Hex Key / Allen Key Sizes Chart
Verified by Rachel Mayfield, Supply Chain Analyst - April 2026
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 Key / Allen Key Sizes Chart 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 |
|---|---|
| #4 (0.112") | 3/32" |
| #6 (0.138") | 7/64" |
| #8 (0.164") | 9/64" |
| #10 (0.190") | 5/32" |
| 1/4" | 3/16" |
| 5/16" | 1/4" |
| 3/8" | 5/16" |
| 7/16" | 3/8" |
| 1/2" | 3/8" |
| 5/8" | 1/2" |
| 3/4" | 5/8" |
| 7/8" | 3/4" |
| 1" | 3/4" |
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)
References
- ISO 2936:2014 - Hexagon socket screw keys
- ISO 4762:2004 - Hexagon socket head cap screws
- ASME B18.3 - Socket cap, shoulder, and set screws
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.