Thread Identifier
Enter what you measured on an unknown thread and get the standard threads closest to it, across metric, inch and pipe threads. The diameter alone narrows it down; adding the pitch usually settles it.
Closest standard threads
| Thread | Series | Major Ø (mm) | Minor Ø (mm) | Pitch mm (TPI) | Your Ø − standard (mm) |
|---|
The best match is highlighted. A bolt normally measures 0.05 to 0.3 mm under its nominal diameter, and a tapped hole slightly over the basic minor diameter; the ranking allows for that.
How to measure a thread
- Diameter. Measure across the crests of a bolt with a caliper. For a nut or a threaded hole, measure the bore across the crests of the internal thread. On a taper pipe thread, measure at the largest full thread.
- Pitch. Use a thread pitch gauge and try leaves until one sits in the thread with no light showing. Without a gauge, measure the length of ten threads and divide by ten, or count the threads in one inch.
- Metric or inch. A pitch that comes out at a round metric value such as 1.0, 1.25 or 1.5 mm is metric. A whole number of threads per inch such as 13, 16, 20 or 24 is an inch thread.
- Parallel or taper. Compare the diameter at the start and at the end of the thread. If it changes, it is a taper pipe thread (NPT or BSP taper).
- Flank angle. If a 14 threads per inch pipe thread could be either NPT or BSP, look at the crests: flat for NPT, rounded for BSP.
Pairs that are easily confused
These metric and inch threads are close enough in size for a nut of one to start on a bolt of the other. None of the pairs is interchangeable: the joint either binds after a few turns or goes together loosely and strips under load.
| Metric | Inch | Major Ø (mm) | Inch − metric Ø (mm) | Pitch (mm) | Pitch difference |
|---|---|---|---|---|---|
| M5×0.8 | #10-32 UNF | 5.000 / 4.826 | -0.174 | 0.800 / 0.794 | -1% |
| M6×1 | 1/4-28 UNF | 6.000 / 6.350 | +0.350 | 1.000 / 0.907 | -9% |
| M8×1.25 | 5/16-18 UNC | 8.000 / 7.938 | -0.062 | 1.250 / 1.411 | +13% |
| M10×1.5 | 3/8-16 UNC | 10.000 / 9.525 | -0.475 | 1.500 / 1.587 | +6% |
| M12×1.75 | 1/2-13 UNC | 12.000 / 12.700 | +0.700 | 1.750 / 1.954 | +12% |
| M16×2 | 5/8-11 UNC | 16.000 / 15.875 | -0.125 | 2.000 / 2.309 | +15% |
| M20×2.5 | 3/4-10 UNC | 20.000 / 19.050 | -0.950 | 2.500 / 2.540 | +2% |
| M24×3 | 1-8 UNC | 24.000 / 25.400 | +1.400 | 3.000 / 3.175 | +6% |
M5 and #10-32 are the closest pair: the pitches differ by less than 1% and the diameters by 0.17 mm. A thread that goes together by hand and then tightens unexpectedly, or that feels loose all the way, is the usual sign of a mismatch.
Other charts and tools
Sources: ISO 724, ASME B1.1, ASME B1.20.1 and ISO 228-1 (basic dimensions). How this site checks its data. Last reviewed 29 September 2026.