Stainless Steel Tube Size Chart: OD, Wall Thickness, ID & Tolerances

Stainless steel round tubing is commonly specified by outside diameter × wall thickness, with length, material grade and delivery requirements stated separately. These dimensions influence fitting compatibility, internal flow area, fabrication and inspection.

For distributors, equipment manufacturers and engineering procurement teams, selecting the correct size requires more than matching a diameter. This guide from Shaanxi Shangyou Stainless Steel Co., Ltd. explains how to read stainless steel tubing dimensions, calculate inside diameter and specify tolerances clearly.

The essential relationship is:

Inside diameter = Outside diameter − 2 × Wall thickness

However, an ID calculated from nominal dimensions is a reference value. A guaranteed bore requires defined acceptance limits and inspection requirements.

For example, a tube specified as:

12 mm OD × 1.5 mm wall × 6,000 mm length

has a nominal calculated ID of:

12 − (2 × 1.5) = 9 mm

The same OD with a different wall thickness produces a different bore. A 12 × 1 mm tube has a calculated ID of 10 mm, while a 12 × 2 mm tube has a calculated ID of 8 mm.

These tubes may share an outside diameter, but their flow areas and wall-related performance differ.

The following table presents selected inch-based tubing combinations found in published manufacturer catalogs. It is a dimensional reference, not a complete standard size series or a pressure-rating table. Availability depends on the material, manufacturing route and supplier.

Calculation basis: 1 inch = 25.4 mm; ID = OD − 2t. Converted dimensions are rounded to three decimal places.

The ID values above are calculated from the original inch dimensions before rounding. Consequently, recalculating an ID using the rounded metric columns may produce a small rounding difference.

Metric tubing uses specified metric outside diameters. A metric size should not be treated as interchangeable with the nearest fractional-inch size.

The following selected combinations are represented in published hydraulic and instrumentation tubing programs. The ID values are calculated from nominal OD and wall thickness.

Three common purchasing distinctions are:

• 6 mm tubing and 1/4-inch tubing: 6.00 mm versus 6.35 mm OD.

• 12 mm tubing and 1/2-inch tubing: 12.00 mm versus 12.70 mm OD.

• 25 mm tubing and 1-inch tubing: 25.00 mm versus 25.40 mm OD.

For compression fittings, identify the exact tube OD and the fitting manufacturer’s approved tubing requirements. A close numerical match does not establish compatibility.

Tube and pipe dimensions follow different designation systems.

OD-sized tubing uses the stated outside diameter directly. For example, 1-inch OD tubing has a specified OD of 25.4 mm.

Pipe commonly uses Nominal Pipe Size — NPS, with a corresponding DN designation, and a specified schedule. For smaller pipe sizes, the nominal size number differs from the actual outside diameter. ASME’s stainless steel pipe dimensional standard distinguishes these pipe designations from tube OD sizing.

Schedules such as 5S, 10S, 40S and 80S belong to pipe dimensional specifications. They should not be substituted for an explicit tube wall thickness.

A schedule number does not represent one fixed wall thickness across all pipe diameters.

A nominal wall is the stated reference thickness. The permitted actual thickness depends on the governing specification and its applicable tolerances.

For example, if a particular order permits ±10% on a 1.00 mm nominal wall, the permitted range is 0.90–1.10 mm. The order therefore does not guarantee a 1.00 mm minimum.

ASTM A269/A269M covers nominal-wall-thickness austenitic stainless steel tubing for general service.

A minimum-wall order specifies a lower wall-thickness requirement under the applicable standard’s measurement and acceptance rules.

This distinction is especially relevant to heat-exchanger tubing. ASTM A213/A213M includes minimum-wall tubing and permits average-wall ordering when specified in the order. Buyers should state the required wall basis explicitly.

Gauge terminology can be ambiguous because different gauge systems use different thickness mappings.

For purchasing, state the actual wall thickness in millimeters or decimal inches. If a gauge designation is also required, identify the gauge system and its corresponding thickness.

A specification such as “0.065-inch wall” is clearer than an unspecified gauge number.

There is no single tolerance that applies to every stainless steel tube.

The applicable limits can depend on:

• Product standard and edition.

• Tube diameter and wall thickness.

• Seamless, welded or cold-worked construction.

• Nominal-, average- or minimum-wall ordering.

• Thin-wall provisions and geometric requirements.

• Additional requirements agreed in the purchase order.

Swagelok’s published tubing data uses an ASTM A269 example with an OD tolerance of ±0.005 inch and a wall tolerance of ±10%. These values illustrate the calculation and should not be extended automatically to every size or product condition.

