Bolt Spanner Size Chart
Quick answer: bolt diameter is not wrench size. A 1/2-inch standard hex bolt uses a 3/4-inch wrench; a 1/2-inch heavy-hex bolt uses 7/8 inch. For current ISO metric hex bolts, M10 uses 16 mm and M12 uses 18 mm. Older DIN M10/M12 heads commonly use 17 mm/19 mm.
Inch Bolt Wrench Sizes
These nominal across-flats sizes distinguish regular hex bolts from heavy-hex bolts. A dash in the heavy-hex column means that size is not listed in this selected heavy-hex reference. Bolt head dimensions can vary by product standard and head style, so check the fastener markings or drawing when precision matters.
| 1/4 in | 7/16 in | Not listed |
|---|---|---|
| 5/16 in | 1/2 in | Not listed |
| 3/8 in | 9/16 in | 11/16 in |
| 7/16 in | 5/8 in | Not listed |
| 1/2 in | 3/4 in | 7/8 in |
| 9/16 in | 13/16 in | Not listed |
| 5/8 in | 15/16 in | 1-1/16 in |
| 3/4 in | 1-1/8 in | 1-1/4 in |
| 7/8 in | 1-5/16 in | 1-7/16 in |
| 1 in | 1-1/2 in | 1-5/8 in |
| 1-1/8 in | 1-11/16 in | 1-13/16 in |
| 1-1/4 in | 1-7/8 in | 2 in |
| 1-3/8 in | 2-1/16 in | 2-3/16 in |
| 1-1/2 in | 2-1/4 in | 2-3/8 in |
| 1-3/4 in | 2-5/8 in | 2-3/4 in |
| 2 in | 3 in | 3-1/8 in |
| 2-1/4 in | 3-3/8 in | 3-1/2 in |
| 2-1/2 in | 3-3/4 in | 3-7/8 in |
| 2-3/4 in | 4-1/8 in | 4-1/4 in |
| 3 in | 4-1/2 in | 4-5/8 in |
20 rows
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Metric Wrench and Hex-Key Sizes
The ISO column is for metric hex-head bolts. The legacy DIN column helps identify older hardware; the wrench widths differ at M10, M12, M14, and M22. The last column is for ISO 4762 socket-head cap screws, which use an internal hex key rather than an external wrench.
| M3 | 5.5 mm | 5.5 mm | 2.5 mm |
|---|---|---|---|
| M4 | 7 mm | 7 mm | 3 mm |
| M5 | 8 mm | 8 mm | 4 mm |
| M6 | 10 mm | 10 mm | 5 mm |
| M8 | 13 mm | 13 mm | 6 mm |
| M10 | 16 mm | 17 mm | 8 mm |
| M12 | 18 mm | 19 mm | 10 mm |
| M14 | 21 mm | 22 mm | 12 mm |
| M16 | 24 mm | 24 mm | 14 mm |
| M18 | 27 mm | 27 mm | 14 mm |
| M20 | 30 mm | 30 mm | 17 mm |
| M22 | 34 mm | 32 mm | 17 mm |
| M24 | 36 mm | 36 mm | 19 mm |
| M27 | 41 mm | 41 mm | 19 mm |
| M30 | 46 mm | 46 mm | 22 mm |
15 rows
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Before Choosing a Tool
- Identify the bolt diameter and the head style separately.
- Check whether metric hardware follows current ISO dimensions or older DIN dimensions.
- For socket-head cap screws, use the hex-key column; it does not apply to every recessed fastener.
- Seat the correct tool fully and avoid forcing a loose-fitting size.
Related references: bolt and wrench dimensions, socket sizes in order, and American wrench sizes.
Sources
Inch-series hex and heavy-hex dimensions follow ASME B18.2.1 and are cross-checked against Northwest Fastener’s tool-size chart. Current metric hex-bolt dimensions are from ISO 4014; the legacy DIN differences are documented by Würth. Internal hex-key sizes are for ISO 4762 socket-head cap screws; Wiha’s metric socket-head screw key chart show the corresponding key sizes. The ASME inch-series heavy-hex range is also cross-checked against Schuster’s ASME B18.2.1 product table.
