Nut and Bolt Size Converter
Convert metric (M6-M24) to imperial bolt and nut sizes with wrench equivalents, thread pitch, and grade markings.
Covers SAE J429, ISO 898, and ASTM grades.
Enter a bolt diameter in mm or inches — see the closest equivalent in the other system.
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 | — |
| 8 mm | 5/16" | M5 | — |
| 10 mm | 3/8" | M6 | — |
| 13 mm | 1/2" | M8 | 5/16" |
| 17 mm | 11/16" | M10 | 3/8" |
| 19 mm | 3/4" | M12 | 1/2" |
| 22 mm | 7/8" | M14 | 9/16" |
| 24 mm | 15/16" | M16 | 5/8" |
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.
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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