NPS 18 (DN 450) pipe — dimensions and weight
Outside diameter 457.2 mm (18 in), unchanged by schedule. This page lists all 16 tabulated schedules that fit on that diameter, with wall thickness, bore, mass per metre and internal volume.
Nominal pipe size is a label, not a dimension. The outside diameter of NPS 18 is 18 in (457.2 mm), and the bore depends entirely on the wall you choose — 448.8 mm on the thinnest tabulated wall down to 366.7 mm on the heaviest. The naming convention was fixed decades ago so that a fitting size could serve every wall thickness; it is not a measurement, and treating it as one is how a line gets sized off a table for the wrong pipe.
NPS 18 (DN 450) — every tabulated schedule
| Schedule | Wall mm | Inside dia. mm | Metal area mm² | Mass / length | Water capacity L/m |
|---|---|---|---|---|---|
| 5S | 4.19 | 448.8 | 5,964.5 | 46.821 kg/m · 31.463 lb/ft | 158.209 |
| 10S | 4.78 | 447.6 | 6,787.2 | 53.279 kg/m · 35.802 lb/ft | 157.386 |
| 10 | 6.35 | 444.5 | 8,994.1 | 70.603 kg/m · 47.443 lb/ft | 155.179 |
| 20 | 7.92 | 441.4 | 11,185.4 | 87.805 kg/m · 59.002 lb/ft | 152.988 |
| 30 | 11.10 | 435.0 | 15,556.0 | 122.114 kg/m · 82.057 lb/ft | 148.617 |
| 40S | 9.52 | 438.1 | 13,396.1 | 105.159 kg/m · 70.664 lb/ft | 150.777 |
| 40 | 14.27 | 428.7 | 19,863.2 | 155.927 kg/m · 104.778 lb/ft | 144.310 |
| STD | 9.52 | 438.1 | 13,396.1 | 105.159 kg/m · 70.664 lb/ft | 150.777 |
| 60 | 19.05 | 419.1 | 26,222.1 | 205.844 kg/m · 138.320 lb/ft | 137.951 |
| 80S | 12.70 | 431.8 | 17,734.8 | 139.218 kg/m · 93.550 lb/ft | 146.438 |
| 80 | 23.80 | 409.6 | 32,405.0 | 254.379 kg/m · 170.935 lb/ft | 131.768 |
| XS | 12.70 | 431.8 | 17,734.8 | 139.218 kg/m · 93.550 lb/ft | 146.438 |
| 100 | 29.36 | 398.5 | 39,465.8 | 309.806 kg/m · 208.180 lb/ft | 124.707 |
| 120 | 34.92 | 387.4 | 46,332.1 | 363.707 kg/m · 244.400 lb/ft | 117.841 |
| 140 | 39.67 | 377.9 | 52,041.2 | 408.523 kg/m · 274.515 lb/ft | 112.132 |
| 160 | 45.24 | 366.7 | 58,547.1 | 459.595 kg/m · 308.833 lb/ft | 105.626 |
Wall thicknesses derived from ASME B36.10M / B36.19M. Outside diameter 457.2 mm. Mass computed for Carbon Steel at 7850 kg/m³.
STD and XS are derived rather than tabulated: STD equals Sch 40 up to NPS 10 and is fixed at 0.375 in above it; XS equals Sch 80 up to NPS 8 and is fixed at 0.500 in above. Both appear here as separate rows because both are orderable designations.
How NPS 18 pipe weight is calculated
Sch 40 at this size, step by step
m = π/4 · (D² − d²) · 10⁻⁶ · ρ
D is outside diameter, d is bore (D − 2t), ρ is density in kg/m³, m is mass in kg per metre.
Worked through with the default dimensions:
D = 457.2 mm (18 in, fixed for every schedule)t = 14.27 mm (Sch 40)d = D − 2t = 457.2 − 28.55 = 428.7 mmA = π/4 × (457.2² − 428.7²) = 19,863.2 mm²m = 19,863.2 × 10⁻⁶ × 7,850 = 155.927 kg/m (104.778 lb/ft)
- D
- 457.2 mm — fixed by the standard for NPS 18
- t
- 14.27 mm for Schedule 40 at this size
- d
- 428.7 mm — the bore, and the dimension that governs flow
- ρ
- 7,850 kg/m³ for carbon steel
Internal volume and surface area
Mass is only one of the quantities that fall out of the same geometry. Two others decide real work on a line, and neither appears on a printed pipe chart:
| Wall vs range | Wallmm | Boremm | Masskg/m |
|---|---|---|---|
| 25 % | 0.57 | 456.1 | 6.41 |
| 50 % | 0.97 | 455.3 | 10.95 |
| 100 % | 1.78 | 453.6 | 20.00 |
| 200 % | 3.40 | 450.4 | 38.02 |
| 500 % | 8.24 | 440.7 | 91.29 |
| 1,000 % | 16.32 | 424.6 | 177.50 |
Sampled across the lightest to heaviest tabulated wall at this size. Mass rises roughly linearly with wall while the bore falls, which is the trade-off a schedule choice actually represents.
