Spanner size chart: metric wrench and Allen key sizes from M3 to M24
Which spanner or Allen key fits a metric bolt or nut, by thread size and type of head. Each column links to the standard it comes from.
Metric wrench size chart for bolts and nuts
Width across flats in millimetres. ISO hex bolts and nuts changed size at M10, M12, M14 and M22, so the old DIN size is shown alongside.
| Thread | Hex bolt and nutISO 4017 / 4032, mm | Hex bolt and nut, old DINDIN 933 / 934, mm | Flange nutDIN 6923, mm |
|---|---|---|---|
| M3 | 5.5 | 5.5 | – |
| M4 | 7 | 7 | – |
| M5 | 8 | 8 | 8 |
| M6 | 10 | 10 | 10 |
| M8 | 13 | 13 | 13 |
| M10 | 16 | 17 | 15 |
| M12 | 18 | 19 | 18 |
| M14 | 21 | 22 | 21 |
| M16 | 24 | 24 | 24 |
| M18 | 27 | 27 | – |
| M20 | 30 | 30 | 30 |
| M22 | 34 | 32 | – |
| M24 | 36 | 36 | – |
Allen key size chart, metric
Hexagon socket size in millimetres, which is the Allen key you need.
| Thread | Socket head cap screwISO 4762, mm | Low headDIN 7984, mm | Button headISO 7380-1, mm | CountersunkISO 10642, mm | Set screwISO 4026, mm |
|---|---|---|---|---|---|
| M3 | 2.5 | 2 | 2 | 2 | 1.5 |
| M4 | 3 | 2.5 | 2.5 | 2.5 | 2 |
| M5 | 4 | 3 | 3 | 3 | 2.5 |
| M6 | 5 | 4 | 4 | 4 | 3 |
| M8 | 6 | 5 | 5 | 5 | 4 |
| M10 | 8 | 7 | 6 | 6 | 5 |
| M12 | 10 | 8 | 8 | 8 | 6 |
| M14 | 12 | – | – | – | – |
| M16 | 14 | 12 | 10 | 10 | 8 |
| M18 | – | – | – | – | – |
| M20 | 17 | 14 | – | 12 | 10 |
| M22 | – | – | – | – | – |
| M24 | 19 | 17 | – | – | 12 |
A dash means the standard has no such size.
Bolt and spanner sizes: why M10 can take 16 or 17 mm
When the German DIN standards for hex bolts and nuts were replaced by ISO, the width across flats changed at four sizes: M10 from 17 to 16 mm, M12 from 19 to 18 mm, M14 from 22 to 21 mm and M22 from 32 to 34 mm. Both are still sold, so check the head before you reach for the spanner. Every other size kept its spanner. The Allen key sizes of socket screws did not change.
Spanner sizes: common questions
- What size spanner do I need for an M8 bolt?
- 13 mm, for both ISO and DIN hex bolts and nuts. An M8 socket head cap screw takes a 6 mm Allen key.
- What size spanner fits an M12 nut?
- 18 mm for an ISO 4032 nut, 19 mm for an older DIN 934 nut.