Thread Identification Guide
Measure the major diameter with calipers and count threads per inch (or measure pitch), then check it against the common standard sizes below.
How to measure it first
1. Measure major (outside) diameter with calipers.
2. Count threads per inch against a rule, or use a thread pitch gauge, or measure the distance across 10 threads and divide by 10 for better accuracy than counting one.
3. Enter what you measured below to check it against common standards.
Common UNC & UNF sizes
| Size | Major dia (in) | UNC | UNF |
|---|---|---|---|
| #4 | 0.112 | 40 TPI | 48 TPI |
| #6 | 0.138 | 32 TPI | 40 TPI |
| #8 | 0.164 | 32 TPI | 36 TPI |
| #10 | 0.190 | 24 TPI | 32 TPI |
| 1/4" | 0.250 | 20 TPI | 28 TPI |
| 5/16" | 0.3125 | 18 TPI | 24 TPI |
| 3/8" | 0.375 | 16 TPI | 24 TPI |
| 7/16" | 0.4375 | 14 TPI | 20 TPI |
| 1/2" | 0.500 | 13 TPI | 20 TPI |
| 9/16" | 0.5625 | 12 TPI | 18 TPI |
| 5/8" | 0.625 | 11 TPI | 18 TPI |
| 3/4" | 0.750 | 10 TPI | 16 TPI |
| 7/8" | 0.875 | 9 TPI | 14 TPI |
| 1" | 1.000 | 8 TPI | 12 TPI |
Common metric coarse sizes
| Size | Major dia (mm) | Coarse pitch (mm) | Equivalent TPI |
|---|---|---|---|
| M3 | 3.0 | 0.50 | 50.8 |
| M4 | 4.0 | 0.70 | 36.3 |
| M5 | 5.0 | 0.80 | 31.8 |
| M6 | 6.0 | 1.00 | 25.4 |
| M8 | 8.0 | 1.25 | 20.3 |
| M10 | 10.0 | 1.50 | 16.9 |
| M12 | 12.0 | 1.75 | 14.5 |
| M14 | 14.0 | 2.00 | 12.7 |
| M16 | 16.0 | 2.00 | 12.7 |
| M20 | 20.0 | 2.50 | 10.2 |
A metric thread will never measure to a clean whole-number TPI — if your measured TPI is close to but not exactly a UNC/UNF value above, check it against the metric table instead.
The fastest way to tell metric from imperial
If threads per inch comes out close to a whole or half number, it's almost certainly one of the imperial standards above. If it lands on an odd fraction that doesn't match any UNC/UNF value, measure pitch directly in mm instead (or convert with the Metric ↔ Imperial Thread Converter) and check the metric table — a genuinely metric thread will almost never happen to measure to a clean imperial TPI.
Coarse vs. fine complicates diameter-only guesses
Major diameter alone often matches more than one thread standard — a 1/4" UNC and 1/4" UNF share the same 0.250" major diameter and only differ in pitch, and several metric sizes are close to but not identical to nearby imperial diameters. Always confirm with a pitch or TPI measurement, not diameter alone.
When this guide isn't enough
These are the sizes that come up most often in general shop work, not an exhaustive list — pipe threads (NPT/NPTF/BSPT), UNEF, and less common metric fine pitches aren't covered here. If nothing above is a close match, the thread is likely one of those less common standards, and a thread pitch gauge set plus a proper thread identification chart is worth the investment before guessing on anything that has to actually fit.