Applying those tolerances gives:

The ID envelope is calculated from the OD and wall limits, using a circular, uniform-wall model. It is not a separately specified ASTM ID tolerance.

The calculations are:

Minimum calculated ID = Minimum OD − 2 × Maximum wall

0.4950 − (2 × 0.0385) = 0.4180 inch

Maximum calculated ID = Maximum OD − 2 × Minimum wall

0.5050 − (2 × 0.0315) = 0.4420 inch

This example demonstrates why a tube with a nominal ID of 0.430 inch should not be assumed to have that exact bore throughout its length.

EN ISO 1127 uses separate classes for diameter and wall-thickness tolerances. Selected class values are shown below.

“Minimum tolerance magnitude” means using the larger of the percentage-based allowance and the stated absolute allowance.

For a 25 mm OD tube ordered to D3:

• 0.75% × 25 mm = 0.1875 mm.

• The applicable allowance is therefore ±0.30 mm.

For a 1.5 mm wall ordered to T3:

• 10% × 1.5 mm = 0.15 mm.

• The applicable allowance is therefore ±0.20 mm.

These classes apply only when called up by the relevant product specification or order. They are not automatic requirements for all metric stainless steel tubing.

OD and wall-thickness limits do not describe every feature of a tube’s geometry.

Ovality describes the difference between the maximum and minimum diameters at the same cross-section. If ovality is specified as a percentage, the calculation basis must also be identified.

Wall-thickness variation describes differences in wall thickness around the circumference or along the length. A satisfactory average thickness does not establish the local minimum wall.

Eccentricity occurs when the inner and outer profiles are not concentric. This can affect machining allowance and internal clearance.

Internal weld profile can influence usable bore in welded tubing. For applications involving close-fitting inserts, hygienic flow paths or precision assembly, specify the required internal weld condition and bore acceptance criteria.

For precision tube fittings, permissible ovality also needs attention. Tubing that satisfies a product standard may still fall outside a particular fitting manufacturer’s requirements.

When internal clearance is critical, define a minimum clear bore, the inspection method and the required inspection coverage. Measurements taken only at the tube ends may not identify restrictions elsewhere.

A dimensional match does not establish that two tubes are interchangeable. The product standard also determines material, manufacturing and testing requirements.

These product categories have different scopes. Their requirements should be checked individually before substitution.

For sanitary tubing, a description such as “polished tube” is incomplete. ASTM A270 recognizes specified finish options, including maximum roughness requirements. The order should identify the surfaces covered and the required finish.

Where applicable, ASTM A1016/A1016M supplies general requirements alongside the product specification. Its precedence provisions give the product specification priority over conflicting general requirements and recognize more stringent purchase-order requirements.

The order should also identify the governing unit system rather than mixing rounded metric and inch limits.

An effective inspection plan connects each critical dimension to a suitable measurement method and acceptance limit.

Measurement equipment must be appropriate for the tube diameter, wall thickness and required accuracy. Thin walls and small bores can require specialized methods.

For critical orders, request a dimensional inspection report linked to the relevant heat, lot or bundle identification. A material test certificate alone should not be assumed to contain individual OD, wall and ID measurements.

A size chart establishes geometry. Pressure suitability additionally depends on material properties, temperature, allowable stress, minimum wall after tolerances, applicable design rules, fabrication allowances and connection ratings.

Before selecting a heavier wall, also check the resulting bore. At a fixed OD, increasing wall thickness reduces the internal flow area.

A useful inquiry should allow suppliers to quote against the same technical requirements.

Include:

1. Material grade: such as TP316L, with the required designation.

2. Product standard and edition.

3. Manufacturing route: seamless or welded, as required.

4. OD × wall thickness, including the wall basis.

5. Length and quantity.

6. Dimensional and geometric tolerances.

7. Delivery condition and surface finish.

8. Testing, inspection documents and traceability requirements.

9. Destination and required delivery date.

An illustrative RFQ description is:

ASTM A269/A269M TP316L seamless stainless steel tubing, specified edition, 12.00 mm OD × 1.00 mm nominal wall × 6,000 mm length, bright annealed condition. Quantity: 1,000 m. Please state the applicable OD, wall, length and ovality limits in the quotation. Include the proposed testing, inspection documentation and dimensional inspection scope. Any additional bore requirement will be provided on the drawing.

This format makes technical differences between quotations easier to identify.

For procurement inquiries, send your tube specification or drawing to Shaanxi Shangyou Stainless Steel Co., Ltd., together with the quantity, application and inspection requirements.

Request a Tube Quotation | Submit Your Drawing | Request Dimensional Inspection Details