Nut and Bolt Size Converter
Convert between metric and imperial bolt and nut sizes.
Includes wrench sizes, thread pitch, and grade markings.
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.