What a Pipe Schedule Actually Is
Schedule sets the wall, never the outside diameter. Where the number came from, why Schedule 40 is a different thickness in every size, and what it does not tell you.
- Schedule is
- A wall thickness series
- Outside diameter
- Fixed by NPS, never by schedule
- Schedules in B36.10M
- 14
- Higher schedule
- Thicker wall, smaller bore
Short answer. A pipe schedule is a wall thickness designation. Within one nominal pipe size the outside diameter is fixed and the schedule sets how far the wall extends inward — so a higher schedule means a thicker wall, a smaller bore, more weight and a higher pressure rating, with the outside of the pipe unchanged. That fixed outside diameter is what lets one set of fittings, flanges and supports serve every schedule in a size.
Where the number comes from
The schedule number originated as an approximation of 1000 × P/S — a thousand times the working pressure divided by the allowable stress. Schedule 40 corresponded roughly to a pipe whose pressure-to-stress ratio was 0.04.
Schedule ≈ 1000 × P / S (historical origin, not a design method)
That relationship is history, not a calculation you should use. The published walls have been rounded to manufacturing convenience for a century, and modern design derives the required wall from the B31.3 equation and then picks the schedule that satisfies it. The number is a label for a wall, nothing more.
One size, every schedule
NPS 4 pipe is 4.500 in (114.3 mm) on the outside in every schedule. Here is what changes and what does not:
Dimensions are given in inches, with the millimetre equivalent beneath in grey.
| Schedule | Outside diameter | Wall | Bore | Weight | Flow area | Barlow P at S = 20 ksi |
|---|---|---|---|---|---|---|
| Sch 5 | 4.500114.30 mm | 0.0832.11 mm | 4.334110.08 mm | 3.925.83 kg/m | 14.75 in² | 738 psig |
| Sch 10 | 4.500114.30 mm | 0.1203.05 mm | 4.260108.20 mm | 5.618.35 kg/m | 14.25 in² | 1,067 psig |
| Sch 40 | 4.500114.30 mm | 0.2376.02 mm | 4.026102.26 mm | 10.7916.06 kg/m | 12.73 in² | 2,107 psig |
| STD | 4.500114.30 mm | 0.2376.02 mm | 4.026102.26 mm | 10.7916.06 kg/m | 12.73 in² | 2,107 psig |
| Sch 80 | 4.500114.30 mm | 0.3378.56 mm | 3.82697.18 mm | 14.9822.30 kg/m | 11.50 in² | 2,996 psig |
| XS | 4.500114.30 mm | 0.3378.56 mm | 3.82697.18 mm | 14.9822.30 kg/m | 11.50 in² | 2,996 psig |
| Sch 120 | 4.500114.30 mm | 0.43811.13 mm | 3.62492.05 mm | 19.0028.28 kg/m | 10.31 in² | 3,893 psig |
| Sch 160 | 4.500114.30 mm | 0.53113.49 mm | 3.43887.33 mm | 22.5133.50 kg/m | 9.28 in² | 4,720 psig |
| XXS | 4.500114.30 mm | 0.67417.12 mm | 3.15280.06 mm | 27.5440.99 kg/m | 7.80 in² | 5,991 psig |
The outside diameter column is constant by design. Allowable pressure is Barlow's 2St/D at 20,000 psi allowable stress, for illustration only.
The three schedule families
| Family | Members | Where you meet them |
|---|---|---|
| Numbered schedules | 5, 10, 20, 30, 40, 60, 80, 100, 120, 140, 160 | ASME B36.10M. The main series for carbon and alloy steel pipe. |
| Named walls | STD, XS, XXS | Pre-schedule designations still in wide use. STD tracks Sch 40 and XS tracks Sch 80 in small bore, then diverges. |
| S-schedules | 5S, 10S, 40S, 80S | ASME B36.19M, for stainless. Thinner in some sizes, and carrying stainless material and tolerance requirements. |
See the STD vs XS comparison for exactly where the named walls stop tracking the numbered ones.
Schedule does not mean the same thing in every size
This is the part that trips people up. Schedule 40 in NPS 1 is a 0.133 in wall; Schedule 40 in NPS 24 is 0.688 in. The schedule number describes a series, not a thickness, and the thickness for a given schedule rises with diameter. Two pipes both labelled Schedule 40 can have walls differing by a factor of five, so a wall thickness is never adequately specified by a schedule number alone — the NPS has to come with it.
What schedule does not tell you
- Material. Schedule 40 says nothing about whether the pipe is A53, A106 or A312.
- Manufacturing method. Seamless and welded pipe share the same schedules but carry different joint quality factors.
- Pressure rating. The rating depends on material, temperature and joint factor as well as wall.
- Actual delivered wall. Mill tolerance permits up to 12.5% under nominal.
See also: the full schedule chart · Sch 40 vs Sch 80 · NPS vs DN · all pipe sizes.
Common questions
What does pipe schedule mean?
It is a wall thickness designation. Within a nominal pipe size the outside diameter is fixed, so a higher schedule number means a thicker wall and a smaller bore.
Does a higher schedule mean a bigger pipe?
No — the outside diameter is unchanged. A higher schedule means a thicker wall, so the pipe is heavier and the bore is smaller, but it fits the same fittings and supports.
Is Schedule 40 the same wall thickness in every size?
No. Schedule 40 is 0.133 in in NPS 1 and 0.688 in in NPS 24. The schedule identifies a series, and the wall for that series rises with diameter.
How does schedule relate to pressure rating?
Only through wall thickness. Pressure capacity follows roughly 2St/D, so a thicker wall holds more — but the rating also depends on material, temperature and the joint quality factor.
What is the difference between Schedule 40 and 40S?
40S is the ASME B36.19M stainless designation. In most sizes the wall matches Schedule 40, but B36.19M publishes 40S only through NPS 12 and adds stainless material and tolerance requirements.