Stainless Pipe and stainless tube are the two most confused words in stainless steel procurement, and the confusion costs money because the two products are specified, sized, tolerated, and priced differently. A buyer who orders "316L tube" against a pipe drawing, or "1-inch pipe" when the design means tube, will get a product that does not fit the fittings, does not meet the pressure calculation, and may not even be manufactured to the right dimensional tolerance.
The difference is not a marketing distinction; it is a difference in the dimensional system itself. This page explains how pipe and tube are defined, how the sizing systems work, what tolerances apply to each, and what to put on the enquiry so the supplier delivers exactly the product the design requires.
The core difference: how each product is sized
The cleanest way to separate the two is the sizing system. Pipe is sized by a nominal pipe size (NPS) plus a schedule number, and its outside diameter is fixed by the size regardless of the wall.
Tube is sized by an exact outside diameter and an exact wall thickness, both stated in millimetres or inches, and the outside diameter is not tied to a nominal size table. This one difference drives everything else - the tolerance system, the manufacturing route, the fittings, and the price.
| Characteristic | Pipe | Tube |
| Sizing | NPS (nominal pipe size) + schedule | Exact OD x wall thickness |
| Outside diameter | Fixed per NPS table (e.g. NPS 2 = 60.3 mm OD) | Any specified value (e.g. 25.4 mm x 1.65 mm) |
| Wall designation | Schedule: 10S, 40S, 80S, etc. | Explicit wall in mm or in (sometimes gauge) |
| Dimensional standards | ASME B36.10, B36.19 | No unified size table; OD and wall per order |
| Main product standards | ASTM A312, A358, A790 | ASTM A269, A213, A789 |
| Typical use | Fluid transport under pressure | Heat exchangers, instrumentation, boilers, mechanical parts |
| Fittings | Butt-weld fittings to ASME B16.9 | Instrument fittings, compression fittings, or custom |
Notice what the table does not say: pipe is not "big" and tube is not "small". Small-diameter pipe exists, and large-diameter tube exists. The distinction is purely the dimensional system, and it is the reason the same material can be supplied as both pipe and tube to different standards. A 60.3 mm OD product is pipe when it is ordered as NPS 2 SCH 40S to ASTM A312; the same outside diameter can be tube when it is ordered as 60.3 mm OD x 3.05 mm wall to ASTM A269.
How the pipe sizing system works
Pipe dimensions are governed by ASME B36.10 (carbon and alloy steel) and ASME B36.19 (stainless steel). In this system each nominal size has a fixed outside diameter, and the schedule number defines the wall thickness.
For example, NPS 1 pipe is always 33.4 mm outside diameter whether it is SCH 10S or SCH 80S; only the wall and the bore change. This is what makes pipe fittings and flanges work: a 1-inch butt-weld fitting fits every 1-inch pipe regardless of schedule, because the outside diameter and the socket bore are standardized.
The schedule number is roughly related to the ratio of design pressure to allowable stress, which is why SCH 80S carries a higher pressure rating than SCH 40S at the same size and material.
Because the OD is fixed, a heavier schedule means a smaller bore and a heavier pipe per metre. For stainless steel, the "S" schedules (5S, 10S, 40S, 80S) are the ones used in process plants, and the wall thicknesses are listed in ASME B36.19.
The pipe outside diameter table is identical to B36.10, but the stainless wall thicknesses differ slightly from carbon steel schedules in some sizes, which is why a stainless pipe drawing must cite B36.19 rather than assuming B36.10 walls.
How the tube sizing system works
Tube has no schedule and no fixed outside-diameter table. The buyer specifies the outside diameter and the wall thickness explicitly - 25.4 mm x 1.65 mm, 12.7 mm x 1.0 mm, or 6.35 mm x 0.89 mm - and the mill produces exactly that dimension.
Because the dimensions are explicit, tube tolerances are also expressed as exact values rather than percentages of a schedule: a tube outside diameter tolerance is commonly +/-0.10 mm or +/-0.05 mm depending on the specification and size, whereas a pipe outside diameter tolerance is a percentage of the nominal OD.
This is the tolerance difference that causes most specification mix-ups. ASTM A312 pipe is allowed an outside-diameter tolerance of about +/-1% (typically +/-0.5 mm on a 25 mm pipe), because it is made to a nominal system.
