Bolt Spanner Size Chart
Quick answer: bolt diameter and wrench size are not the same. A 1/2-inch standard hex bolt commonly uses a 3/4-inch wrench, while a 1/2-inch heavy hex bolt uses a 7/8-inch wrench. Metric hex bolts use millimeter spanner sizes such as M8 with 13 mm, M10 with 17 mm, and M12 with 19 mm.
Inch Bolt Spanner Size Chart
| Bolt diameter | Standard hex wrench | Heavy hex wrench | Note |
|---|---|---|---|
| 1/4 in | 7/16 in | – | Small hardware |
| 5/16 in | 1/2 in | – | Common light hardware |
| 3/8 in | 9/16 in | – | Very common garage size |
| 7/16 in | 5/8 in | – | Common inch hardware |
| 1/2 in | 3/4 in | 7/8 in | Heavy hex needs a larger wrench |
| 5/8 in | 15/16 in | 1-1/16 in | Large fastener range |
| 3/4 in | 1-1/8 in | 1-1/4 in | Structural/hardware range |
| 7/8 in | 1-5/16 in | 1-7/16 in | Large fastener range |
| 1 in | 1-1/2 in | 1-5/8 in | Large fastener range |
| 1-1/4 in | 1-7/8 in | 2 in | Large structural fastener |
| 1-1/2 in | 2-1/4 in | 2-3/8 in | Industrial range |
| 2 in | 3 in | 3-1/8 in | Industrial range |
Metric Bolt Spanner And Hex Key Sizes
| Metric bolt diameter | Hex wrench/spanner size | Socket head hex key | Common note |
|---|---|---|---|
| M3 | 5.5 mm | 2.5 mm | Small machine screw range |
| M4 | 7 mm | 3 mm | Small machine screw range |
| M5 | 8 mm | 4 mm | Small machine screw range |
| M6 | 10 mm | 5 mm | Common light hardware |
| M8 | 13 mm | 6 mm | Very common automotive/shop size |
| M10 | 17 mm | 8 mm | Common automotive/shop size |
| M12 | 19 mm | 10 mm | Common automotive/shop size |
| M14 | 21 mm | 12 mm | Large metric fastener |
| M16 | 24 mm | 14 mm | Large metric fastener |
| M20 | 30 mm | 17 mm | Heavy hardware |
| M24 | 36 mm | 19 mm | Heavy hardware |
| M30 | 46 mm | 22 mm | Industrial range |
Spanner Fit Tips
- Use a 6-point socket or ring spanner on stubborn fasteners.
- Check whether the fastener is standard hex, heavy hex, or socket head.
- Do not substitute metric and inch tools when the fit is loose.
- For structural fasteners, confirm the standard and manufacturer drawing.
Related references: bolt chart with wrench size, socket sizes in order, and Allen wrench size chart.
Source Notes
Spanner sizes were checked against inch and metric tool-size charts including Northwest Fastener’s tool size chart and US bolt wrench-size references.
