Pipe Size Converter
NPS to OD to DN, with Schedule 40 inside diameter lookup.
How to use
Pick a nominal pipe size (NPS) from the dropdown. The converter returns the outside diameter, the metric DN equivalent, and the Schedule 40 inside diameter.
- Select the nominal pipe size (NPS) — the size printed on the pipe (e.g. 1/2″, 3/4″, 1″, 2″).
- Read the outside diameter — the physical dimension of the pipe.
- Read the DN equivalent — the metric "Diameter Nominal" used in ISO and European systems.
- Use the Schedule 40 inside diameter for flow-area calculations.
Reviewed 3 June 2026 · methodology cited
About this converter
Nominal Pipe Size (NPS) is the US/Canadian system for naming pipes. It is not the actual outside diameter, and it is not the inside diameter either — it is a label whose origin is historical, going back to the 1920s when pipes were thicker-walled and the NPS corresponded more closely to the inside diameter of standard-wall pipe. As wall thicknesses standardized, the names stuck but the dimensions diverged.
This converter takes any NPS and returns: the actual outside diameter (in inches and millimetres), the metric DN (Diameter Nominal) equivalent used in ISO systems, and the Schedule 40 inside diameter for flow-area calculations. The reference table below covers sizes from 1/8″ NPS through 24″.
Why NPS does not equal OD
For NPS 1/8″ through 12″, the actual OD does not match the NPS number. NPS 1/2″ is actually 0.840″ OD; NPS 1″ is 1.315″ OD; NPS 2″ is 2.375″ OD. The relationship is fixed by ASME B36.10 and B36.19 — these are not formulas you compute, they are values in a published standard.
At NPS 14″ and above, the OD finally matches the NPS — NPS 14″ is 14.000″ OD, NPS 24″ is 24.000″ OD, etc. So the rule is: at 14″ and above, NPS = OD; below 14″, NPS is a name and OD is in the table.
The DN equivalent is the metric naming convention: DN 15 = NPS 1/2″, DN 25 = NPS 1″, DN 50 = NPS 2″. DN is also a label, not a measured dimension. Schedule 40 inside diameter depends on wall thickness, which depends on the schedule — Schedule 40 is the most common general-purpose schedule for water and gas piping in residential and light commercial work.
Full pipe size reference
| NPS | DN | OD (in / mm) | ID Sch 40 (in) |
|---|---|---|---|
| 1/8″ | 6 | 0.405 / 10.29 | 0.269″ |
| 1/4″ | 8 | 0.540 / 13.72 | 0.364″ |
| 3/8″ | 10 | 0.675 / 17.15 | 0.493″ |
| 1/2″ | 15 | 0.840 / 21.34 | 0.622″ |
| 3/4″ | 20 | 1.050 / 26.67 | 0.824″ |
| 1″ | 25 | 1.315 / 33.40 | 1.049″ |
| 1-1/4″ | 32 | 1.660 / 42.16 | 1.380″ |
| 1-1/2″ | 40 | 1.900 / 48.26 | 1.610″ |
| 2″ | 50 | 2.375 / 60.33 | 2.067″ |
| 2-1/2″ | 65 | 2.875 / 73.03 | 2.469″ |
| 3″ | 80 | 3.500 / 88.90 | 3.068″ |
| 4″ | 100 | 4.500 / 114.30 | 4.026″ |
| 6″ | 150 | 6.625 / 168.28 | 6.065″ |
| 8″ | 200 | 8.625 / 219.08 | 7.981″ |
| 10″ | 250 | 10.750 / 273.05 | 10.020″ |
| 12″ | 300 | 12.750 / 323.85 | 11.938″ |
| 14″ | 350 | 14.000 / 355.60 | 13.124″ |
| 16″ | 400 | 16.000 / 406.40 | 15.000″ |
| 18″ | 450 | 18.000 / 457.20 | 16.876″ |
| 20″ | 500 | 20.000 / 508.00 | 18.812″ |
| 24″ | 600 | 24.000 / 609.60 | 22.624″ |
Notes for plumbing and gas work
Always specify both the NPS and the schedule when ordering. NPS 1″ pipe in Schedule 40 has 1.049″ ID; NPS 1″ in Schedule 80 has 0.957″ ID. The OD is identical — 1.315″ — but the wall thickness differs, so the ID and the pressure rating differ. Threaded fittings work with any schedule; flange and weld fittings are schedule-specific.
Copper tubing uses a different naming convention than steel pipe: Type K, L, and M copper are named by wall thickness and the actual OD is 1/8″ larger than the nominal size. A “1/2 inch” copper line has 5/8″ OD. PEX tubing follows a third convention. Always confirm the actual OD against the fitting you plan to use, and never assume that “1/2 inch copper” and “1/2 inch black steel” have the same dimensions — they do not.
Frequently asked questions
Why does 1″ NPS pipe measure 1.315″ outside?
Historically NPS corresponded roughly to the inside diameter of standard-wall pipe of that era. As wall thicknesses standardized in the 1920s and 30s, the manufacturers kept the original NPS labels even though the new dimensions no longer matched. NPS is now purely a name, not a measurement.
What is "Schedule 40"?
Schedule numbers describe wall thickness. The original definition was schedule = (1000 × P/S), where P is the design pressure and S is the allowable stress, but it has become a standardized nomenclature in ASME B36.10. Schedule 40 is the most common for water and gas; Schedule 80 has thicker walls and is used for higher pressures.
How do DN and NPS relate?
DN (Diameter Nominal) is the metric naming convention used in ISO and European systems. It pairs with NPS roughly as DN = NPS × 25 (so DN 15 = NPS 1/2″, DN 25 = NPS 1″). The actual OD is the same in both systems — only the label differs.
Does this apply to copper tubing?
No. Copper tubing (Type K, L, M, DWV) uses a different naming convention. A "1/2 inch" copper tube has 5/8″ OD; a "3/4 inch" copper has 7/8″ OD. The fittings are not interchangeable with steel pipe fittings of the same nominal name. Always verify the actual OD against your fitting.
What about PEX and CPVC?
PEX follows yet another convention based on Copper Tube Size (CTS) — actually closer to copper than to steel. CPVC follows the steel-pipe NPS convention for OD. PVC drain-waste-vent (DWV) uses a separate dimensional standard. The reference table on this page covers steel pipe only.
Can I use this to size a plumbing system?
No. Pipe sizing depends on the flow rate, allowable pressure drop, fixture units, and the local plumbing code (NPC in Canada, IPC/UPC in the US, depending on jurisdiction). This is a unit-conversion utility, not a sizing tool. Real installation belongs to a licensed plumber.