Nut and Bolt Size Converter
Convert metric (M3-M30) to imperial bolt sizes.
Returns coarse and fine thread pitch, UNC and UNF threads per inch, and the wrench that fits the hex.
Enter a bolt diameter in mm or inches to see the closest standard size in each system, with its thread pitch and the wrench that fits it.
Bolt and nut sizes use two main systems: metric (ISO) and imperial (SAE/USS). The metric system uses millimeters, while imperial uses fractions of an inch.
Metric bolt naming convention: A metric bolt labeled “M8 x 1.25” means:
- M8 = 8 mm nominal diameter
- 1.25 = 1.25 mm thread pitch (distance between threads)
Imperial bolt naming convention: An imperial bolt labeled “5/16-18” means:
- 5/16 = 5/16 inch (0.3125 in) nominal diameter
- 18 = 18 threads per inch (TPI)
Metric to imperial bolt size equivalents:
| Metric | Diameter (mm) | Closest Imperial | Diameter (inches) |
|---|---|---|---|
| M3 | 3.0 | #4 (4-40) | 0.112 |
| M4 | 4.0 | #8 (8-32) | 0.164 |
| M5 | 5.0 | #10 (10-32) | 0.190 |
| M6 | 6.0 | 1/4" | 0.250 |
| M8 | 8.0 | 5/16" | 0.3125 |
| M10 | 10.0 | 3/8" | 0.375 |
| M12 | 12.0 | 1/2" | 0.500 |
| M14 | 14.0 | 9/16" | 0.5625 |
| M16 | 16.0 | 5/8" | 0.625 |
| M20 | 20.0 | 3/4" | 0.750 |
| M24 | 24.0 | 1" | 1.000 |
Wrench/socket sizes (hex head across flats):
| Metric Wrench | Imperial Wrench | Fits Metric Bolt | Fits Imperial Bolt |
|---|---|---|---|
| 7 mm | 9/32" | M4 | none in common use |
| 8 mm | 5/16" | M5 | #6 nut |
| 10 mm | 3/8" | M6 | #10 nut |
| 13 mm | 1/2" | M8 | 5/16" |
| 17 mm | 11/16" | M10 | none in common use |
| 19 mm | 3/4" | M12 | 1/2" |
| 22 mm | 7/8" | M14 | none in common use |
| 24 mm | 15/16" | M16 | 5/8" |
Several metric wrench sizes have no imperial partner because the two systems never lined up at those diameters. There is no common imperial hex bolt that takes an 11/16" or a 7/8" wrench, for instance, since a 9/16" bolt jumps to 13/16" and a 5/8" bolt to 15/16".
Important note: Metric and imperial bolts are NOT interchangeable even when diameters are close. The thread pitch differs, so a metric nut will not fit an imperial bolt. Always match metric with metric and imperial with imperial.
The gap is smaller than most people expect, which is exactly why it causes trouble. An M8 coarse bolt has a 1.25 mm pitch. Its nearest imperial neighbour, 5/16-18, works out at 25.4 รท 18 = 1.41 mm. That is only 0.16 mm apart, so the first two or three turns go in cleanly by hand before the threads bind and start tearing metal. By the time it feels wrong, the damage is done. The calculator below prints both pitches side by side so you can see how close the two really are.
Coarse and fine threads: Every metric size has a default coarse pitch (the one you get unless the bolt says otherwise) and an optional fine pitch. M8 x 1.25 is coarse, M8 x 1.0 is fine. Imperial does the same thing with UNC (Unified National Coarse) and UNF (Unified National Fine): a 1/4-20 is coarse and a 1/4-28 is fine. Fine threads hold slightly more tension and resist vibration a little better, but they strip more easily in soft materials like aluminium and they are slower to run down. Most general assembly uses coarse.
Bolt grade/strength markings:
- Metric: Grade 8.8 (medium strength), Grade 10.9 (high strength), Grade 12.9 (highest strength)
- Imperial: Grade 2 (low), Grade 5 (medium), Grade 8 (high)
- Grade 8.8 metric is roughly equivalent to Grade 5 imperial.
How we build and check this converter
This converter runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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