Dimensions
| Trade size | NPS 20 · DN 500 |
|---|---|
| Schedule | Schedule 10S |
| Standard | ASME B36.19M (stainless steel pipe) |
| Outside diameter | 20.000" · 508.00 mm |
| Wall thickness | 0.218" · 5.54 mm |
| Inside diameter | 19.564" · 496.92 mm |
| Weight, plain end | not published |
ASME B36.19M (stainless steel pipe) · DN 500
20 inch Schedule 10S pipe
Outside diameter 20.000" (508.00 mm), wall 0.218" (5.54 mm), inside diameter 19.564" (496.92 mm).
Inside diameter
19.564"
496.92 mm · wall 0.218"
Why 20 inch pipe is not 20 inches
Nominal Pipe Size is a label, not a measurement. NPS 20 pipe has an outside diameter of 20.000 inches, and it keeps that outside diameter in every schedule made. What the schedule number changes is the wall: at schedule 10s the wall is 0.218 inches, so the bore comes out at 19.564 inches.
That is the practical consequence. Fittings, flanges, supports and threading dies are sized to the outside diameter, so they fit any schedule of the same trade size. Flow, pressure drop and anything that has to pass through the pipe depend on the inside diameter, which changes with every schedule. Order by trade size and schedule; work your hydraulics from the bore.
Other schedules in NPS 20
Same outside diameter of 20.000 inches throughout - only the wall and the bore move.
| Schedule | Wall (in) | ID (in) | Weight (lb/ft) |
|---|---|---|---|
| Schedule 5 | 0.188 | 19.624 | 39.82 |
| Schedule 10 | 0.250 | 19.500 | 52.7 |
| Schedule 20 | 0.375 | 19.250 | 78.67 |
| Schedule 30 | 0.500 | 19.000 | 104.23 |
| Schedule 40 | 0.594 | 18.812 | 123.23 |
| Schedule 60 | 0.812 | 18.376 | 166.56 |
| Schedule 80 | 1.031 | 17.938 | 209.06 |
| Schedule 100 | 1.281 | 17.438 | 256.34 |
| Schedule 120 | 1.500 | 17.000 | 296.65 |
| Schedule 140 | 1.750 | 16.500 | 341.41 |
| Schedule 160 | 1.969 | 16.062 | 379.53 |
| Standard wall (STD) | 0.375 | 19.250 | 78.67 |
| Extra strong (XS) | 0.500 | 19.000 | 104.23 |
| Schedule 5S | 0.188 | 19.624 | not published |
| Schedule 40S | 0.375 | 19.250 | not published |
| Schedule 80S | 0.500 | 19.000 | not published |
Frequently asked questions
What is the wall thickness of 20 inch schedule 10s pipe?
0.218 inches (5.54 mm). The outside diameter is 20.000 inches (508.00 mm) whatever the schedule, so this wall leaves an inside diameter of 19.564 inches.
What is the inside diameter of 20 inch schedule 10s pipe?
19.564 inches (496.92 mm). Inside diameter is outside diameter minus twice the wall: 20.000 - 2 x 0.218 = 19.564 inches.
Is 20 inch pipe actually 20 inches?
No. NPS 20 is a trade size, not a measurement. The pipe measures 20.000 inches across the outside and 19.564 inches across the bore in this schedule. Only the outside diameter is fixed across schedules.
Where this table stops
This table runs from NPS 1/8 to NPS 24 and stops there. ASME B36.10M itself continues to NPS 80; those larger sizes are not carried here, so a size above NPS 24 is missing rather than undefined.
Where these figures come from
Dimensions are as defined by ASME B36.19M (stainless steel pipe). This site is an independent reference and is not affiliated with, endorsed by or approved by any standards body.
Every value on this page was cross-checked against 2 independent published sources:
- Dimensions of steel and stainless steel pipes, ASME B36.10 and ASME B36.19 - Wermac, edition not stated (read 2026-08-27)
- Nominal Pipe Size - Wikipedia, revision read 2026-08-27 (read 2026-08-27)
Sources disagree here. Published tables split on the last digit here - 5.54 mm (b3610m-wermac), 5.537 mm (b3610m-wikipedia). The figure printed is 0.218 in (5.54 mm), carried by the majority of the sources. The two readings describe the same wall: they differ by less than a thousandth of an inch, far inside the mill tolerance of minus 12.5 percent on wall thickness.
Last-digit inch walls across the whole table: 0.594, 0.688, 0.719, 0.844, 0.938, 1.094, 1.219, 1.438, 1.969, 2.344 in (b3610m-fbc) against 0.593, 0.687, 0.718, 0.843, 0.937, 1.093, 1.218, 1.437, 1.968, 2.343 in (b3610m-wikipedia). Two families of inch figures are in circulation, one a thousandth of an inch below the other, and they come from rounding the millimetre wall the standard tabulates rather than from any difference in the pipe. The higher family is what the majority of the sources read here carry and is what this site prints; the affected rows say so individually. Nothing turns on the choice: the wall tolerance is minus 12.5 percent, which at a half-inch wall is sixty times the gap.
NPS 14 extra strong (XS): 0.469 in (b3610m-fbc) against 0.500 in (b3610m-etb) against 0.500 in (b3610m-wikipedia). 0.500 in is printed here. Two of the three inch sources give it, both millimetre sources give 12.70 mm which is 0.500 in, and extra strong is a 0.500 in wall at every size from NPS 12 up. The 0.469 in reading looks like a misplaced row in that one chart, but it is recorded rather than deleted.
Nearby sizes