NPT Pipe Thread Size Converter
Convert NPT nominal pipe thread sizes into actual outside diameter, metric diameter, threads per inch, thread pitch, and common tap drill reference sizes for shop and plumbing work.
Convert an NPT thread size
Pick the NPT size stamped on the fitting, or enter a measured male thread outside diameter to find the closest nominal size.
Choose an NPT size or enter a measured OD.
NPT size chart
Use this quick chart to compare nominal NPT sizes with their approximate male thread outside diameters.
| NPT size | OD inches | OD mm | TPI | Tap drill |
|---|
How to use this NPT converter
- Select the nominal NPT size from the drop-down to see its common reference dimensions.
- If you have a fitting in hand, measure the outside of the male threads with calipers.
- Enter the measurement to see the closest nominal NPT size and the difference from the chart value.
NPT sizes are nominal. The named size is not the same as the outside diameter you measure on the threads, which is why 1/4 NPT measures about 0.540 inches across.
Useful NPT conversions
- Inches to millimeters: inches × 25.4
- TPI to pitch: 25.4 ÷ TPI = millimeters per thread
- Measured OD matching: compare your caliper reading with the nearest standard NPT outside diameter.
For pressure systems, confirm thread type, sealant, material, and pressure rating. NPT, NPTF, BSPP, and BSPT fittings can look similar but are not always compatible.
NPT converter FAQ
What is 1/2 NPT in inches and millimeters?
1/2 NPT has an approximate male thread outside diameter of 0.840 inches, or 21.34 millimeters, with 14 threads per inch.
What does 18 TPI mean on an NPT fitting?
18 TPI means 18 threads per inch. Converted to metric pitch, that is about 1.411 millimeters from one thread crest to the next.
Can I identify NPT with only an outside diameter?
Outside diameter is a strong clue, but it is best to confirm thread pitch too. Nearby pipe thread standards can have similar diameters with different pitches or thread forms.