel tubing used in applications that require close dimensional tolerances and controlled mechanical properties.

It includes several manufacturing methods and product forms, such as seamless cold-drawn tubes and welded cold-sized tubes.

These tubes are commonly encountered in:

• Hydraulic systems
• Pneumatic systems
• Automotive equipment
• Machine tools
• Industrial machinery

EN 10305 tubing is often specified by:

Outside diameter × wall thickness

For example:

12 × 1.5 mm

or:

25 × 2 mm

The exact dimensional tolerance depends on the applicable part of EN 10305 and the delivery condition.

DIN 2391

DIN 2391 is historically associated with seamless precision steel tubing and remains widely referenced in hydraulic and industrial documentation.

Many modern specifications use corresponding EN standards, particularly EN 10305.

DIN 2391-style tubing is commonly associated with metric hydraulic systems where accurate tube dimensions are important for compression fitting performance.

Typical applications include:

• Hydraulic power units
• Mobile hydraulics
• Machine tools
• Lubrication systems
• Process machinery

DIN 2353

DIN 2353 is closely associated with metric compression tube fittings used in hydraulic and pneumatic systems.

The standard historically defines fitting dimensions and connection configurations for metric tubing.

These fittings are commonly categorized into series such as:

• Light series
• Heavy series

Tube outside diameter is a key selection parameter.

For example, a DIN-style fitting may be specified for:

• 6 mm tube
• 8 mm tube
• 10 mm tube
• 12 mm tube
• 15 mm tube
• 18 mm tube
• 22 mm tube
• 28 mm tube
• 35 mm tube
• 42 mm tube

The correct fitting series must be matched with the required tube OD, wall thickness, pressure, and application.

ISO 8434

ISO 8434 covers metallic tube connections for fluid power and general use.

The standard includes several connection technologies.

ISO 8434-1 is particularly relevant to 24-degree cone connectors commonly associated with metric hydraulic tubing.

These fittings are widely used in:

• Hydraulic equipment
• Industrial machinery
• Mobile equipment
• Agricultural machinery
• Construction equipment

Metric tube OD is used as a primary fitting size reference.

ASTM A269

ASTM A269 covers seamless and welded austenitic stainless steel tubing intended for general service.

Common grades include:

• 304
• 304L
• 316
• 316L

ASTM A269 tubing may be used in:

• Instrumentation
• Chemical processing
• Process systems
• Utility systems
• General industrial service

Although ASTM specifications are frequently associated with inch dimensions, metric dimensions may also be supplied depending on purchase requirements and manufacturer capability.

ASTM A213

ASTM A213 covers seamless ferritic and austenitic alloy-steel boiler, superheater, and heat-exchanger tubes.

It is typically associated with higher-temperature heat-transfer applications rather than general-purpose instrumentation tubing.

Applications include:

• Boilers
• Heat exchangers
• Superheaters
• Process equipment

The applicable tubing standard should always be selected according to the intended service rather than simply according to tube size.

ASTM A632

ASTM A632 covers seamless and welded austenitic stainless steel tubing intended for small-diameter general service.

It may be relevant in applications requiring relatively small tubing dimensions and controlled manufacturing requirements.

EN 10216 and EN 10217

EN 10216 covers seamless steel tubes for pressure purposes, while EN 10217 covers welded steel tubes for pressure purposes.

These standards are more closely associated with pressure tubing and piping applications than precision instrumentation tubing.

Selection depends on factors such as:

• Material grade
• Operating pressure
• Operating temperature
• Manufacturing method
• Inspection requirements
• Project specifications

Why the Tubing Standard Matters

Two tubes can have the same nominal dimensions but different mechanical and dimensional characteristics.

For example, two tubes may both be marked:

12 × 1.5 mm

but differ in:

• Material grade
• Yield strength
• Tensile strength
• Hardness
• OD tolerance
• Wall thickness tolerance
• Surface finish
• Heat treatment
• Testing requirements

Therefore, tube size alone is not sufficient when designing a pressure system.

The applicable material and dimensional standard should always be confirmed before final selection.

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8. How to Select the Correct Metric Tube Size

Choosing the correct metric tube size requires more than selecting an outside diameter from a chart.

A tube must satisfy both the hydraulic or flow requirements and the mechanical requirements of the system.

Important selection factors include:

• Flow rate
• Fluid velocity
• Pressure
• Temperature
• Tube material
• Wall thickness
• Corrosion resistance
• Fitting compatibility
• Bend requirements
• Installation space
• Applicable standards

Determine the Required Flow Capacity

The tube inside diameter determines the available flow area.

