Every Allen key is sized by the distance across the flats of its hexagon - the measurement matters because a hex key only works when it fills the socket completely. This guide lists every standard metric and imperial size, the bolts each one fits, the near-miss pairs that ruin screws, and how to measure a key you cannot identify.
Metric Allen key sizes
Metric is the dominant system in the UK. Almost everything made or sold here in the last fifty years - flat pack furniture, bicycles, 3D printers, modern machinery - takes a metric hex key. These are the standard sizes with the socket screws each one drives:
| Key size | Cap head screw | Button head | Grub screw |
|---|---|---|---|
| 1.27mm | - | - | M2 and smaller |
| 1.5mm | M2 | - | M3 |
| 2mm | M2.5 | M3 | M4 |
| 2.5mm | M3 | M4 | M5 |
| 3mm | M4 | M5 | M6 |
| 4mm | M5 | M6 | M8 |
| 5mm | M6 | M8 | M10 |
| 6mm | M8 | M10 | M12 |
| 8mm | M10 | - | M16 |
| 10mm | M12 | - | M20 |
Notice the same key drives three different screw types at three different thread sizes - a 4mm key fits an M5 cap head, an M6 button head and an M8 grub screw. This is why identifying the screw type matters as much as measuring the thread.
The half sizes: 3.5mm, 4.5mm and 5.5mm
Three metric sizes sit between the standard steps, and most Allen key sets simply leave them out. They turn up on bicycle components, machine tools, optical equipment and a surprising amount of European machinery - and when a 5.5mm socket meets a set that jumps from 5mm to 6mm, the job stops. The 5mm key rattles; the 6mm will not enter. A 13 piece metric set that includes the half sizes costs no more than one that skips them, which makes their absence from most sets hard to defend.
Imperial Allen key sizes
Imperial (inch) sizes rule anything built to American standards and much older British engineering - classic motorcycles, vintage machinery, imported US equipment, and most American-made guitars and amplifiers. The standard range:
| Key size | Decimal (inches) | Millimetre equivalent | Typical cap head |
|---|---|---|---|
| 3/64in | 0.047 | 1.19mm | #3, #4 |
| 1/16in | 0.063 | 1.59mm | #5 |
| 5/64in | 0.078 | 1.98mm | #6 |
| 3/32in | 0.094 | 2.38mm | #8 |
| 7/64in | 0.109 | 2.78mm | #10 |
| 1/8in | 0.125 | 3.18mm | #10, 1/4in button |
| 9/64in | 0.141 | 3.57mm | - |
| 5/32in | 0.156 | 3.97mm | 1/4in button |
| 3/16in | 0.188 | 4.76mm | 1/4in |
| 7/32in | 0.219 | 5.56mm | 5/16in |
| 1/4in | 0.250 | 6.35mm | 3/8in |
| 5/16in | 0.313 | 7.94mm | 1/2in |
| 3/8in | 0.375 | 9.53mm | 5/8in |
If your workshop only ever sees modern metric equipment you may never need these. The moment a classic bike, an old lathe or an American import arrives, nothing else will do - which is why a dedicated imperial set or a combined 26 piece set covering both systems earns its place.
The near-miss pairs that round screws
Some metric and imperial sizes fall close enough that the wrong one enters the socket - and then slips under torque, rounding the screw. These are the pairs responsible for most ruined hex sockets:
| Imperial | Millimetres | Nearby metric | What happens |
|---|---|---|---|
| 5/64in | 1.98mm | 2mm | Metric key is 0.02mm larger - may jam or fit; never assume |
| 5/32in | 3.97mm | 4mm | 4mm key in a 5/32 socket feels tight but can slip loaded |
| 7/32in | 5.56mm | 5.5mm | 5.5mm sits loose in 7/32 - classic rounding risk |
| 5/16in | 7.94mm | 8mm | 8mm will not enter 5/16; 5/16 rattles in 8mm |
| 3/8in | 9.53mm | 10mm | 3/8 in a 10mm socket slips under any real torque |
The rule that prevents all of it: if a key enters with play, stop. A correct key slides in with no rattle and turns with no rotational slack. Slightly loose is the wrong key, whatever the label says.
How to measure an unknown Allen key or socket
For a key: measure across the flats (side to side, not corner to corner) with vernier calipers, then read the tables above - 3.97mm is 5/32in, not a worn 4mm. For a socket: find the largest key that enters and turns without play. If a metric key almost fits, check the imperial column before forcing anything; the almost-fit is how sockets die.
Ball end, short arm: which end does what
Most quality hex keys carry a ball end on the long arm. It exists for access - the ball drives a screw at up to 25 degrees off axis, reaching fixings you cannot approach straight on. It is not for final tightening: the ball contacts less of the socket, so heavy torque on it can round the screw or snap the ball. Reach and run the screw down with the ball, then switch to the short arm - it engages the full hex and takes the torque. Two ends, two jobs.
Frequently asked questions
What size Allen key does flat pack furniture use? Usually 4mm or 5mm, occasionally 3mm on smaller fittings. Most flat pack brands include a folded key, but a proper long-arm key with a ball end makes the job faster and kinder to your hands.
What sizes do bicycles use? Predominantly 4mm and 5mm, with 2mm, 2.5mm, 3mm, 6mm and 8mm across brakes, seatposts, stems and crank bolts. A metric set from 2mm to 8mm covers almost every bike made this century.
Are Allen keys and hex keys the same thing? Yes. Allen is a genericised brand name; hex key is the generic term. L-key, L-wrench and hex wrench all describe the same tool.
Can I use a metric key in an imperial socket? Sometimes it will enter - that is exactly the problem. See the near-miss table above: an almost-fitting key slips under torque and rounds the socket. Match the system to the screw.
All HemBorta Allen key sets - metric, imperial and combined - are available on Amazon; each product page on this site links straight to the listing for your marketplace.
