Sanitary Stainless Steel Tube Size Chart: Imperial vs Metric OD

Sanitary stainless steel tubing must be selected by its dimensional series, outside diameter and wall thickness—not by a size label alone.

In common inch-based sanitary tubing systems, a 2-inch tube has a specified outside diameter of 50.8 mm. In EN 10357 Series A, a DN50 tube has a specified outside diameter of 53 mm. These dimensions describe different products.

The distinction matters when purchasing replacement tubing, connecting imported equipment or matching tubes with hygienic valves, ferrules and fittings. A correct unit conversion does not establish interchangeability between dimensional standards.

Four terms commonly appear on sanitary tube drawings and purchase orders:

• Outside diameter, OD: The external tube diameter, subject to the specified manufacturing tolerance.

• Inside diameter, ID: The internal flow passage.

• Wall thickness, t: The thickness of the tube wall.

• Nominal diameter, DN: A size designation used within a particular dimensional system.

For a circular tube with uniform wall thickness:

ID = OD − 2t

DN should not be interpreted as a direct measurement in millimetres. Its relationship to OD depends on the applicable standard and series.

For reliable identification, record both the size designation and the specified OD.

In common inch-series sanitary tubing, the stated tube size corresponds to the specified outside diameter.

The conversion is:

1 inch = 25.4 mm exactly

These sizes appear across commercial sanitary and high-purity tubing programs. Available wall thicknesses, finishes and specification coverage vary by product. The table is an OD reference, rather than a complete ASME BPE or ASTM dimensional specification.

Some catalogues display rounded metric conversions. For example, 3/4-inch tubing may be shown as 19.1 mm, although its exact inch-to-millimetre conversion is 19.05 mm.

When the inch dimension governs, retain that basis in drawings and orders. A rounded display value should not silently become a different manufacturing target.

EN 10357 includes dimensional series that must be identified in the order. Series A is commonly associated with the former DIN 11850 Series 2 dimensions.

The following table shows selected Series A sizes.

These dimensions are documented in hygienic-component manufacturers’ technical information. Guth identifies Series A with the former DIN 11850 Series 2, while Alfa Laval publishes the corresponding outside-diameter sequence.

An order stating only “EN 10357, DN50” leaves the dimensional series unclear.

For example, the cited manufacturer information lists:

• Series A, DN50: 53 mm OD

• Series B, DN50: 52 mm OD

The difference affects tube-end matching and component selection. State the series together with the actual OD and wall thickness.

ISO 2037 remains a familiar designation in installed hygienic systems and supplier catalogues. However, ISO 2037:1992 is listed as withdrawn by ISO. For new procurement, identify the current project specification and use legacy references carefully when matching existing equipment.

Manufacturer catalogues using the ISO 2037 designation include the following OD values:

The catalogue dimensions above are shown in Alfa Laval’s ISO 2037 product data. This is a comparison of selected published products, not a claim that all legacy ISO 2037 products share identical wall thicknesses or connection details.

Some outside diameters match the inch series exactly; others differ. Even where OD matches, wall thickness, bore, tolerances and connection geometry still need verification.

Small nominal differences should not be dismissed because manufacturing tolerances might overlap. Compatibility must be established against the applicable drawings and acceptance requirements.

Sanitary tubing and scheduled process pipe use different sizing conventions.

A 2-inch sanitary tube in the inch-OD series has an OD of 50.8 mm. An NPS 2 process pipe has an OD of approximately 60.3 mm. Manufacturer ANSI/ASME pipe tables document this larger pipe outside diameter.

Consequently, an order for “2-inch stainless steel pipe” can produce the wrong product when the requirement is sanitary tubing.

Use a description that identifies the product form and dimensional basis, such as:

Sanitary stainless steel tube, 2.000 in OD × specified wall thickness

If a sanitary tube must connect to scheduled pipe, select an appropriate transition component with defined end dimensions.

Outside diameter is only one part of the size specification.

Consider an inch-series tube with:

• OD: 2.000 in

• Nominal wall: 0.065 in

Its calculated nominal ID is:

2.000 − 2 × 0.065 = 1.870 in

Converted to millimetres:

• OD: 50.800 mm

• Wall: 1.651 mm

• Calculated ID: 47.498 mm

A metric tube specified as 53 mm OD × 1.5 mm wall has a calculated nominal ID of 50 mm.

These products therefore differ in both their external connection dimensions and internal flow passage. The wall values used here are examples found in published sanitary tube programs; they are not universal requirements for every tube of those sizes.

Two components with the same OD can still have different IDs if their wall thicknesses differ.

For hygienic welded assemblies, check the tube-to-fitting match against the required internal alignment and weld acceptance criteria. Excessive internal mismatch can create an undesirable step in the product-contact passage.

The welding specification should establish the permitted condition. Matching the OD alone does not complete the assessment.

