Article
A pipe schedule is a way of stating wall thickness. It is not a thickness in itself: Schedule 40 is a different wall thickness at every pipe size. That is the first thing to understand, and it explains most of the confusion around schedules.
Nominal pipe size and outside diameter
Pipe sizes are given as NPS (nominal pipe size) in inches, or DN in millimetres. For sizes up to 12 inches the NPS does not match any real dimension. What is fixed for each NPS is the outside diameter. A 2 inch pipe is 60.3 mm on the outside whatever its schedule; a heavier schedule simply adds wall thickness inwards and reduces the bore.
This is what lets fittings and flanges fit pipe of any schedule at a given size: the outside diameter is the same.
Wall thickness for common sizes
Values from ASME B36.10M, in millimetres.
| NPS | OD | SCH 40 | SCH 80 | SCH 160 | XXS |
|---|---|---|---|---|---|
| 1/2" | 21.3 | 2.77 | 3.73 | 4.78 | 7.47 |
| 1" | 33.4 | 3.38 | 4.55 | 6.35 | 9.09 |
| 2" | 60.3 | 3.91 | 5.54 | 8.74 | 11.07 |
| 4" | 114.3 | 6.02 | 8.56 | 13.49 | 17.12 |
| 6" | 168.3 | 7.11 | 10.97 | 18.26 | 21.95 |
STD, XS and XXS
The older weight classes are still in daily use. STD (standard) is the same as Schedule 40 up to 10 inches, and a fixed 9.53 mm wall from 12 inches up. XS (extra strong) matches Schedule 80 up to 8 inches. XXS (double extra strong) is a separate, very heavy wall. Above the sizes where they coincide, STD and SCH 40 diverge, which catches people out on large-bore orders.
The stainless steel S series
ASME B36.19M covers stainless steel pipe and adds light-wall schedules with an S suffix: 5S, 10S, 40S and 80S. Because stainless steel resists corrosion, it rarely needs the corrosion allowance a heavier carbon steel wall provides, so thinner walls are common. 40S and 80S match Schedule 40 and 80 in most sizes up to 10 inches, but not in every size above that.
Choosing a schedule
The schedule is the result of a calculation, not a preference. The designer works out the minimum wall for the pressure, temperature, material allowable stress and corrosion allowance under the relevant code, such as ASME B31.3, then picks the lightest schedule that meets it after mill tolerance. Mill tolerance matters: ASTM pipe standards generally allow the wall to be up to 12.5% under nominal.
Calculating weight from the schedule
Once the OD and wall are known, weight per metre follows directly. Our pipe weight calculator works it out for carbon, stainless and alloy steel, and our article on calculating steel pipe weight shows the formula.
When you enquire, give the NPS and the schedule (or the wall in millimetres), and say whether it is B36.10 carbon or B36.19 stainless dimensions. Browse pipes and tubes.
Frequently asked questions
Does schedule change the outside diameter?
No. For a given size the OD is fixed; a higher schedule means a thicker wall and smaller bore.
What is the difference between SCH 40 and SCH 40S?
For sizes up to 10" they are the same; above that, 40S (B36.19M) can be thinner than SCH 40 (B36.10M).
Is SCH 80 the same as XS?
Up to 8" yes; above that, wall thicknesses can differ.
Key takeaways
- Schedule sets wall thickness, not OD.
- S schedules (5S, 10S, 40S, 80S) are for stainless pipe.
- Give wall thickness in mm to avoid confusion.
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.