Fasteners reference
Metric or inch bolt? Check these two measurements
A practical way to separate close metric and Unified inch fasteners without relying on the wrench that fits the head.
Reviewed against GaugeFox data and the sources below · Updated August 4, 2026
Use diameter and pitch together
Measure across the widest part of the threaded shaft, from crest to crest. Do not measure the smooth shank, the root between threads, or the head. Record the number before rounding it to a familiar size.
Next, check the spacing between thread crests. Metric threads are named by the distance from one crest to the next in millimeters. Unified inch threads are normally named by the number of threads in one inch. A 1/4-20 bolt therefore has 20 threads per inch, not a 20 mm pitch.
- Measure the major diameter across the thread crests.
- Match several thread crests to a pitch gauge instead of judging one gap.
- Keep both the metric and inch candidate visible when both are close.
- Confirm by threading into a known mating part by hand. Stop if it binds.
Pairs that are easy to confuse
The diameter alone can be misleading. M8 and 5/16 inch differ by only about 0.06 mm, which is smaller than the error caused by a poor viewing angle on many phone screens. Their pitch separates them.
| Metric | Inch candidate | Diameter difference | Pitch comparison |
|---|---|---|---|
| M6 × 1.0 | 1/4-20 UNC | 0.35 mm | 1.00 mm vs 1.27 mm |
| M8 × 1.25 | 5/16-18 UNC | 0.06 mm | 1.25 mm vs 1.41 mm |
| M10 × 1.5 | 3/8-16 UNC | 0.48 mm | 1.50 mm vs 1.59 mm |
| M12 × 1.75 | 1/2-13 UNC | 0.70 mm | 1.75 mm vs 1.95 mm |
Why the wrench size cannot settle it
The same thread can use different head widths. Current ISO metric hex heads, older DIN heads, flange bolts, heavy-hex hardware, and locknuts do not all follow one head table. Inch hardware has the same problem.
Treat the wrench or socket fit as a measured tool size. Use it to build a shortlist, then confirm the shaft diameter and thread pitch. That is why a 1 1/8 inch wrench can suggest a regular 3/4 inch bolt or nut while other hardware patterns can share a nearby width.
Reference trail
Sources and review method
These source pages identify the standards or industry references used for this guide. Full standards may require purchase. The guide is an identification aid, not a substitute for a calibrated inspection or the complete applicable standard.
Read the editorial standardUse it at the bench
Put these references on your Android screen.
GaugeFox adds calibrated on-screen gauges, searchable nearby sizes, and a local Parts Bin to the reference data in this guide.