Reference & Charts
Wrench Size Chart
The across-flats wrench (spanner/socket) size for standard hex-head bolts and nuts, metric and inch in one searchable chart. Search a fastener size (e.g. M10, 1/2) or filter by system. Across-flats is the flat-to-flat hex dimension — the size stamped on the wrench.
23 entries · Last verified: 2026-07-17
"Across flats" (A/F) is the distance between two opposite flats of the hex — the number stamped on a wrench or socket. This chart gives the standard across-flats size for regular hex-head bolts and hex nuts so you can grab the right wrench by fastener size. Search a size (type "M10" or "1/2") or filter by system.
One caveat worth knowing: a few metric sizes changed. Current ISO (4014/4017/4032) uses 16 mm for M10 and 18 mm for M12, but older DIN 931/934 fasteners use 17 mm and 19 mm — you'll still meet both. Heavy-hex structural fasteners are larger across the flats than the regular hex here. When in doubt, measure the flats.
Showing 23 of 23
| Fastener | System | Wrench size (across flats) |
|---|---|---|
| M3 | Metric | 5.5 mm |
| M4 | Metric | 7 mm |
| M5 | Metric | 8 mm |
| M6 | Metric | 10 mm |
| M8 | Metric | 13 mm |
| M10 | Metric | 16 mm |
| M12 | Metric | 18 mm |
| M14 | Metric | 21 mm |
| M16 | Metric | 24 mm |
| M18 | Metric | 27 mm |
| M20 | Metric | 30 mm |
| M22 | Metric | 34 mm |
| M24 | Metric | 36 mm |
| 1/4" | Inch | 7/16" (0.4375") |
| 5/16" | Inch | 1/2" (0.5") |
| 3/8" | Inch | 9/16" (0.5625") |
| 7/16" | Inch | 5/8" (0.625") |
| 1/2" | Inch | 3/4" (0.75") |
| 9/16" | Inch | 13/16" (0.8125") |
| 5/8" | Inch | 15/16" (0.9375") |
| 3/4" | Inch | 1-1/8" (1.125") |
| 7/8" | Inch | 1-5/16" (1.3125") |
| 1" | Inch | 1-1/2" (1.5") |
Frequently asked
- Is the wrench size the same for the bolt head and the nut?
- For standard hex bolts and hex nuts of the same thread size, yes — the across-flats (and so the wrench) matches. Heavy-hex, flange, and some locknuts differ, so confirm on the actual fastener.
- Why do some M10/M12 bolts need a 17 mm or 19 mm wrench?
- Those are older DIN 931/934 fasteners. Current ISO standards reduced M10 to a 16 mm and M12 to an 18 mm across-flats, but plenty of DIN-spec hardware (17 mm / 19 mm) is still in service. Keep both wrenches handy or measure the flats.
- Does this cover socket head cap screws?
- No — socket head cap screws use a hex key (Allen) in the head, not an external wrench. See the Socket Head Cap Screw Dimensions reference for those hex-key sizes.
Sources
- ISO 4014 / ISO 4017 — hexagon head bolts and screws (across-flats)
- ISO 4032 — hexagon regular nuts (across-flats)
- ASME B18.2.1 / B18.2.2 — inch hex bolts and nuts (finished hex across-flats)
- Note: older DIN 931/934 differ for some sizes; heavy-hex fasteners are larger