Metric Stainless Steel Tubing Sizes & Specifications
A practical guide to outside diameter, wall thickness, stainless steel grades, dimensional standards, and fitting compatibility.
Understanding Metric Stainless Steel Tubing
Metric stainless steel tubing is identified using measurements expressed in millimeters. Before selecting tubing for an industrial installation, it is useful to understand how outside diameter (OD), wall thickness (WT), and inside diameter (ID) relate to each other and to the components being joined.
Applications and connection methods vary. Industrial vacuum and pneumatic conveying tubing should not automatically be assumed suitable for high-pressure hydraulic, instrumentation, or process service simply because it is stainless steel or shares a nominal dimension with another product.
Outside Diameter, Wall Thickness & Inside Diameter
Outside Diameter (OD)
The distance across the outside of the tube. OD is a key measurement when matching straight tubing to elbows, reducers, couplings, and adapters.
Wall Thickness (WT)
The thickness of the metal wall. Available thicknesses depend on the product specification, material, and manufacturing method.
Inside Diameter (ID)
The open internal dimension of the tube. For a round tube with uniform wall thickness, ID can be calculated from OD and WT.
Dimensional Tolerances
Manufacturing tolerances describe allowable variations in size. The relevant tolerance requirements depend on the selected product and joining method.
Illustrative Metric Dimension Examples
The following examples demonstrate the arithmetic only. They are not a list of stocked or available products.
| Example OD | Example WT | Calculated ID | Note |
|---|---|---|---|
| 6 mm | 1 mm | 4 mm | Illustrative dimensions only |
| 12 mm | 1 mm | 10 mm | Illustrative dimensions only |
| 25 mm | 2 mm | 21 mm | Illustrative dimensions only |
Metric Tubing Compared with Nominal Pipe Sizes
Metric tubing is commonly specified by its actual outside diameter and wall thickness, such as 12 mm OD × 1 mm WT. Pipe may instead be designated using a nominal pipe size (NPS or DN) together with a schedule or another wall designation. Nominal pipe designations are not interchangeable with measured outside diameters without checking the applicable size tables.
Schedule 40 and Schedule 80 are pipe-wall designations, not universal descriptions of metric tubing wall thickness. Do not assume that a metric tube and a scheduled pipe will fit the same coupling or adapter.
| Term | What It Identifies | What to Verify |
|---|---|---|
| OD | Outside measurement of the tube | Actual OD and allowable variation |
| WT | Thickness of tube wall | Specified nominal or minimum wall |
| ID | Internal opening | Calculated or measured inside diameter |
| NPS / DN | Nominal pipe designation | Corresponding actual OD |
| Schedule | Pipe wall designation | Actual wall thickness for that nominal size |
Stainless Steel Grades: 304, 316 & 316L
Stainless steel grade is a separate selection consideration from diameter and wall thickness. Common grade designations include 304, 316, and 316L. Chemical exposure, temperature, cleaning processes, fabrication requirements, and applicable specifications should be considered when comparing grades.
304 Stainless Steel
A common austenitic stainless steel grade used for a range of industrial applications. Suitability depends on the actual environment and service conditions.
316 / 316L Stainless Steel
316 contains molybdenum and can offer improved resistance in certain corrosive environments compared with 304. 316L has a lower carbon limit relevant to some fabrication and welding considerations.
A grade designation alone does not establish pressure capability, food-contact suitability, sanitary finish, certification, or expected component service life. Review the specific product documentation when these requirements apply.
How ISO and ASTM Standards Relate to Tubing
Technical standards provide definitions and requirements for particular types of tubing. Their applicability depends on the material, intended service, manufacturing specification, and purchasing requirements.
| Standard | General Subject | Selection Consideration |
|---|---|---|
| ISO 1127 | Dimensions, tolerances, and conventional masses per unit length for stainless steel tubes | Check which dimensional requirements and editions apply |
| ISO 4200 | Dimensions and masses per unit length of plain-end steel tubes | Confirm the relevant sizing designation |
| ASTM A269/A269M | Seamless and welded austenitic stainless steel tubing for general service | Confirm product specification and documented compliance when needed |
| ASTM A270/A270M | Seamless and welded austenitic and ferritic/austenitic stainless steel sanitary tubing | Relevant when sanitary tubing requirements apply |
Dimensional Tolerances, Ovality & Connection Compatibility
Two tubes with the same nominal OD may differ slightly in actual diameter, roundness, wall thickness, or end preparation. These differences can matter when joining tubing to other components.
- OD tolerance: Check the allowable variation for the tubing specification.
- Ovality: Check roundness requirements where the connection relies on contact around the tube circumference.
- Tube-end condition: Check for burrs, damage, deformation, and the specified end preparation.
- Coupling dimensions: Verify compatibility with the actual tubing OD and the coupling manufacturer's requirements.
- Seal and gasket material: Consider the conveyed material, temperature, and operating conditions.
For larger industrial vacuum systems using compression couplings, the applicable coupling dimensions and instructions should be confirmed before installation. Couplings intended for industrial vacuum or conveying service should not be assumed suitable for high-pressure service without the manufacturer's documentation.
Wall Thickness, Temperature & Pressure: What to Check
Working-pressure capability cannot be established from OD and wall thickness alone. Material grade, manufacturing specification, minimum wall, temperature, joint design, applicable code, and installation details may all be relevant.
This article does not provide working-pressure or burst-pressure ratings. Industrial vacuum tubing offered for dust collection or pneumatic conveying must not be treated as pressure-rated hydraulic tubing unless an applicable product specification explicitly supports that use.
When an application requires a pressure rating, request the manufacturer's technical data and have the intended design reviewed by a qualified professional.
What to Include in a Metric Tubing Inquiry
Dimensions & Material
- Outside diameter in millimeters
- Wall thickness in millimeters
- Stainless steel grade, if specified
- Length and quantity
- Required end configuration
Application & Fitment
- Intended system and conveyed material
- Operating temperature and environment
- Vacuum or pressure requirements, if applicable
- Existing fittings or coupling specifications
- Drawings or measurements where available
Not every metric diameter, grade, wall thickness, or certification is necessarily available. Product suitability, specifications, and availability should be confirmed for the requested application.
Related Industrial Vacuum Tubing & Fittings
For industrial vacuum, dust collection, and pneumatic conveying components, the following pages provide additional product categories and information.
Frequently Asked Questions
How are metric stainless steel tubing sizes written?
Usually with an outside diameter and a wall thickness in millimeters, such as 12 mm OD × 1 mm WT. Confirm how the manufacturer expresses the dimensions.
Is metric tubing the same as nominal pipe?
No. Metric tubing may be specified by actual OD and WT, while pipe may use nominal sizes and schedules. Always compare the actual measurements.
Can I calculate inside diameter from OD and wall thickness?
For a round tube with uniform wall thickness, the nominal inside diameter equals OD minus twice the wall thickness.
Does thicker wall tubing automatically have a higher approved pressure rating?
No. A documented pressure rating depends on the complete product specification, joint design, service conditions, and applicable requirements.
Are all metric sizes compatible with Morris-type compression couplings?
Compatibility must be confirmed using the actual outside diameter, coupling specifications, gasket selection, and intended service. Do not assume interchangeability based on size naming alone.
Questions About Tubing Dimensions or Fittings?
Send your dimensions, quantities, material preferences, and fitting requirements to Processing Equipment Inc. for review. Product options and availability depend on the requested specifications.