Thread Identifier
Measure across the threads of a bolt or screw and count the threads. Enter the numbers to find its size (inch UNC, UNF or metric), plus the tap drill and wrench to use.
| Thread | Type | Diameter | Threads | Match |
|---|
How to measure a bolt
- Diameter: measure across the tops of the threads with calipers, or hold the bolt against a mm ruler at actual size. Don't measure the smooth shank or the head.
- Inch threads: count the peaks along exactly 1 inch (or ½ inch and double it). That's the TPI.
- Metric threads: measure across 10 peaks and divide by 10. That's the pitch in mm.
- Not sure which? Enter it both ways: the calculator compares inch and metric threads at once.
Have the screw but no calipers? Hold it against the on-screen screw size checker.
Threads that are easy to mix up
| Inch | Metric | The difference |
|---|---|---|
| #10-32 | M5 × 0.8 | 4.83 mm vs 5.0 mm; 0.79 mm vs 0.8 mm pitch |
| 1/4-20 | M6 × 1.0 | 6.35 mm vs 6.0 mm; 1.27 mm vs 1.0 mm pitch |
| 5/16-18 | M8 × 1.25 | 7.94 mm vs 8.0 mm; 1.41 mm vs 1.25 mm pitch |
| 3/8-16 | M10 × 1.5 | 9.53 mm vs 10.0 mm; 1.59 mm vs 1.5 mm pitch |
| 1/2-13 | M12 × 1.75 | 12.7 mm vs 12.0 mm; 1.95 mm vs 1.75 mm pitch |
A nut that starts on but tightens after a turn or two is almost always the wrong one of a pair like these. Stop before you strip it.
Full tables: thread pitch chart, tap drill chart, screw and bolt sizes and wrench sizes.
Frequently asked questions
How do I identify a bolt thread?
Measure the outside diameter of the threads with calipers or a ruler, then count the threads. For inch bolts, count the peaks in one inch (threads per inch); for metric, measure the distance between peaks (the pitch, in mm). A 6.3 mm bolt with 20 threads per inch is a 1/4-20; a 6.0 mm bolt with 1.0 mm between threads is an M6.
Why does my bolt measure smaller than its size?
Bolt threads are made slightly undersize so they fit into a nut. A 1/4 inch bolt usually measures about 0.240 to 0.248 in, and an M6 about 5.8 to 5.97 mm. The calculator allows for this.
How do I measure thread pitch without a gauge?
Lay a ruler along the bolt and count the threads across 1 inch, or across 10 mm. For a fine thread, count over a longer length and divide: 13 threads over 1/2 inch is 26 per inch. For metric, measure the length of 10 threads and divide by 10.
Is my bolt metric or SAE?
Look at the head first: metric bolts are usually stamped 8.8, 10.9 or 12.9, while SAE bolts have radial lines or no marks. If there are no marks, measure: metric diameters are whole millimeters (5, 6, 8, 10 mm), inch diameters land on fractions (6.35 mm is 1/4 in, 7.94 mm is 5/16 in).
Need a hole size instead? Use the drill size calculator.