Wrench size conversion
Metric and SAE wrenches do not match one-to-one. Use this chart to pick the closest imperial size for a metric bolt, or the metric size for an imperial fastener, then check the fit.
| Metric mm | Closest SAE inch | Notes |
|---|---|---|
| 8 mm | 5/16 in | Metric fastener, ~0.315 in |
| 10 mm | 3/8 in | Metric fastener, ~0.394 in |
| 13 mm | 1/2 in | Metric fastener, ~0.512 in |
| 14 mm | 9/16 in | Metric fastener, ~0.551 in |
| 17 mm | 11/16 in | Metric fastener, ~0.669 in |
| 19 mm | 3/4 in | Metric fastener, ~0.748 in |
| 21 mm | 13/16 in | Metric fastener, ~0.827 in |
| 22 mm | 7/8 in | Metric fastener, ~0.866 in |
| 24 mm | 15/16 in | Metric fastener, ~0.945 in |
| 27 mm | 1-1/16 in | Metric fastener, ~1.063 in |
| 30 mm | 1-3/16 in | Metric fastener, ~1.181 in |
| 32 mm | 1-1/4 in | Metric fastener, ~1.260 in |
Notes
- SAE sizes are approximations; a metric fastener may feel tight in the nearest SAE wrench.
- For safety-critical or precision work, use the exact system the fastener was made in.
- Hex head widths are what matter — these pairings assume standard bolt-head dimensions.
Frequently asked questions
- Is 10 mm the same as 3/8 inch?
- Close but not exact: 10 mm is 0.394 in while 3/8 in is 0.375 in. They often swap in a pinch but a 10 mm wrench fits a 10 mm head best.
- What SAE size matches 13 mm?
- 13 mm is nearest to 1/2 in (0.500 in); 13 mm = 0.512 in, so a 1/2 in wrench is the usual substitute.
- Why do metric and SAE not line up?
- They come from two different standards (ISO millimetres vs fractional inches) with unrelated step sizes, so only some pairings are close.
- Can I use a SAE wrench on a metric bolt?
- For non-critical jobs, the nearest size works; for torque or safety parts, use the matching system to avoid rounding the head.