Allen / Hex Key Size Finder
Pick the right hex key for any socket-head cap screw
किसी भी socket head cap screw के लिए सही allen key
Allen keys (also called hex keys) come in a set of standard sizes designed to match socket-head screws. For ISO-standard socket-head cap screws (Unbrako's default), the rule is: M3 uses 2.5 mm, M4 uses 3 mm, M5 uses 4 mm, M6 uses 5 mm, M8 uses 6 mm, M10 uses 8 mm, M12 uses 10 mm, M16 uses 14 mm, M20 uses 17 mm. These are fixed by ISO 4762 and don't vary between manufacturers. Hex socket set screws (grub screws) use a different set — usually one size smaller: M4 set screw uses 2 mm, M6 uses 3 mm, M8 uses 4 mm, M10 uses 5 mm. Countersunk socket screws (ISO 10642) match cap screws up to M8 but then diverge. This finder covers all three types and warns when ball-end L-keys or T-handles are a better choice for the application.
Calculator
For higher torque applications, use a T-handle or 3/8"-drive hex bit in a torque wrench — straight L-keys can slip and round the socket at high torque.
ƒFormula
No formula — ISO 4762 lookup tableAllen key sizes are standardised by ISO 4762 (socket head cap screws) and ISO 4027 (hex set screws). There is no calculation — always refer to the table. Manufacturers like Unbrako, Holo-Krome, and Bossard all follow the same sizes.
Units: Across flats (AF) of hex socket — millimetre (mm)
Allen key ki size kaise decide karein?
Unbrako socket head cap screws (SHCS) ke liye allen key ki size ISO 4762 standard se fix hoti hai — manufacturer change ho to bhi size same rehti hai. Common pairs: M6 cap screw → 5 mm allen, M8 → 6 mm, M10 → 8 mm, M12 → 10 mm, M16 → 14 mm. Set screws (grub screws) usually ek size chhoti key use karte hain: M6 set screw → 3 mm allen, M8 → 4 mm, M10 → 5 mm. Countersunk socket screws ki size cap screw jaisi hi hoti hai chhote sizes mein, lekin bade sizes mein different hai. Ball-end L-keys tight corners mein helpful hain; T-handle keys heavy torque ke liye.
Worked Examples
Assembling a fixture with Unbrako M8 cap screws
Problem: Four M8 × 30 mm Unbrako grade 12.9 cap screws in a bracket.
Solution: M8 SHCS uses a 6 mm allen key. For 42 Nm torque (grade 12.9 dry), use a T-handle 6 mm allen or a 3/8-inch drive hex bit with a torque wrench. Straight L-keys slip on high torque.
Loosening a stuck grub screw on a pulley
Problem: The M6 grub screw locking a pulley onto a shaft is stuck — which key?
Solution: M6 hex set screw uses a 3 mm allen key. Heat the pulley area gently to break any thread-lock, then use a ball-end 3 mm for corner access. Replace if the socket is rounded.
Countersunk screw confusion
Problem: M10 countersunk socket screw (CSK) — same 8 mm key as cap screw?
Solution: For ISO 10642 M10 countersunk socket screw, the hex key is 6 mm (not 8 mm like the cap screw). Always verify with the actual screw — countersunk variants have a smaller hex socket.
Allen Key (Hex Key) Size Chart — All Socket Screw Types
Allen key chart — सभी socket screws के लिए
| Bolt Size | Cap Screw (SHCS) | Set Screw / Grub | Countersunk (CSK) |
|---|---|---|---|
| M1.6 | 1.5 mm | 0.7 mm | — |
| M2 | 1.5 mm | 0.9 mm | 1.3 mm |
| M2.5 | 2 mm | 1.3 mm | 1.5 mm |
| M3 | 2.5 mm | 1.5 mm | 2 mm |
| M4 | 3 mm | 2 mm | 2.5 mm |
| M5 | 4 mm | 2.5 mm | 3 mm |
| M6 | 5 mm | 3 mm | 4 mm |
| M8 | 6 mm | 4 mm | 5 mm |
| M10 | 8 mm | 5 mm | 6 mm |
| M12 | 10 mm | 6 mm | 8 mm |
| M14 | 12 mm | 6 mm | 10 mm |
| M16 | 14 mm | 8 mm | 10 mm |
| M18 | 14 mm | — | 12 mm |
| M20 | 17 mm | 10 mm | 12 mm |
| M22 | 17 mm | — | 14 mm |
| M24 | 19 mm | 12 mm | 14 mm |
| M27 | 19 mm | — | — |
| M30 | 22 mm | 14 mm | — |
Sizes are ISO 4762 (SHCS), ISO 4027 (set screws), ISO 10642 (CSK). Unbrako, Holo-Krome, and all major brands follow these. Ball-end L-keys trade some torque capacity for angle tolerance — use straight keys for high-torque applications.
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Grade 12.9 SHCS M3–M36, set screws, countersunk screws — all ISO standard.
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