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Hex key (Allen key) sizes for metric socket head cap screws per ISO 4762 and imperial socket head cap screws per ASME B18.3. Hex keys are also called Allen keys - named after the Allen Manufacturing Company, which patented the fastener in 1910. The key size is measured across flats, matching the recessed hexagon in the screw head.
Metric hex key sizes (ISO 4762)
| Socket Head Screw Size |
Hex Key Size (mm) |
| M1.6 |
1.5 |
| M2 |
1.5 |
| M2.5 |
2.0 |
| M3 |
2.5 |
| M4 |
3.0 |
| M5 |
4.0 |
| M6 |
5.0 |
| M8 |
6.0 |
| M10 |
8.0 |
| M12 |
10.0 |
| M14 |
12.0 |
| M16 |
14.0 |
| M20 |
17.0 |
| M24 |
19.0 |
| M30 |
22.0 |
Imperial hex key sizes (ASME B18.3)
| Socket Head Screw Size |
Hex Key Size |
Hex key (decimal in) |
Hex key (mm) |
| #4 (0.112") | 3/32" | 0.0938 | 2.381 |
| #6 (0.138") | 7/64" | 0.1094 | 2.778 |
| #8 (0.164") | 9/64" | 0.1406 | 3.572 |
| #10 (0.190") | 5/32" | 0.1562 | 3.969 |
| 1/4" | 3/16" | 0.1875 | 4.762 |
| 5/16" | 1/4" | 0.25 | 6.350 |
| 3/8" | 5/16" | 0.3125 | 7.938 |
| 7/16" | 3/8" | 0.375 | 9.525 |
| 1/2" | 3/8" | 0.375 | 9.525 |
| 5/8" | 1/2" | 0.5 | 12.700 |
| 3/4" | 5/8" | 0.625 | 15.875 |
| 7/8" | 3/4" | 0.75 | 19.050 |
| 1" | 3/4" | 0.75 | 19.050 |
Imperial Allen key sizes in mm
Every imperial key on this chart converted exactly at 25.4 mm to the inch, with the closest metric key beside it and the gap between the two.
Imperial key |
Size in mm |
Nearest metric key on this chart (mm) |
Metric minus imperial (mm) |
|---|
| 3/32" | 2.381 | 2.5 | +0.119 |
| 7/64" | 2.778 | 3 | +0.222 |
| 9/64" | 3.572 | 4 | +0.428 |
| 5/32" | 3.969 | 4 | +0.031 |
| 3/16" | 4.762 | 5 | +0.238 |
| 1/4" | 6.350 | 6 | -0.350 |
| 5/16" | 7.938 | 8 | +0.062 |
| 3/8" | 9.525 | 10 | +0.475 |
| 1/2" | 12.700 | 12 | -0.700 |
| 5/8" | 15.875 | 17 | +1.125 |
| 3/4" | 19.050 | 19 | -0.050 |
The pairs that get mixed up are the ones within a tenth of a millimetre: 5/32 inch and 4 mm (0.031 mm apart), 5/16 inch and 8 mm (0.062 mm apart), 3/4 inch and 19 mm (0.050 mm apart). The wrong key from one of those pairs will go into the socket and feel usable, then round the corners under load. Every other pair differs by enough that the wrong key either will not enter or is obviously loose.
How to use the hex key chart
Find the screw size in the left column and read the required hex key size on the right. For a stripped or damaged hex socket, try one size smaller with tape wrapped around the key to fill the gap, or use a bolt extractor.
Hex keys come in short-arm, long-arm, ball-end, and T-handle variants. Ball-end keys allow angled access up to 25 degrees off-axis but reduce the maximum torque by about 80%. T-handles give higher torque but need clearance above the fastener.
Avoid mixing metric and imperial keys. A 5/32 inch key (3.97 mm) looks very close to a 4 mm metric key but will round off the recess in either direction. If unsure, the screw grade stamp on the head usually indicates metric (e.g. 8.8, 10.9, 12.9) vs imperial (e.g. Grade 5, Grade 8).
Related standards
- ISO 2936 - Assembly tools for screws and nuts: hexagon socket screw keys
- ISO 4762 - Hexagon socket head cap screws
- DIN 911 - Hexagon socket screw keys (short series)
- DIN 912 - Hexagon socket head cap screws (equivalent to ISO 4762)
- ASME B18.3 - Socket cap, shoulder, and set screws (inch series)
Hex key + Allen key sets
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Frequently Asked Questions
What size Allen key fits an M6 screw?