For a circular tube, internal flow area is:

Flow Area = 3.1416 × ID² ÷ 4

For example, a tube with an ID of 10 mm has an approximate flow area of:

Flow Area = 3.1416 × 10² ÷ 4

Flow Area ≈ 78.5 mm²

Increasing the inside diameter can significantly increase the available flow area and reduce fluid velocity.

Consider Fluid Velocity

Fluid velocity is related to flow rate and tube internal area.

If the tube is too small, velocity may become excessive.

High fluid velocity can result in:

• Increased pressure drop
• Higher energy consumption
• Noise
• Vibration
• Heat generation
• Erosion in severe applications

For hydraulic systems, different types of lines typically operate within different velocity ranges.

Suction lines generally require larger tubing because lower velocity helps reduce the risk of excessive inlet pressure loss and cavitation.

Pressure lines can usually tolerate higher fluid velocity.

Return lines typically operate between suction-line and pressure-line velocity ranges.

The exact design limit should be determined according to the system requirements and industry practice.

Select the Tube Outside Diameter

Once the required internal flow area is known, an appropriate tube OD can be selected.

For example, suppose the design requires approximately a 10 mm inside diameter.

Possible tubing combinations include:

Tube Size	OD	Wall	ID
12 × 1 mm	12 mm	1 mm	10 mm
14 × 2 mm	14 mm	2 mm	10 mm
16 × 3 mm	16 mm	3 mm	10 mm

All three have approximately the same inside diameter but very different outside diameters and wall thicknesses.

The final choice depends on pressure, material, available fittings, and installation requirements.

Select the Correct Wall Thickness

Wall thickness affects the tube’s ability to withstand internal pressure.

For the same outside diameter, increasing wall thickness generally increases pressure capability.

Consider a 12 mm OD tube:

Tube Size	Wall	ID
12 × 1 mm	1 mm	10 mm
12 × 1.5 mm	1.5 mm	9 mm
12 × 2 mm	2 mm	8 mm

The thicker-wall tube provides more material to resist pressure but has a smaller internal flow area.

This creates a design trade-off between:

• Pressure capability
• Flow capacity
• Weight
• Cost
• Bendability

Check Operating Pressure

The selected tubing must have an allowable working pressure greater than the maximum design pressure of the system.

Pressure selection should consider:

• Normal operating pressure
• Maximum allowable pressure
• Pressure spikes
• Temperature derating
• Cyclic loading
• Material strength
• Safety factor

Do not determine pressure capability only from OD and wall thickness.

The allowable working pressure should be verified against the applicable tubing specification, design code, or manufacturer’s pressure table.

Check Operating Temperature

Material strength can change with temperature.

For example, stainless steel tubing may have a lower allowable working pressure at elevated temperatures compared with room temperature.

Very low temperatures can also influence:

• Material toughness
• Seal performance
• Thermal contraction
• Fitting behavior

Temperature effects should therefore be included when selecting tubing for steam, cryogenic, furnace, or high-temperature process systems.

Select the Tube Material

The fluid and surrounding environment strongly influence material selection.

316/316L stainless steel is commonly chosen for:

• Corrosive environments
• Chemical service
• Instrumentation
• Offshore systems
• Process plants

Carbon steel is commonly used for:

• Hydraulic systems
• Machinery
• General industrial service

Copper may be used for:

• Refrigeration
• HVAC
• Utility service

Polymer tubing may be used for:

• Pneumatic systems
• Laboratory systems
• Chemical distribution
• Low-pressure utility systems

Material compatibility should always be checked against the process fluid.

Check Tube Fitting Compatibility

Tube OD must match the fitting size.

For example:

A 12 mm fitting is intended for a tube with approximately 12 mm OD.

It should not normally be assembled on:

• 1/2 in tube
• 13 mm tube
• 10 mm tube

Wall thickness also matters because some fittings have minimum wall requirements or pressure limitations.

Important tubing characteristics for fitting compatibility include:

• Tube OD
• OD tolerance
• Wall thickness
• Hardness
• Ovality
• Surface condition
• Material

Consider Tube Bending

The tube must be suitable for the required routing.

A larger OD or thicker-wall tube generally requires:

• Larger bending radius
• More bending force
• Larger bending equipment

Selecting tubing that is unnecessarily large can make installation more difficult.

Conversely, tubing that is too thin may experience:

• Flattening
• Wrinkling
• Excessive ovality
• Wall thinning

during bending.

Example 1: Instrumentation Tubing

Suppose an instrumentation system requires a compact stainless steel tube for transmitting process pressure to an instrument.

A possible choice might be:

6 × 1 mm 316L stainless steel

Dimensions:

• OD = 6 mm
• Wall = 1 mm
• ID = 4 mm

This small tube size may be suitable for impulse or instrumentation service where flow demand is low.

