How pipe sizing actually works
Nominal Pipe Size is a label, not a measurement. A "2-inch" pipe has no dimension equal to 2 inches — its outside diameter is 60.3 mm (2.375 in) and its bore depends on the schedule. The system is built around a fixed OD per nominal size so that flanges, fittings and supports interchange across every wall thickness:
The schedule number itself came from an approximation of 1000 × P/S (pressure over allowable stress) — which is why higher schedules mean thicker walls and higher pressure ratings, and why the numbering jumps in the odd steps it does.
Worked example
4" NPS, SCH 40:
- OD = 114.3 mm (fixed by NPS 4, every schedule)
- Wall = 6.02 mm → ID = 114.3 − 12.04 = 102.26 mm
- Flow area = π/4 × 102.26² = 8,212 mm²
- Mass = π × (114.3 − 6.02) × 6.02 × 7850 ÷ 10⁶ = 16.08 kg/m — matching the published table exactly
Field notes
- Never size a flow calculation on nominal diameter. Using 100 mm instead of the real 102.26 mm bore of 4" SCH 40 shifts velocity by 4% and Reynolds number with it — feed the true ID into the pipe velocity and Reynolds number calculators.
- STD, XS and XXS are the older designations. STD = SCH 40 up to 10", XS = SCH 80 up to 8"; beyond those sizes they diverge and the distinction starts to matter.
- Stainless uses the S schedules (B36.19M) with thinner walls — a 40S and a 40 are not always the same pipe.
- Weights here are bare carbon steel at 7,850 kg/m³. Add lining, cladding, insulation and contents separately for support loading.
Frequently asked questions
What is the inside diameter of 2 inch SCH 40 pipe?
2" NPS SCH 40 has an outside diameter of 60.3 mm and a wall thickness of 3.91 mm, giving an inside diameter of 52.48 mm (2.067 inches).
Why does the outside diameter stay the same across schedules?
Nominal pipe size fixes the outside diameter so that the same fittings and flanges fit every schedule. Increasing the schedule number thickens the wall inward, which reduces the bore — the OD never changes.
What is the difference between SCH 40 and STD?
They are identical from 1/2" up to 10". Above 10", STD stays at a fixed 9.53 mm wall while SCH 40 keeps increasing. The same applies to XS and SCH 80, which match up to 8".
What does the S suffix mean in 10S or 40S?
The S schedules come from ASME B36.19M for stainless steel pipe. Because stainless is stronger, these walls are thinner — 40S matches SCH 40 in the smaller sizes but diverges in larger ones.
Dimensional data compiled from ASME B36.10M for reference and education; confirm against the current standard and your piping specification before use. See our disclaimer.