Metric and imperial fasteners are not interchangeable. Some pairs sit within a fraction of a millimetre of each other in diameter, which is exactly what makes them dangerous: the bolt starts, turns half a turn, and strips the thread.
This page maps the nearest equivalents so you can identify a part, not substitute one.
Nearest diameter equivalents
| Metric | Diameter (in) | Nearest imperial | Diameter (in) | Difference |
|---|---|---|---|---|
| M2 | 0.0787 | #2 | 0.0860 | 9.3% |
| M2.5 | 0.0984 | #3 | 0.0990 | 0.6% |
| M3 | 0.1181 | #4 | 0.1120 | 5.2% |
| M4 | 0.1575 | #8 | 0.1640 | 4.1% |
| M5 | 0.1969 | #10 | 0.1900 | 3.5% |
| M6 | 0.2362 | 1/4" | 0.2500 | 5.8% |
| M8 | 0.3150 | 5/16" | 0.3125 | 0.8% |
| M10 | 0.3937 | 3/8" | 0.3750 | 4.8% |
| M12 | 0.4724 | 1/2" | 0.5000 | 5.8% |
| M16 | 0.6299 | 5/8" | 0.6250 | 0.8% |
| M20 | 0.7874 | 3/4" | 0.7500 | 5.0% |
The M8 trap
M8 and 5/16" differ by 0.8% in diameter — close enough that the bolt will enter the hole. But the pitch is not close at all:
- M8 coarse: 1.25 mm between threads
- 5/16-18: 18 threads per inch = 1.41 mm between threads
That is a 13% pitch mismatch. It will turn, it will feel wrong, and it will destroy the internal thread. The same trap exists with M16 and 5/8", and with M2.5 and #3.
How to tell them apart
- Head markings. Metric bolts carry numerals: 8.8, 10.9, 12.9. Imperial bolts carry radial lines: none for Grade 2, three for Grade 5, six for Grade 8.
- Pitch gauge. The only reliable test. A thread gauge costs a few dollars and settles the question in seconds.
- Wrench size. If the socket that fits is 13 mm, the bolt is metric. If it is 1/2", it is imperial. Metric and imperial socket sizes rarely coincide exactly.
Wrench sizes across systems
| Metric wrench | Inches | Nearest imperial wrench | Fits well? |
|---|---|---|---|
| 8 mm | 0.315 | 5/16" (0.313) | Yes |
| 10 mm | 0.394 | 3/8" (0.375) | No, rounds corners |
| 13 mm | 0.512 | 1/2" (0.500) | No, rounds corners |
| 16 mm | 0.630 | 5/8" (0.625) | Marginal |
| 19 mm | 0.748 | 3/4" (0.750) | Yes |
Length
Metric lengths are given in millimetres, imperial in fractions of an inch. One inch is 25.4 mm exactly.
| Inches | mm | Nearest stock metric length |
|---|---|---|
| 1/4" | 6.35 | 6 mm |
| 3/8" | 9.53 | 10 mm |
| 1/2" | 12.7 | 12 mm |
| 3/4" | 19.05 | 20 mm |
| 1" | 25.4 | 25 mm |
| 1 1/2" | 38.1 | 40 mm |
| 2" | 50.8 | 50 mm |
Where length is measured from depends on the head: under the head for hex and socket cap, from the top of the head for countersunk.
Related: tap drill chart and standards cross-reference.