An M6 socket head cap screw (ISO 4762) needs a 5 mm hex key (Allen key). This is standard across metric socket head screws from DIN 912 and ISO 4762.
What Allen key do I need for M8?
An M8 socket head cap screw needs a 6 mm hex key. For M10, you need 8 mm. For M12, 10 mm. Sizes follow ISO 4762 / DIN 912 for all standard socket head cap screws.
Are hex keys and Allen keys the same thing?
Yes. Hex key is the generic name, Allen key is a brand-derived term (from the Allen Manufacturing Company, which patented the fastener in 1910). Both refer to the same six-sided tool used to drive hexagon socket screws.
What does the Hex Key / Allen Key Sizes Chart provide information about?
The Hex Key / Allen Key Sizes Chart provides details on the appropriate hex key sizes required for metric socket head cap screws according to ISO 4762 and imperial socket head cap screws per ASME B18.3.
Are there different types or variants of hex keys mentioned in the chart?
Yes, the Hex Key / Allen Key Sizes Chart includes both metric and imperial versions, catering to various screw sizes used across different industries.
How do I choose the right size hex key for a socket head cap screw according to the chart?
The chart specifies that you should match the hex key size with the corresponding screw size. For instance, an M4 screw requires a 3.0 mm hex key, while a #6 imperial screw needs a 7/64 inch hex key.
What standards are referenced in the Hex Key / Allen Key Sizes Chart?
The chart references ISO 4762 for metric screws and ASME B18.3 for imperial screws to ensure compliance with international quality specifications.
Where can UK engineers find suppliers for hex keys based on this chart?
UK engineers can consult local hardware stores or online platforms like Screwfix, RS Components, or Amazon Industrial & Scientific for supplies that conform to the Hex Key / Allen Key Sizes Chart standards.
What are some factors affecting the cost and availability of hex keys in the UK market?
Cost and availability may vary based on demand, material costs (like steel quality), quantity ordered, and whether they're stocked items or need custom manufacturing. Suppliers like Grainger or MRO Supply can offer competitive pricing and reliable delivery options.
How does the Hex Key / Allen Key Sizes Chart assist in inventory management for UK engineering firms?
By providing precise size requirements based on industry standards, the chart helps UK engineering firms maintain an accurate and efficient inventory of hex keys needed for maintenance and manufacturing tasks.
What are the imperial Allen key sizes?
Imperial Allen key sizes for socket head cap screws follow ASME B18.3 and step in fractional inches. A #4 screw takes a 3/32 inch key, #6 takes 7/64 inch, #8 takes 9/64 inch and #10 takes 5/32 inch. Larger screws move up through 3/16, 5/16, 3/8, 1/2 inch and beyond. Avoid mixing imperial and metric keys, as a 5/32 inch key (3.97 mm) is close enough to a 4 mm key to round off the recess.
What are the hex key sizes?
Metric hex key sizes follow ISO 4762 and are measured across flats to match the screw's recessed hexagon. M3 takes 2.5 mm, M4 takes 3.0 mm, M5 takes 4.0 mm, M6 takes 5.0 mm, M8 takes 6.0 mm, M10 takes 8.0 mm and M12 takes 10.0 mm. Larger sizes run M16 at 14.0 mm, M20 at 17.0 mm and M24 at 19.0 mm. Imperial keys follow ASME B18.3.
What are the Allen key sizes?
Allen keys are hex keys, named after the Allen Manufacturing Company that patented the fastener in 1910. Metric sizes (ISO 4762) match the screw: M4 takes 3.0 mm, M5 takes 4.0 mm, M6 takes 5.0 mm, M8 takes 6.0 mm and M10 takes 8.0 mm. The key size equals the across-flats measurement of the recessed hexagon in the screw head.
What are the UK Allen key sizes?
UK socket head cap screws use metric Allen keys to ISO 4762, sized across flats. Common sizes are 2.5 mm for M3, 4.0 mm for M5, 5.0 mm for M6, 6.0 mm for M8 and 8.0 mm for M10. Imperial keys to ASME B18.3 still appear on older or US-sourced kit. Do not mix the two, since a 5/32 inch key (3.97 mm) will strip a 4 mm recess.
What is a 5/32 Allen key in mm?
A 5/32 inch hex key is 3.969 mm. The nearest metric key is 4 mm, which is 0.031 mm larger, so the two are not interchangeable.