- Internal volume — 144.310 L/m in Sch 40 ( 38.123 US gal/ft). Multiply by the line length to size a hydrostatic test or a flush.
- External surface — 1.4363 m² per metre of pipe, which is what a coating or insulation quantity is priced against. It depends only on the outside diameter, so it is identical in every schedule at NPS 18.
- Water mass — 144.31 kg per metre of water at 1,000 kg/m³. This is the number that decides whether a support is adequate during a test, not in service.
Download this size
NPS 18 pipe — CSV
Exports the schedule table above. The full pipe dataset, every size against every schedule, is on the downloads page.
Frequently asked questions
What is the outside diameter of NPS 18 pipe?
NPS 18 pipe has an outside diameter of 457.2 mm (18 in), and that diameter is fixed for every schedule at this size. Nominal pipe size is a label, not a dimension: NPS 18 does not measure 18 inches anywhere. The naming convention was chosen so that the outside diameter stays constant while the wall varies.
How much does NPS 18 pipe weigh?
In Schedule 40 it weighs 155.927 kg/m (104.778 lb/ft) with a 14.27 mm wall and a 428.7 mm bore. Not every schedule is tabulated for this size.
How many schedules are available for NPS 18?
This page lists 16 tabulated schedules, including the derived STD and XS walls. Note that STD and Sch 40 are the same thickness at the small sizes and different from NPS 12 upward, and XS and Sch 80 are the same up to NPS 8 and different above — the two confusions that most often produce a wrong order.
What is the internal diameter of NPS 18 pipe?
It depends entirely on the wall. The bore is the outside diameter minus twice the wall, so at NPS 18 it ranges from about 448.8 mm on the thinnest tabulated wall down to 448.8 mm on the heaviest. Flow capacity follows the bore, not the nominal size, which is why two pipes both sold as "NPS 18" can carry very different flows.
Nearby nominal sizes
Adjacent sizes in the NPS series. Outside diameters step non-uniformly — the jump from NPS 14 to NPS 16 is 2 in, the same as from NPS 2 to NPS 4.
- NPS 12 (DN 300) — 16 schedules
- NPS 14 (DN 350) — 16 schedules
- NPS 16 (DN 400) — 16 schedules
- NPS 20 (DN 500) — 16 schedules
- NPS 22 (DN 550) — 15 schedules
- NPS 24 (DN 600) — 16 schedules
Schedules at this size
Each schedule page lists every size that carries that wall.
- Sch 5S pipe — 4.19 mm wall here
- Sch 10S pipe — 4.78 mm wall here
- Sch 10 pipe — 6.35 mm wall here
- Sch 20 pipe — 7.92 mm wall here
- Sch 30 pipe — 11.10 mm wall here
- Sch 40S pipe — 9.52 mm wall here
- Sch 40 / STD pipe — 14.27 mm wall here
- Sch 60 pipe — 19.05 mm wall here
- Sch 80S pipe — 12.70 mm wall here
- Sch 80 / XS pipe — 23.80 mm wall here
- Sch 100 pipe — 29.36 mm wall here
- Sch 120 pipe — 34.92 mm wall here
- Sch 140 pipe — 39.67 mm wall here
- Sch 160 pipe — 45.24 mm wall here
- STD (Standard) pipe — 9.52 mm wall here
- XS (Extra Strong) pipe — 12.70 mm wall here
Pipe reference and tools
- Pipe overview — schedules, sizes and how weight is computed
- All pipe schedules — Sch 5S to XXS
- All nominal sizes — NPS 1/8 to NPS 36
- DN to NPS conversion — metric to imperial designations
- Pipe weight formula — worked derivation
- Pipe weight calculator — any OD and wall
- All calculators — metal weight and pipe weight
- Download the data — CSV, generated from the same source
- Cross-reference tables — DN / NPS / material equivalents
- Methodology and sources — where every number comes from
Outside diameter for NPS 18 is the standard's governed value of 18 in, converted at exactly 25.4 mm per inch. Wall thicknesses are the tabulated B36.10M / B36.19M values. Mass, bore, internal volume and surface areas are computed on this page.
- ASME B36.10M — Welded and Seamless Wrought Steel Pipe (dimensional tables).
- ASME B36.19M — Stainless Steel Pipe (dimensional tables).
- BIPM / NIST definitional constants: international inch = 25.4 mm, foot = 0.3048 m, pound = 0.45359237 kg.
Dataset version 2026.09.1 · dimensions last verified against source 2026-09-21. Found an error? Report it — corrections are issued with a version bump.