ASTM A269 tube holds a much tighter outside diameter tolerance - commonly +/-0.10 mm on small sizes - because tube is often used in heat exchangers where the tube must seal into a tubesheet and where the internal diameter governs flow. The same material, the same mill, can ship to both standards, and the tolerance system is what changes.
Standards to use for each product
The material grade can be identical, but the product standard is selected by the application:
- Pipe: ASTM A312 (seamless and welded austenitic pipe), A358 (electric-fusion-welded large-diameter pipe), A790 (duplex and super duplex pipe), A928 (EFW duplex)
- Tube: ASTM A269 (seamless and welded austenitic tube for general and heat-exchanger service), A213 (seamless ferritic and austenitic boiler tube), A789 (seamless and welded duplex tube), A511/A554 (mechanical tube)
- Bare tubing for instrumentation: A269 is the default; for hydraulic and instrumentation lines A269 with a smooth bore is commonly specified
When the enquiry says "316L tube", the supplier will ask for the OD x wall and the standard; when it says "316L pipe", the supplier will ask for NPS and schedule. A complete enquiry states both the product word and the standard, because "tube to A312" is contradictory and "pipe to A269" is also contradictory - the standard itself defines which product it covers.
Pressure ratings and the practical consequence
For pressure design, pipe is calculated through the piping code using the schedule wall and the allowable stress. Tube used in a heat exchanger or an instrument line is calculated with the explicit wall, and because the wall is exact rather than nominal, the design margin is more visible. In practice the same material at the same wall carries the same pressure whether it is called pipe or tube; the difference is in how the dimensions are ordered and verified, not in the physics.
What changes commercially is the price: pipe is quoted per metre by NPS and schedule, tube is quoted per metre by OD x wall, and the same physical tube can be cheaper or dearer than the pipe of equivalent dimensions depending on the mill's product mix.
What to write on the enquiry: the five fields that end the confusion
A pipe/tube enquiry is unambiguous when it contains these five items:
- Product word and standard: "pipe to ASTM A312" or "tube to ASTM A269" - never just "316L pipe/tube"
- Size in the correct system: NPS + schedule for pipe (e.g. NPS 2 SCH 40S), OD x wall in mm for tube (e.g. 25.4 x 1.65 mm)
- Grade and condition: e.g. TP316L, annealed, or "cold drawn, annealed" for tube
- Length and end finish: fixed 6 m lengths, plain end or beveled end, cut lengths for tube
- Certificate requirement: EN 10204 3.1, and the dimensional tolerance class if the application is critical (e.g. heat-exchanger tube)
Buyers who write these five fields on the enquiry get quotations that are comparable and deliveries that fit. Buyers who write "316L seamless 25.4 mm pipe" or "2 inch tube" get a telephone call back, a clarification, and often a wrong or delayed shipment. The cost of the clarification is small; the cost of the wrong dimension arriving on site is not.
FAQ
Q: Is pipe stronger than tube of the same material?
A: No. Strength depends on the grade and wall thickness, not on the product word. Pipe and tube in the same grade and wall carry the same pressure; the difference is the sizing and tolerance system, not the strength.
Q: Can I use tube instead of pipe for a process line?
A: Only if the dimensions are stated in the pipe system (NPS + schedule) and the standard covers the product. A tube of matching OD and wall can substitute for pipe where the code allows, but the fittings, flanges, and tolerances are different, so substitution should be checked against the design before purchase.
Q: What is the difference between NPS and OD?
A: NPS is the nominal pipe size - a name, not a dimension. NPS 2 pipe is 60.3 mm outside diameter, not 50.8 mm. The OD is fixed by the size table in ASME B36.10/B36.19. Never multiply NPS by 25.4 to get the pipe OD.
Q: Why are tube tolerances tighter than pipe tolerances?
A: Because tube is used where the dimension is functional - heat-exchanger tube seals into tubesheets, instrument tube must fit compression fittings. Pipe tolerances are percentages of nominal sizes; tube tolerances are exact values such as +/-0.10 mm.
Q: Does A312 cover tube?
A: No. ASTM A312 covers seamless and welded austenitic pipe only. Tube is covered by A269, A213, A789 and related standards. Ordering "tube to A312" will not produce a compliant tube certificate.
Q: What does SCH 40S mean on stainless pipe?
A: SCH 40S is the stainless steel schedule 40 wall per ASME B36.19. The "S" distinguishes the stainless schedule table from the carbon steel B36.10 table, which differs in some sizes.
