“Pipe” and “tube” are often used as if they mean the same thing, but in stainless steel purchasing they are specified differently, made to different standards and used for different jobs. Mixing them up is one of the most common reasons an order arrives in the wrong size. This guide explains the difference and how to specify each correctly.
1. The short answer
- Pipe is ordered by a nominal size (NPS or DN) and a schedule, and is used mainly to carry fluids and gases under pressure. Its outside diameter is fixed for each nominal size; the schedule sets the wall thickness, so the inside diameter changes with the schedule.
- Tube is ordered by its actual outside diameter and wall thickness (for example 25.4 × 1.65 mm, or 1″ × 0.065″), and is used where the exact OD matters — heat exchangers, instrumentation, sanitary lines, structural and decorative work.
2. Side-by-side comparison
| Pipe | Tube | |
|---|---|---|
| How the size is stated | Nominal pipe size (NPS, in inches) or DN, plus schedule (e.g. NPS 2 Sch 40S) | Actual OD × wall thickness, in mm or inches; sometimes wall by gauge (BWG) |
| What stays constant | The OD for a given NPS; the wall changes with schedule | The OD you order; the wall you order |
| Typical stainless standards | ASTM A312 (seamless and welded), A358 (welded large diameter), A790 (duplex); dimensions per ASME B36.19 / B36.10 | ASTM A269 (general service), A213 (seamless boiler, superheater and heat exchanger), A249 (welded heat exchanger), A270 (sanitary), A554 (welded mechanical and decorative); EN 10216-5 / EN 10217-7 |
| Main uses | Process piping, water and fluid transport, pressure systems connected with flanges and pipe fittings | Heat exchangers, condensers, instrumentation and hydraulic lines, sanitary lines, medical and precision parts, handrails, furniture and structures |
| Tolerances | Set by the pipe standard; suited to welded and flanged joints | Usually tighter OD and wall tolerances, needed for tubesheets, compression and clamp fittings |
| Shapes | Round | Round, and also square, rectangular, oval and special profiles |
3. Why the “same” size is not the same
A nominal pipe size is a name, not a measurement. For example, NPS 1 pipe has an outside diameter of 33.4 mm (1.315 in) whatever its schedule, while a 1-inch tube has an outside diameter of 25.4 mm. Order “1 inch” without saying pipe or tube and you may receive a product that does not fit your fittings.
The same applies in metric systems: a DN size identifies a pipe series, while tube is described by its real OD. Always state the actual OD and wall, or the NPS/DN and schedule, together with the standard.
4. Which one should you specify?
- Choose pipe when the line connects to pipe flanges, butt-weld or threaded pipe fittings and valves sized by NPS or DN — typical of process plants, water treatment and general fluid transport. See seamless pipe, welded pipe and the ASTM A312 guide.
- Choose tube when the part must fit a tubesheet, compression or clamp fitting, or a precision assembly — heat exchanger tube, sanitary tube, coiled instrumentation tubing, capillary tube.
- Choose structural or decorative tube for frames, handrails and furniture, including square, rectangular and oval sections.
5. What to write in your enquiry
- Pipe: standard (e.g. ASTM A312), grade, seamless or welded, NPS or DN, schedule, length, end finish (plain, bevelled, threaded)
- Tube: standard (e.g. ASTM A269, A213, A270), grade, seamless or welded, OD × wall, tolerance class if any, length or coil length, surface finish
- Both: quantity per size, testing and documents (EN 10204 3.1 or 3.2), packing and delivery term
How TeCarve can help
TeCarve is the sole export arm of two stainless steel tube factories in Foshan and Wenzhou, China, supplying both pipe and tube across 19 product lines. Send us your drawing or the size you have — NPS and schedule, or OD and wall — and we will confirm the matching standard before quoting. Every order ships with an EN 10204 3.1 mill test certificate. Response within 24 hours at sales@tecarve.com.
Frequently asked questions
Is stainless steel pipe stronger than tube?
Not inherently. Strength depends on the grade, the wall thickness and the condition. Pipe schedules often give heavier walls for pressure service, while tube can be specified with whatever wall the design needs.
Can I use tube instead of pipe?
Sometimes, but only if the OD matches your fittings and the tube standard is accepted for the pressure and service. Check the design code before substituting.
What does “Sch 40S” mean?
It is a stainless steel pipe schedule that sets the wall thickness for a given nominal size under ASME B36.19 (see our pipe size and schedule chart). The “S” denotes the stainless steel schedule series.
Why does pipe OD not match its nominal size?
For NPS 12 and smaller, the nominal size is a historical designation rather than a measured dimension, so the actual OD differs from the number. From NPS 14 upward, the OD equals the nominal size in inches.
Related reading: ASTM A269 tubing guide · Stainless steel pipe size & schedule chart · Seamless vs welded stainless steel pipe & tube · Stainless steel pipe price guide · Mill test certificates and EN 10204 3.1 explained
