Thread ID Guide
Pipe thread sizes do not match what you measure. A "1/4 inch" NPT fitting is over half an inch across, which is why so many people buy the wrong plug twice. Measure the outside of the threads, pick the closest number below, and this tells you the thread, what to seal it with, and what to buy.
Industrial Supply World is reader-supported. Some links below are affiliate links or go to hpcouplers.com, our own store. That never changes the answer the tool gives you.
Male or female?
What does it measure?
Do the threads taper?
Hold a straight edge along the threads. If they get slightly smaller toward the end, it is tapered pipe thread (NPT). If they are the same diameter end to end, it is a straight thread and needs a washer or O-ring to seal, not tape.
NPT sizes: what the name says vs what you measure
| NPT size | Male thread OD | Threads per inch | Where it shows up |
|---|---|---|---|
| 1/8" NPT | 0.405" (about 13/32) | 27 | Gauges, small valves, 1/8" body couplers |
| 1/4" NPT | 0.540" (about 17/32) | 18 | Almost every air tool, plug and coupler |
| 3/8" NPT | 0.675" (about 11/16) | 18 | 3/8" hose ends, compressor outlets, filters |
| 1/2" NPT | 0.840" (about 27/32) | 14 | Compressor tank ports, 1/2" hose, shop drops |
| 3/4" NPT | 1.050" (about 1-1/16) | 14 | Main air lines, tank drains, large filters |
Female NPT measures a touch smaller across the opening than the male OD above. If your number is within 1/32" of a row, that is your size.
Four things that trip people up
- Thread size is not body size. A 1/4" body coupler can have a 1/4" or 3/8" NPT thread. The thread is what you screw in; the body is the air passage. Use the Interchange Finder for the body and this page for the thread.
- NPT and BSP are not the same. British pipe thread (BSPT/BSPP) is 55°, NPT is 60°, and the thread counts differ on most sizes (BSP 1/4" is 19 TPI, NPT 1/4" is 18). They will start together and leak forever. Anything sold in the US for air is NPT unless it came in with imported equipment.
- Tape goes on male threads only, wrapped in the direction the fitting tightens, two to three wraps, leaving the first thread bare so no tape gets into the air line.
- Straight threads do not seal on tape. If the thread is straight, it seals on a washer, O-ring or flare face. Tape will not fix a straight-thread leak; a new sealing washer will.
Send us a photo
Photograph the fitting next to a ruler or caliper and email it to support@industrialsupplyworld.com. We will tell you the thread, the body size and the part number that fits.
Industrial Supply World is run by Mid-States Industrial Sales, a 40-year industrial distributor in Elgin, Illinois, which also operates hpcouplers.com.