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
References
- ISO 4014:2011 - Hexagon head bolts
- ISO 4762:2004 - Hexagon socket head cap screws
- British Standards Institution
Frequently Asked Questions
What size spanner fits an M10 bolt?
An M10 hex bolt has an across-flats (A/F) dimension of 16 mm, so you need a 16 mm spanner or socket. This applies to standard hex bolts per ISO 4014/4017. Some flanged or non-standard bolts may differ.
What Allen key do I need for an M8 socket head cap screw?
An M8 socket head cap screw (ISO 4762) requires a 6 mm hex key (Allen key). The head diameter is 13 mm and the head height is approximately 8 mm.
What is the difference between A/F and A/C bolt head measurements?
A/F (across flats) is the distance between two parallel flat faces of the hex head - this is your spanner size. A/C (across corners) is the distance between two opposite corners of the hexagon. A/C is always larger than A/F. For standard hex bolts, A/C = A/F x 1.155.
What does the Bolt Head Sizes Chart provide for engineers working with metric bolts and screws in the UK?
The Bolt Head Sizes Chart provides specific dimensions such as across flats (A/F) and head height measurements from M3 to M30, adhering to ISO standards 4014/4017 for hex bolts and ISO 4762 for socket head cap screws.
How do engineers determine the correct spanner or socket size needed based on the Bolt Head Sizes Chart?
Engineers can find the appropriate spanner or socket size by consulting the A/F (across flats) column in the chart, which indicates the size required to fit the hexagonal head of bolts and screws.
What types of fasteners are included in the Bolt Head Sizes Chart for UK engineers?
The chart includes metric hex bolts as per ISO 4014/4017 and socket head cap screws following ISO 4762, covering sizes from M3 to M30.
When selecting a bolt size using the Bolt Head Sizes Chart, what are some critical dimensions engineers should consider?
Engineers should pay attention to both the A/F (across flats) dimension and the height of the hex head or socket cap screw when choosing the right bolt size for their application from the chart.
Where can UK engineers find suppliers who provide bolts according to the Bolt Head Sizes Chart standards?
UK engineers can contact local fastener distributors that supply metric bolts adhering to ISO 4014/4017 and socket head cap screws following ISO 4762, ensuring compatibility with the dimensions listed in the chart.
What is the standard for hex bolt heads as referenced by the Bolt Head Sizes Chart?
The standard for hex bolt heads in the Bolt Head Sizes Chart is ISO 4014/4017, providing dimensions that ensure interchangeability across different manufacturers and applications.
How does the cost or availability of bolts impact UK engineers using the Bolt Head Sizes Chart?
Availability and costs can vary based on bolt size and material; engineers should consult local suppliers for pricing and stock levels to ensure they have the necessary fasteners according to the dimensions specified in the chart.