Article
To many people, pipe and tube look the same: round, hollow and made of steel. In the steel trade they are different products, sized and specified in different ways. Mixing them up is one of the most common causes of wrong deliveries.
The core difference
- Pipe is designed to carry fluids. It is ordered by nominal pipe size (NPS) and schedule. The inside diameter matters most for flow, and the actual outside diameter is fixed by the size standard.
- Tube is used for heat exchangers, instruments and structural parts. It is ordered by actual outside diameter and wall thickness, with tighter dimensional tolerances.
Example
| Description | Actual OD | Wall |
|---|---|---|
| 2" NPS SCH 40S pipe | 60.3 mm | 3.91 mm |
| 2" OD tube, 16 BWG | 50.8 mm | 1.65 mm |
Both might be called "2 inch", but they will not fit the same fittings.
Sizing rules
- For pipe from 1/8" to 12", the NPS is only a name; the OD is larger than the nominal size. From 14" upwards, the NPS equals the OD in inches.
- Tube OD is the true dimension, in inches or millimetres, and wall thickness is given in mm, inches or BWG gauge.
Standards
| Product | Common stainless specifications |
|---|---|
| Pipe | ASTM A312, A358, A790 (duplex); dimensions ASME B36.19M |
| Tube | ASTM A213 (boiler/heat exchanger), A249 (welded), A269 (general service), A270 (sanitary) |
Tolerances
Tube is made to tighter OD and wall tolerances because it often fits into tube sheets, compression fittings or precision equipment. Pipe tolerances are wider but suit welding to buttweld fittings and flanges.
How to avoid mix-ups
Always state the product (pipe or tube), the specification and either NPS + schedule (for pipe) or OD × wall (for tube). Adding the actual OD in millimetres removes any doubt.
Frequently asked questions
Is a 2 inch pipe 2 inches in diameter?
No. A 2" NPS pipe has an actual outside diameter of 60.3 mm (2.375").
How is tube sized?
By its actual outside diameter and wall thickness.
Which ASTM standard covers stainless tube for general service?
ASTM A269 covers seamless and welded austenitic stainless tubing for general service.
Key takeaways
- Pipe = NPS + schedule; tube = actual OD × wall.
- From 14" up, pipe NPS equals the OD.
- Always state the product, specification and actual OD.
Need these products?
Sawariya Metal Industries supplies stainless steel pipes, flanges and buttweld pipe fittings from Mumbai to buyers in India and overseas. Send your size, grade, standard and quantity through our enquiry form and we will reply with a clear quotation and the test documents you need. You can also browse our stainless steel pipes, flanges and buttweld fittings.