Bolt Head Sizes Chart

Verified by Rachel Mayfield, Supply Chain Analyst - April 2026

Head dimensions for metric hex bolts (ISO 4014/4017) and socket head cap screws (ISO 4762) from M3 to M30. Bolt Head Sizes Chart The A/F (across flats) dimension tells you the spanner or socket size needed. Head height is measured from the bearing surface to the top of the head. Socket head cap screw dimensions include the head diameter and the hex key (Allen key) size required.

Bolt Size	Hex Head A/F (mm)	Hex Head Height (mm)	Socket Head Dia (mm)	Socket Key Size (mm)
M3	5.5	2.0	5.5	2.5
M4	7.0	2.8	7.0	3.0
M5	8.0	3.5	8.5	4.0
M6	10.0	4.0	10.0	5.0
M8	13.0	5.3	13.0	6.0
M10	16.0	6.4	16.0	8.0
M12	18.0	7.5	18.0	10.0
M14	21.0	8.8	21.0	12.0
M16	24.0	10.0	24.0	14.0
M18	27.0	11.5	27.0	14.0
M20	30.0	12.5	30.0	17.0
M22	32.0	14.0	33.0	17.0
M24	36.0	15.0	36.0	19.0
M27	41.0	17.0	40.0	19.0
M30	46.0	18.7	45.0	22.0

How to use the bolt head sizes chart

To find the correct spanner size for a hex bolt, look up the bolt size (thread diameter) in the left column and read the A/F dimension. This is the nominal spanner size. For example, an M10 hex bolt needs a 16 mm spanner or socket.

For socket head cap screws, the key size column shows the hex key (Allen key) needed. An M8 socket head cap screw needs a 6 mm hex key.

Head height matters for counterbore design and clearance calculations. The counterbore depth should be at least equal to the head height plus 0.5 mm clearance. Counterbore diameter should be the head diameter plus 1 mm.

Related standards

• ISO 4014 - Hexagon head bolts, product grades A and B (partially threaded)
• ISO 4017 - Hexagon head screws, product grades A and B (fully threaded)
• ISO 4762 - Hexagon socket head cap screws
• ISO 4032 - Hexagon regular nuts, product grades A and B
• BS EN ISO 4014 - UK adoption of hex bolt dimensions