20 inch pipe size
508 mm outside diameter
NPS 20 · DN500 · 20.000 in OD · 477.8 mm bore in schedule 40
What this pipe is for
Large-diameter transmission piping.
Here the number is the size
NPS 20 pipe is 20.000 inches across the outside — the NPS number exactly. From NPS 14 upward the designation finally becomes a measurement, which it is not at any smaller size.
The numbers
| NPS | 20 |
| DN (metric designation) | 500 |
| Outside diameter | 508 mm · 20.000 in |
| Wall, schedule 40 | 15.09 mm |
| Bore, schedule 40 | 477.8 mm · 18.812 in |
| Name against reality | the NPS number is the OD |
Outside diameter is the number that matters. It is fixed for a given NPS regardless of wall thickness, which is what lets a fitting made for schedule 10 thread onto schedule 80. The bore changes with the schedule; the outside never does.
Nearby sizes
| NPS | DN | OD | Sch 40 bore |
|---|---|---|---|
| 10 | 250 | 273 mm | 254.5 mm |
| 12 | 300 | 323.8 mm | 303.2 mm |
| 14 | 350 | 355.6 mm | 333.3 mm |
| 16 | 400 | 406.4 mm | 381.0 mm |
| 18 | 450 | 457.2 mm | 428.7 mm |
| 20 | 500 | 508 mm | 477.8 mm |
| 24 | 600 | 610 mm | 575.0 mm |
Frequently asked questions
What is the outside diameter of 20 inch pipe?
508 mm, or 20.000 inches. At this size the NPS number is the outside diameter, exactly.
What is the DN equivalent of NPS 20?
DN 500. DN is the metric designation and is just as nominal — DN 500 pipe is also 508 mm across, not 500. The two systems label the same pipe.
What is the inside diameter of 20 inch schedule 40 pipe?
About 477.8 mm (18.812 in), from an outside diameter of 508 mm and a 15.09 mm wall. Inside diameter changes with the schedule; outside diameter does not, which is why fittings are sized on it.
Why is NPS 20 not 20 inches?
It is. From NPS 14 upward the number is the outside diameter in inches — the scale becomes a measurement at exactly this point.