Wrench Sizes: Metric to SAE Conversion Chart
A wrench is sized by the distance across the flats of the nut or bolt head, not by the thread diameter. A 10 mm wrench has a 10 mm opening (0.394 in), a 3/4 in wrench has a 3/4 in opening (19.05 mm), and the two systems only line up in a few places. Use the chart below to find the nearest inch size for a metric wrench, and the gap tells you whether the substitute is close enough or just a stopgap.
Metric to SAE wrench size chart
| Metric | Inches | Nearest SAE | Gap |
|---|---|---|---|
| 6 mm | 0.236 in | 1/4 in (6.35 mm) | SAE 0.35 mm larger |
| 7 mm | 0.276 in | 1/4 in (6.35 mm) | SAE 0.65 mm smaller |
| 8 mm | 0.315 in | 5/16 in (7.94 mm) | SAE 0.06 mm smaller |
| 9 mm | 0.354 in | 3/8 in (9.52 mm) | SAE 0.52 mm larger |
| 10 mm | 0.394 in | 3/8 in (9.52 mm) | SAE 0.48 mm smaller |
| 11 mm | 0.433 in | 7/16 in (11.11 mm) | SAE 0.11 mm larger |
| 12 mm | 0.472 in | 1/2 in (12.70 mm) | SAE 0.70 mm larger |
| 13 mm | 0.512 in | 1/2 in (12.70 mm) | SAE 0.30 mm smaller |
| 14 mm | 0.551 in | 9/16 in (14.29 mm) | SAE 0.29 mm larger |
| 15 mm | 0.591 in | 9/16 in (14.29 mm) | SAE 0.71 mm smaller |
| 16 mm | 0.630 in | 5/8 in (15.88 mm) | SAE 0.12 mm smaller |
| 17 mm | 0.669 in | 11/16 in (17.46 mm) | SAE 0.46 mm larger |
| 18 mm | 0.709 in | 11/16 in (17.46 mm) | SAE 0.54 mm smaller |
| 19 mm | 0.748 in | 3/4 in (19.05 mm) | SAE 0.05 mm larger |
| 21 mm | 0.827 in | 13/16 in (20.64 mm) | SAE 0.36 mm smaller |
| 22 mm | 0.866 in | 7/8 in (22.22 mm) | SAE 0.22 mm larger |
| 24 mm | 0.945 in | 15/16 in (23.81 mm) | SAE 0.19 mm smaller |
| 27 mm | 1.063 in | 1-1/16 in (26.99 mm) | SAE 0.01 mm smaller |
A wrench that is larger than the head is loose on it. A wrench that is smaller than the head will not go on at all.
Which pairs can you swap?
The pairs with a gap of about 0.12 mm or less are the near matches: 8 mm and 5/16 in, 11 mm and 7/16 in, 16 mm and 5/8 in, 19 mm and 3/4 in, and 27 mm and 1-1/16 in. For these, a wrench from one system goes onto a head from the other system with little or no play.
Every other pair differs by 0.19 mm or more. A wrench that much oversize touches only the corners of the head instead of the flats, which rounds soft, rusty or already worn heads. Use the exact size whenever you can, and treat a swap as a stopgap.
Wrench size for metric bolts
| Bolt | Standard wrench | Common alternative |
|---|---|---|
| M4 | 7 mm | |
| M5 | 8 mm | |
| M6 | 10 mm | |
| M8 | 13 mm | 12 mm on some Japanese-market hardware |
| M10 | 16 mm | 17 mm on DIN bolts, 14 mm on some Japanese-market hardware |
| M12 | 18 mm | 19 mm on DIN bolts, 17 mm on some Japanese-market hardware |
| M14 | 21 mm | 22 mm on DIN bolts |
| M16 | 24 mm | |
| M20 | 30 mm |
The standard column follows the ISO hex bolt dimensions; DIN dimensions are slightly larger on M10, M12 and M14. If a wrench from the standard column feels loose, try the alternative before you force it.
Wrench size for inch bolts
| Bolt diameter | Wrench size |
|---|---|
| 1/4 in | 7/16 in |
| 5/16 in | 1/2 in |
| 3/8 in | 9/16 in |
| 7/16 in | 5/8 in |
| 1/2 in | 3/4 in |
| 5/8 in | 15/16 in |
| 3/4 in | 1-1/8 in |
These are the widths for regular hex cap screws. Nuts and structural bolts can have larger heads, so measure when you are not sure.
How to find the size of a nut or bolt
- Close an adjustable wrench or a caliper on two opposite flats of the head.
- Read the opening in millimeters or inches.
- Pick the metric or SAE wrench that matches it within about 0.1 mm.
- Check that the wrench seats fully on the flats before you turn it.
If the head is rounded, a socket with a 6-point opening or a chain wrench grips better than a 12-point wrench.
Which set to buy
Most metric sets run from about 8 to 19 mm and most SAE sets from 5/16 to 1 in. A combination set has an open end on one side and a box end on the other. Ratcheting wrench sets save time on long runs of bolts, and when a bolt needs a specific tightening force, a torque wrench with the right drive size takes over from the wrench.
Related guides
Related guides
Frequently asked questions
What size is a 10 mm wrench in inches?
0.394 in, or about 25/64 in. There is no standard SAE wrench for it: 3/8 in is 0.375 in and too small, and 7/16 in is 0.438 in and loose.
Is a 13 mm wrench the same as a 1/2 in wrench?
No. 13 mm is 0.512 in and 1/2 in is 0.500 in (12.7 mm). A 1/2 in wrench is too small for a 13 mm head, and a 13 mm wrench is loose on a 1/2 in head.
What size wrench fits an M8 bolt?
A 13 mm wrench fits a standard ISO M8 hex bolt. Some Japanese-market hardware uses a smaller 12 mm head.
Why do loose wrenches round off bolts?
A wrench that is even a fraction of a millimeter oversize touches only the corners of the head instead of the flats, which rounds soft or rusty heads.
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