Sanitary clamp connections introduce another diameter: the outside diameter of the ferrule’s clamp end.

Three measurements must be distinguished:

1. Tube outside diameter.

2. Ferrule clamp-end outside diameter.

3. Internal flow bore.

These dimensions serve different purposes.

Dixon’s sanitary fitting sizing guide, for example, lists the same clamp-end outside diameter for its 1-inch and 1-1/2-inch sanitary sizes, even though their internal passages differ.

A clamp that physically closes around two ferrules does not establish that the complete connection is correctly matched.

Check:

• Ferrule standard and dimensional series.

• Clamp-end diameter and profile.

• Tube-end OD and wall.

• Gasket profile and bore.

• Component alignment.

• Applicable pressure and temperature ratings.

For replacement work, measure the straight tube section as well as the ferrule. A ferrule-face measurement alone can lead to an incorrect size identification.

The dimensional series and the product specification should be stated together.

ASTM A270/A270M covers specified seamless, welded and heavily cold-worked welded stainless steel sanitary tubing with special surface finishes. Its scope includes dairy and food applications, with pharmaceutical quality available through supplementary requirements.

The standard also states that its SI and inch-pound systems are to be used independently; values in the two systems are not necessarily exact equivalents.

This reinforces the need to identify which unit system governs the purchase order.

ASME BPE addresses equipment for bioprocessing, pharmaceutical and other applications with high hygienic requirements. Its scope includes materials, design, fabrication, inspection and testing.

A tube matching a familiar BPE outside diameter has not, by that fact alone, demonstrated compliance with all applicable BPE requirements.

EN 10357 specifies dimensions, tolerances, materials, surface characteristics and marking for longitudinally welded stainless steel tubes used in food and chemical industries.

The order should identify the applicable dimensional series and required surface or delivery condition.

Older equipment drawings may reference DIN 11850, ISO 2037 or other established systems.

Use the original equipment documentation to establish the required interface. A newer standard designation should not be substituted without checking the dimensions and technical requirements.

A practical identification process combines measurement with documentation.

Use a suitable calibrated instrument on an accessible, undamaged straight section. Measure in more than one direction to identify ovality.

Avoid using a distorted cut end, weld bead or ferrule face as the only measurement location.

Use an accessible end measurement or an appropriate qualified thickness-measurement method.

Where possible, compare the result with the original tube marking or material records.

Look for the material grade, product standard, dimensions, heat number and surface or execution designation.

A marking such as “316L” identifies the alloy grade but does not identify the dimensional series.

Confirm the valve, fitting, ferrule or equipment nozzle dimensions. For existing installations, the interface drawing can resolve ambiguities that measurement alone cannot.

Replace vague descriptions such as “approximately 2-inch metric tube” with a definite OD, wall thickness and dimensional standard.

A complete sanitary tube specification should address the finished product and its intended connections.

Where roughness is controlled, state the required Ra, its units and the relevant measurement requirements.

An abrasive grit designation alone does not establish one guaranteed surface roughness. Also distinguish mechanical polishing, electropolishing and other finishing processes where the application requires them.

A 20-foot tube is 6,096 mm long. A 6 m tube is 6,000 mm long.

The 96 mm difference can matter in prefabricated assemblies, cutting plans and replacement work. Specify the finished length rather than treating these commercial descriptions as identical.

An OD chart does not establish a pressure rating or hygienic suitability.

Wall thickness, material, joints, clamps and gaskets must meet the operating conditions. Line sizing should also consider product flow, pressure loss and the intended cleaning process.

They are different size descriptions. Inch-series 2-inch tubing has a specified OD of 50.8 mm, while EN 10357 Series A DN50 tubing has a specified OD of 53 mm. State the standard and actual OD before selecting a replacement.

No. It may refer to a metric dimensional standard, a legacy product family or an inch-series tube described in millimetres. The applicable standard, series and dimensions must be identified.

Their specified ODs differ: 1-1/2 inch converts exactly to 38.1 mm. Assess the actual dimensional requirements and mating components before treating the products as interchangeable.

Yes, some sanitary fitting sizes share a clamp-end diameter. The tube bore, ferrule details and gasket still need to match the intended connection.

ISO lists ISO 2037:1992 as withdrawn. The designation remains in supplier catalogues and installed equipment documentation, so legacy dimensions may still be relevant when specifying replacement components.

No. Wall thickness, internal alignment, ferrule or weld geometry, gasket selection, surface condition and assembly quality also affect the finished connection.

A reliable sanitary tube order connects the size designation to a definite dimensional series, OD, wall thickness and mating interface.

Use exact unit conversions for reference, retain the governing specification, and verify the complete connection. This approach makes imperial and metric sanitary tube comparisons useful for purchasing, fabrication and maintenance.