Final suitability depends on the required pressure rating, process fluid, temperature, and fitting specification.

Example 2: General Process Tubing

Consider:

12 × 1.5 mm 316L stainless steel

Dimensions:

• OD = 12 mm
• Wall = 1.5 mm
• ID = 9 mm

This provides a larger internal passage while remaining compact enough for many instrumentation and process applications.

Example 3: Hydraulic Tubing

Consider:

25 × 3 mm carbon steel tubing

Dimensions:

• OD = 25 mm
• Wall = 3 mm
• ID = 19 mm

This larger tube may be suitable for hydraulic fluid transfer where greater flow capacity is required.

The final pressure capability must be checked against the tubing material specification and fitting system.

Practical Tube Selection Checklist

Before ordering metric tubing, verify:

• Correct OD
• Correct wall thickness
• Required ID
• Material grade
• Maximum working pressure
• Operating temperature
• Fluid compatibility
• Tube standard
• Fitting compatibility
• Minimum bend radius
• Surface finish
• Cleanliness requirements
• Dimensional tolerances

A correctly selected tube should satisfy the complete system requirements rather than simply match a nominal diameter.

9. How to Measure and Identify Metric Tubing

Correctly measuring tubing is important when replacing an existing tube, selecting fittings, checking fabrication work, or identifying whether a tube is metric or imperial.

The most important measurements are:

• Outside diameter
• Wall thickness
• Inside diameter
• Tube material
• Surface condition

A caliper or micrometer is normally sufficient for basic dimensional identification.

Measure the Outside Diameter

The first step is to measure the tube outside diameter.

Use a vernier caliper, digital caliper, or micrometer and measure across the widest outside surfaces of the tube.

For example, if the measured OD is approximately:

12.0 mm

the tube is likely a 12 mm metric tube.

If the measured OD is approximately:

12.7 mm

it may instead be a 1/2 in imperial tube because:

1/2 in = 12.70 mm

This distinction is important because metric and imperial fittings are not automatically interchangeable.

Measure at More Than One Position

Tubing may become slightly oval due to:

• Manufacturing tolerances
• Bending
• Handling
• Clamping
• Mechanical damage

For this reason, measure the OD in at least two directions.

For example:

• Measurement 1 = 12.01 mm
• Measurement 2 = 11.98 mm

A small variation may be normal, depending on the applicable tubing tolerance.

If the difference is excessive, the tube may be oval or damaged.

Measure the Wall Thickness

Wall thickness can be measured directly at the open end of the tube using a suitable caliper or micrometer.

For example:

• OD = 12 mm
• Wall thickness = 1.5 mm

The tube can then be identified as:

12 × 1.5 mm

If the tubing is already installed and the end is not accessible, wall thickness may need to be determined from documentation, ultrasonic measurement, or other suitable inspection methods.

Calculate the Inside Diameter

If the OD and wall thickness are known, the inside diameter can be calculated using:

ID = OD – (2 × Wall Thickness)

For example:

Tube size:

18 × 2 mm

The inside diameter is:

ID = 18 – (2 × 2)

ID = 14 mm

Another example:

25 × 2.5 mm

gives:

ID = 25 – (2 × 2.5)

ID = 20 mm

This calculated ID is useful for estimating flow area and comparing different tubing sizes.

Measure the Inside Diameter Directly

When the tube end is accessible, ID may also be measured directly.

However, direct measurement may be less accurate when:

• The tube end is burred
• The tube is distorted
• The cut is not square
• The tube is very small
• The internal surface is irregular

For precision work, calculating ID from verified OD and wall thickness can often provide a more reliable nominal value.

How to Identify Metric vs. Imperial Tubing

The easiest way to distinguish metric and imperial tubing is by measuring the outside diameter accurately.

Some common comparisons are:

Measured OD	Likely Tube Size
6.00 mm	6 mm metric
6.35 mm	1/4 in
8.00 mm	8 mm metric
9.53 mm	3/8 in
10.00 mm	10 mm metric
12.00 mm	12 mm metric
12.70 mm	1/2 in
15.88 mm	5/8 in
16.00 mm	16 mm metric
19.05 mm	3/4 in
20.00 mm	20 mm metric
25.00 mm	25 mm metric
25.40 mm	1 in

Some sizes are very close.

For example:

• 16 mm = 16.00 mm
• 5/8 in = 15.875 mm

The difference is only about:

0.125 mm

A visual check is therefore not sufficient. A calibrated measuring tool should be used.

Check Tube Markings

Many manufacturers print or engrave information directly on tubing.

Tube markings may include:

• Manufacturer name
• Material grade
• Tube size
• Wall thickness
• Heat number
• Specification
• Lot number

For example, a stainless steel tube may be marked with information similar to:

316L – 12 × 1.5 mm

This is useful for confirming both dimensions and material.

