Hex Bolt Wrench Size Chart
Across-flats wrench sizes for metric and inch hex bolts, in millimeters and inches.
The wrench size of a hex bolt or nut is the distance across the flats of the hexagon. This chart gives it for metric sizes (ISO) and inch sizes (ASME) in both millimeters and inches, so you can find the right wrench or socket, or see at a glance whether a metric wrench will fit an inch bolt.
The table
| Bolt size | Hex across flats (mm) | Hex across flats (in) | Note |
|---|---|---|---|
| M3 (metric) | 5.5 | 0.217 | |
| M4 (metric) | 7 | 0.276 | |
| M5 (metric) | 8 | 0.315 | |
| M6 (metric) | 10 | 0.394 | |
| M8 (metric) | 13 | 0.512 | |
| M10 (metric) | 16 | 0.63 | 17 (DIN) |
| M12 (metric) | 18 | 0.709 | 19 (DIN) |
| M14 (metric) | 21 | 0.827 | 22 (DIN) |
| M16 (metric) | 24 | 0.945 | |
| M20 (metric) | 30 | 1.181 | |
| M24 (metric) | 36 | 1.417 | |
| M30 (metric) | 46 | 1.811 | |
| M36 (metric) | 55 | 2.165 | |
| 1/4 in (inch) | 11.11 | 0.4375 | Wrench: 7/16 in |
| 5/16 in (inch) | 12.7 | 0.5 | Wrench: 1/2 in |
| 3/8 in (inch) | 14.29 | 0.5625 | Wrench: 9/16 in |
| 7/16 in (inch) | 15.88 | 0.625 | Wrench: 5/8 in |
| 1/2 in (inch) | 19.05 | 0.75 | Wrench: 3/4 in |
| 9/16 in (inch) | 20.64 | 0.8125 | Wrench: 13/16 in |
| 5/8 in (inch) | 23.81 | 0.9375 | Wrench: 15/16 in |
| 3/4 in (inch) | 28.58 | 1.125 | Wrench: 1-1/8 in |
| 7/8 in (inch) | 33.34 | 1.3125 | Wrench: 1-5/16 in |
| 1 in (inch) | 38.1 | 1.5 | Wrench: 1-1/2 in |
Source and notes: ISO 4017 and ISO 4032 for metric bolts and nuts (DIN 931, 933 and 934 use larger flats for M10, M12 and M14), ASME B18.2.1 for inch hex bolts and cap screws. The wrench size is the across-flats dimension.
How to use this table
- Find the bolt size (metric rows are marked “metric”, inch rows “inch”).
- Read the across-flats size in the unit your wrenches use.
- Check the note column for sizes where older DIN hardware differs.
Worked example: which wrench fits?
- A 1/2 inch bolt has 19.05 mm flats and takes a 3/4 inch wrench. An older DIN M12 nut is 19 mm across, only 0.05 mm different, so the same wrench fits both.
- An ISO M12 nut is 18 mm across: a 3/4 inch wrench is 1.05 mm too big for it and will slip and round the corners.
- An ISO M10 head is 16 mm across; the DIN version is 17 mm. A 5/8 inch wrench is 15.88 mm, too small for either.
Notes and cautions
- Use a six-point socket or a box wrench on tight bolts: a twelve-point or open-end wrench can round the corners.
- The wrench for a nut is the same as for the bolt of that size.
- Hex key (Allen) sizes for socket head screws are on the socket head cap screw page.
Related tools and guides
- Metric Hex Bolt, Nut and Washer Dimensions
- Socket Head Cap Screw Dimensions
- Unified Inch Thread Table (UNC and UNF)
- All reference tables
FAQ
What size wrench for an M8 bolt?
13 mm.
What size wrench for a 1/2 inch bolt?
3/4 inch (19.05 mm).
Why are some metric nuts 17 mm instead of 16 mm?
The older DIN standard used larger flats for M10, M12 and M14 (17, 19 and 22 mm). ISO later reduced them (16, 18 and 21 mm), so both are still in use.
Sources
- ISO 4017 and ISO 4032 (metric hex bolts and nuts); ASME B18.2.1 (inch hex bolts and cap screws); DIN 931, 933 and 934.
- ISO 4017 and ISO 4032 for metric bolts and nuts (DIN 931, 933 and 934 use larger flats for M10, M12 and M14), ASME B18.2.1 for inch hex bolts and cap screws. The wrench size is the across-flats dimension.