Pipe Size Converter
Convert nominal pipe size (NPS) to its metric DN equivalent and actual outside diameter in inches and millimetres, from 1/8 inch to 24 inch.
Select a pipe size to see all equivalent measurements.
Pipe sizes use nominal names that do NOT match actual dimensions.
NPS (Nominal Pipe Size) is the North American standard (in inches). DN (Diamètre Nominal) is the metric equivalent (in mm). OD (Outside Diameter) is the actual measured outer diameter.
NPS to DN and OD reference:
| NPS | DN | OD (inches) | OD (mm) |
|---|---|---|---|
| 1/8" | DN6 | 0.405" | 10.29 mm |
| 1/4" | DN8 | 0.540" | 13.72 mm |
| 3/8" | DN10 | 0.675" | 17.15 mm |
| 1/2" | DN15 | 0.840" | 21.34 mm |
| 3/4" | DN20 | 1.050" | 26.67 mm |
| 1" | DN25 | 1.315" | 33.40 mm |
| 1-1/4" | DN32 | 1.660" | 42.16 mm |
| 1-1/2" | DN40 | 1.900" | 48.26 mm |
| 2" | DN50 | 2.375" | 60.33 mm |
| 2-1/2" | DN65 | 2.875" | 73.03 mm |
| 3" | DN80 | 3.500" | 88.90 mm |
| 3-1/2" | DN90 | 4.000" | 101.60 mm |
| 4" | DN100 | 4.500" | 114.30 mm |
| 5" | DN125 | 5.563" | 141.30 mm |
| 6" | DN150 | 6.625" | 168.28 mm |
| 8" | DN200 | 8.625" | 219.08 mm |
| 10" | DN250 | 10.750" | 273.05 mm |
| 12" | DN300 | 12.750" | 323.85 mm |
| 14" | DN350 | 14.000" | 355.60 mm |
| 16" | DN400 | 16.000" | 406.40 mm |
| 18" | DN450 | 18.000" | 457.20 mm |
| 20" | DN500 | 20.000" | 508.00 mm |
| 24" | DN600 | 24.000" | 609.60 mm |
Look at where the table changes character. From 1/8" to 12" the nominal size is a label with no direct relationship to anything you can measure: a 12" pipe is 12.750" across. At 14" it snaps into line, and from there up NPS is exactly the outside diameter in inches. Those larger sizes are called “iron pipe size” only by habit; the number is real.
The reason for the mess is historical. The nominal sizes were originally the approximate inside diameter of the wrought-iron pipe of the day, when there was only one wall thickness. Once thinner and thicker walls arrived the inside diameter stopped being fixed, but the outside had to stay put so existing fittings and threads still worked. So the label froze against a bore that no longer exists, and the outside diameter became the number that actually matters.
DN is not a conversion of NPS, it is a parallel label. DN25 is not 25 mm across, it is simply the metric name for the pipe the Americans call 1 inch, whose true outside diameter is 33.4 mm. Trying to convert between the two arithmetically will not work; you look them up in the table above, which is exactly what this page is for. The one thing you can rely on is that a DN and its NPS partner are the same physical pipe, so a DN50 fitting goes onto 2" pipe.
For wall thickness and bore, which vary by schedule for any of these sizes, see the pipe schedule converter.
How we build and check this converter
This converter runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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