However, markings can become difficult to read after long-term service, painting, corrosion, or handling.

Dimensional verification may still be necessary.

Identify the Tube Material

Dimensions alone do not identify the material.

For example, both stainless steel and carbon steel tubing may be supplied in:

12 × 1.5 mm

Material identification may involve:

• Tube markings
• Material certificates
• Project documentation
• Positive Material Identification
• Visual inspection
• Magnet testing as a preliminary check

For critical applications, material should be verified using approved documentation or testing methods rather than appearance alone.

Check Surface Condition

Before installing a mechanical tube fitting, inspect the outside surface.

Look for:

• Deep scratches
• Flat spots
• Dents
• Corrosion
• Weld defects
• Paint
• Heavy oxidation
• Out-of-round condition

A damaged sealing or gripping area can reduce fitting performance.

Surface condition is especially important in compression fittings because the ferrules interact directly with the outside surface of the tube.

Check the Tube End

The tube end should normally be:

• Cut square
• Free of excessive burrs
• Clean
• Round
• Free from deformation

A poorly prepared tube end can create problems during fitting installation, welding, or inspection.

After cutting, remove internal and external burrs without excessively chamfering or damaging the tube.

Measure Before Selecting Replacement Fittings

When replacing a fitting on an existing system, do not select the fitting based only on appearance.

For example, a tube that appears to be approximately 1/2 in could actually be:

• 12 mm
• 12.7 mm
• Another nearby size

Always measure the tube OD before ordering fittings.

This is especially important in facilities that contain equipment from different regions because the same system may contain both metric and imperial components.

Common Measurement Mistakes

Several errors can lead to incorrect tube identification.

Measuring the Inside Diameter Instead of OD

Tube fittings are commonly selected using outside diameter.

A technician may measure the internal opening and mistakenly use this value as the tube size.

For example, a tube with:

• OD = 12 mm
• ID = 9 mm

is a 12 mm tube, not a 9 mm tube.

Assuming Metric and Imperial Sizes Are Equivalent

A 12 mm tube is not the same as a 1/2 in tube.

Likewise:

• 6 mm is not 1/4 in
• 10 mm is not 3/8 in
• 20 mm is not 3/4 in
• 25 mm is not exactly 1 in

These sizes may be close, but they are not identical.

Ignoring Wall Thickness

Two tubes with the same OD can have very different internal dimensions.

For example:

Tube Size	ID
12 × 1 mm	10 mm
12 × 1.5 mm	9 mm
12 × 2 mm	8 mm

Wall thickness affects both flow capacity and pressure capability.

Measuring Over Paint or Coating

Paint, plating, contamination, or protective coating may increase the measured OD.

If possible, measure on a clean, representative section of the tube.

Measuring a Bent or Damaged Section

Avoid measuring directly on:

• A bend
• A dent
• A clamped area
• A flattened section

Use a straight, undamaged section whenever possible.

Practical Identification Example

Suppose an unknown tube is measured and the results are:

• Outside diameter = 15.98 mm
• Wall thickness = 1.49 mm

The nominal size is likely:

16 × 1.5 mm

The approximate ID is:

ID = 16 – (2 × 1.5)

ID = 13 mm

The next step is to verify:

• Material
• Applicable tubing standard
• Fitting compatibility
• Pressure rating

before using the tube in a pressure system.

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Conclusion

A Metric Tubing Size Chart is a useful reference for identifying and comparing tubing based on outside diameter, wall thickness, and inside diameter.

Metric tubing is normally designated using:

OD × Wall Thickness

For example:

12 × 1.5 mm

indicates:

• 12 mm outside diameter
• 1.5 mm wall thickness
• 9 mm inside diameter

Common metric tubing sizes range from small instrumentation tubing such as 3 mm, 4 mm, and 6 mm OD to larger hydraulic and industrial tubing such as 25 mm, 32 mm, 42 mm, and 50 mm OD.

However, tube size alone does not determine whether a tube is suitable for an application.

Engineers and technicians should also consider:

• Material grade
• Pressure capability
• Temperature
• Flow rate
• Fluid velocity
• Corrosion resistance
• Tube hardness
• Surface finish
• Dimensional tolerances
• Bend radius
• Applicable standards
• Fitting compatibility

Metric and imperial tubing should also be treated as separate sizing systems. Although some dimensions are close, such as 16 mm and 5/8 in tubing, they are not necessarily interchangeable.

For reliable system design and assembly, always verify the actual tube OD and wall thickness and confirm the selected tubing against the applicable material standard, design requirements, and fitting manufacturer’s technical data.

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