"Pipe" and "tube" are used as if they mean the same thing. In the metal trade they don't. A buyer asking for "3/4 inch tube" and one asking for "3/4 inch pipe" want two very different products. Getting the pipe vs tube difference right avoids costly mistakes and shows buyers you know the business.
The core difference: how they are sized
- Pipe is specified by nominal pipe size (NPS) and schedule. The NPS is a name, not an exact measurement. For a given NPS, the outside diameter stays fixed while wall thickness changes with schedule.
- Tube is specified by actual outside diameter (OD) and wall thickness, both measured precisely, in mm or inches, with wall in mm, BWG or SWG.
Example: 1" NPS pipe has an OD of 33.4 mm. 1" OD tube has an OD of exactly 25.4 mm. Order one and receive the other, and nothing will fit.
For a full size table, see our NPS and DN pipe size chart.
Function: transport vs precision
- Pipes carry fluids and gases — water, oil, steam, gas, chemicals. What matters most is internal flow capacity and pressure rating.
- Tubes are used where precise outside dimensions matter — heat exchangers, boilers, condensers, instrumentation, hydraulic lines, structural and mechanical parts.
Standards for pipe and tube
| Product | Common ASTM standards |
|---|---|
| Carbon steel pipe | A53, A106, API 5L |
| Stainless steel pipe | A312 (seamless and welded), A358 (EFW large dia), A790 (duplex) |
| Stainless tube, general service | A269 |
| Boiler, superheater and heat exchanger tube | A213 (seamless alloy and stainless), A249 (welded stainless) |
| Carbon steel heat exchanger tube | A179 (cold drawn), A192 (boiler), A210 |
| Sanitary tube | A270 |
| Mechanical tube | A513, A519, A554 (welded stainless mechanical) |
Pipe dimensions follow ASME B36.10M (carbon and alloy) and ASME B36.19M (stainless). Tube dimensions come from the product standard and the order.
Tolerances
Tubes generally have tighter tolerances on OD and wall than pipes, because they must fit tube sheets, ferrules and compression fittings. For example, heat exchanger tubes under A213 or A249 have strict OD and minimum-wall requirements, and are often supplied with eddy current or hydro testing on every tube.
Pipe tolerances on wall thickness are typically –12.5% under nominal, which is accounted for in pressure design.
Shapes
Pipe is almost always round. Tube can be round, square, rectangular or oval — square and rectangular hollow sections are common in structures, furniture and automotive frames.
Ends and joining
- Pipe: plain end (PE), bevelled end (BE) for butt welding, threaded end (TE), grooved end. Joined with butt weld, socket weld, threaded or flanged fittings. See butt weld vs socket weld fittings.
- Tube: joined with compression fittings, flare fittings, orbital welding or expansion into tube sheets.
Price and quantity
Tube is usually sold in shorter fixed lengths and often costs more per kilogram because of tighter tolerances, cold drawing, surface finish and 100% testing. Heat exchanger tubes may be supplied in exact cut lengths or as U-tubes. Pipe is usually sold in 6 m or random lengths by weight or metre.
To work out weight per metre for either product, enter OD and wall in our free pipe weight calculator.
How to read enquiries correctly
Look for clues:
- "SCH 40", "SCH 80", "NB", "NPS", "DN" → pipe.
- "OD × WT", "BWG", "SWG", "U-tube", "A213", "A249", "A269" → tube.
- "Heat exchanger", "boiler", "condenser", "instrumentation" → almost always tube.
When unclear, reply with a short clarification: "Please confirm: 3/4" NPS pipe (OD 26.7 mm) or 3/4" OD tube (19.05 mm)?" A question like that impresses technical buyers and prevents wrong production.
Seamless and welded options
Both pipes and tubes can be seamless or welded. Seamless products are made by piercing a solid billet and are preferred for high pressure and critical service. Welded products are formed from strip and welded longitudinally, which makes them cheaper and more uniform in wall thickness. For heat exchanger tubes, welded and cold-drawn tubes to ASTM A249 are very common, while boiler and high-pressure tubes are often seamless to ASTM A213. Our seamless vs welded pipe guide compares the two routes in detail.
Frequently asked questions
Is tube stronger than pipe?
Neither is inherently stronger. Strength depends on material, OD and wall thickness. Pipe is designed and rated around pressure; tube around dimensional precision.
Why is pipe OD bigger than its nominal size?
For NPS 1/8" to 12", the nominal size is a historical name roughly linked to the inside diameter of old standard-weight pipe. From NPS 14" upward, the OD equals the nominal size in inches.
What is BWG?
Birmingham Wire Gauge is a system for tube wall thickness. Higher BWG numbers mean thinner walls — for example, 16 BWG is 1.65 mm and 14 BWG is 2.11 mm.
Are A312 and A269 interchangeable?
No. A312 is pipe for high-temperature and general corrosive service, sized by NPS and schedule. A269 is tubing for general service, sized by OD and wall.
Key takeaways
- Pipe = NPS and schedule; tube = exact OD and wall thickness.
- Pipe carries fluids; tube is for precision applications like heat exchangers and instruments.
- Different ASTM standards apply — check the spec in every enquiry.
- When in doubt, ask the buyer to confirm OD in mm.
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