Socket Head Cap Screw Dimensions
Head diameter, head height, hex key size and counterbore sizes for socket head cap screws, M3 to M36.
The socket head cap screw (ISO 4762, often called an Allen screw) is the usual fastener in machine tools, fixtures and compact assemblies. Its head is a plain cylinder with a hexagon socket, so it can sit in a counterbore and be driven in tight spaces. The head height equals the thread diameter, which makes it easy to design for.
The table
| Thread | Coarse pitch (mm) | Head diameter dk (mm) | Head height k (mm) | Hex key size s (mm) | Counterbore diameter, practical minimum (mm) | Counterbore depth, practical minimum (mm) |
|---|---|---|---|---|---|---|
| M3 | 0.5 | 5.5 | 3 | 2.5 | 6 | 3.5 |
| M4 | 0.7 | 7 | 4 | 3 | 7.5 | 4.5 |
| M5 | 0.8 | 8.5 | 5 | 4 | 9 | 5.5 |
| M6 | 1 | 10 | 6 | 5 | 10.5 | 6.5 |
| M8 | 1.25 | 13 | 8 | 6 | 13.5 | 8.5 |
| M10 | 1.5 | 16 | 10 | 8 | 16.5 | 10.5 |
| M12 | 1.75 | 18 | 12 | 10 | 18.5 | 12.5 |
| M14 | 2 | 21 | 14 | 12 | 21.5 | 14.5 |
| M16 | 2 | 24 | 16 | 14 | 24.5 | 16.5 |
| M20 | 2.5 | 30 | 20 | 17 | 30.5 | 20.5 |
| M24 | 3 | 36 | 24 | 19 | 36.5 | 24.5 |
| M30 | 3.5 | 45 | 30 | 22 | 45.5 | 30.5 |
| M36 | 4 | 54 | 36 | 27 | 54.5 | 36.5 |
Source and notes: ISO 4762 (hexagon socket head cap screws), maximum head diameter and nominal head height (equal to the thread diameter). Counterbore sizes are a practical rule: the head diameter plus 0.5 mm, and a depth of the head height plus 0.5 mm; the standard recommends sizes by series, so check it for tight fits.
How to use this table
- Find the thread size.
- Use the hex key size to pick the tool, and the head diameter to check the space around the screw.
- For a sunk head, make the counterbore at least the head diameter plus a little clearance, and at least as deep as the head height.
Worked example: an M8 screw sunk flush
- M8 socket head: head diameter 13 mm, head height 8 mm, hex key 6 mm.
- A counterbore of 13.5 mm diameter and 8.5 mm deep leaves the head 0.5 mm below the surface.
- The through hole below the counterbore is the medium clearance hole for M8: 9 mm.
Notes and cautions
- Counterbore sizes here are a practical rule (head diameter + 0.5 mm; head height + 0.5 mm). ISO 4762 and the maker’s catalog give recommended series, which are a little larger for easy assembly.
- Low-head, button-head (ISO 7380) and countersunk (ISO 10642) socket screws have different heads.
- Standard socket cap screws are class 12.9 (alloy steel) or A2-70 and A4-80 stainless; 12.9 screws are strong but hard, so avoid side loads and shock.
Related tools and guides
- Metric Bolt Clearance Hole Sizes
- Estimated Bolt Tightening Torque (Metric)
- Metric Thread Table (Coarse)
- Drilling, Tapping and Fasteners (guide)
- All reference tables
FAQ
What size hex key for an M6 socket screw?
5 mm. For M3 it is 2.5 mm, M4 is 3 mm, M5 is 4 mm, M8 is 6 mm and M10 is 8 mm.
How tall is a socket head cap screw head?
The head height equals the nominal thread diameter: 6 mm for an M6.
Sources
- ISO 4762 (hexagon socket head cap screws); ISO 4762 Annex and the maker’s catalog for counterbore recommendations.
- ISO 4762 (hexagon socket head cap screws), maximum head diameter and nominal head height (equal to the thread diameter). Counterbore sizes are a practical rule: the head diameter plus 0.5 mm, and a depth of the head height plus 0.5 mm; the standard recommends sizes by series, so check it for tight fits.