Hex Key Size Table
| Metric (mm) | Decimal (in) | Near-match SAE |
|---|---|---|
| 0.7 | 0.028 | — |
| 0.9 | 0.035 | — |
| 1.3 | 0.051 | — |
| 1.5 | 0.059 | — |
| 2.0 | 0.079 | — |
| 2.5 | 0.098 | — |
| 3.0 | 0.118 | — |
| 4.0 | 0.157 | 5/32 in |
| 5.0 | 0.197 | — |
| 6.0 | 0.236 | — |
| 8.0 | 0.315 | 5/16 in |
| 10.0 | 0.394 | — |
| 12.0 | 0.472 | — |
| 14.0 | 0.551 | — |
| 17.0 | 0.669 | — |
| 19.0 | 0.748 | 3/4 in |
A hex key is sized by the distance across flats - the width of the hexagon between two parallel sides, not corner to corner - which is why a key either fits a socket with almost no play or visibly will not enter: there is no in-between fit. The table carries the standard metric ladder from 0.7 mm (eyeglass and electronics territory) to 19 mm (large machine fasteners), each with its decimal-inch equivalent, and it is worth reading twice because three pairs sit within one percent of each other: 4 mm versus 5/32 inch, 8 mm versus 5/16 inch, and 19 mm versus 3/4 inch.
Those near-matches are the practical heart of the chart. A 5/32-inch key in a 4 mm socket turns - barely - and rounds the corners a little each time, which is how good fasteners become bad sockets. The inch ladder also climbs in fractions where the metric one steps in even tenths, so neither ladder lands on the other's values; the near-match pairs are coincidence, not design. The system carries a famous generic name: Allen key, from the Allen Manufacturing Company of Hartford, which made the tools so common that the brand became the word - the same path as dump truck and aspirin.
How to use
- Measure a fastener or key by its flats: caliper jaws across two parallel faces of the hex, never tip to tip - corner-to-corner reads about 15% larger and sends you up a size.
- Match exactly or not at all: the chart pairs 4 mm with 5/32 inch for identification, but substitution rounds hex corners - use the right system for the hardware, especially above finger-tight torque.
- Size ball-end keys for reach, not power: the ball tip engages at up to 30 degrees off-axis for blind or recessed screws, but torque drops fast with angle - thread the screw with the ball, finish it straight.
Frequently asked questions
Can I use a 5/32 inch hex key on a 4 mm screw?
Only once, and regret it after: 5/32 inch is 3.969 mm, within about 0.8% of 4 mm, so it seats and turns - and quietly rounds the socket corners as it does. The three near-match pairs on the chart (4 mm vs 5/32 in, 8 mm vs 5/16 in, 19 mm vs 3/4 in) exist because the inch ladder steps in fractions and the metric one in round millimeters, and at three points the ladders nearly touch. Identification is what the match is for: a 5/32 key tells you the foreign bolt is metric 4 mm. Light duty - a screw already loose, a low-torque set screw - forgives a near-match; anything you are tightening hard does not, because a rounded socket is fixed by drilling or extraction, not by buying a better wrench. The same logic runs backwards for metric keys on inch hardware.
How do I measure what size hex key I need?
Measure across the flats of either the key or the socket: put caliper jaws on two parallel sides of the hexagon and read the distance - that number is the nominal size, in millimeters for metric hardware and inches for imperial. Two traps account for most mis-measurements: measuring corner to corner instead (about 15% oversize - the diagonal of a hexagon is not its width) and measuring a worn socket, where rounded corners read small and steer you down a size. Without calipers, trial-fit works in one direction: a key that slides in with zero wiggle is the size; one that clicks over detents as you turn is one step too small; one that will not enter is one step too big. The table then names what your measurement found, on whichever ladder your hardware lives.
What is a ball-end hex key good for?
Access, not force. The rounded ball tip enters the socket at an angle - typically up to 30 degrees off-axis - which reaches recessed or shrouded screws a straight key cannot touch, and lets you spin a long run of thread in a hurry without realigning your wrist. The cost is mechanical: the ball contacts the hex corners instead of the flats, so torque capacity drops sharply as the angle grows, and under hard load the ball can pop out or round the socket. That is why the classic ball-end key puts the ball on the long arm (reach where you need it) and keeps a conventional chamfered end on the short arm (where the real tightening happens). Use the ball to find and start, the straight end to finish - and skip ball-end entirely on corroded or overtightened fasteners that need every bit of torque you have.
Why are hex keys called Allen keys?
Allen was a brand, not a type: the Allen Manufacturing Company of Hartford, Connecticut produced hex sockets and keys so widely that the name slid into the language as the generic word for the tool - the same journey that turned aspirin and escalator from trademarks into nouns. The fastener itself predates and outgrows the brand: hex-socket screws distribute torque across six faces instead of two, which is why they replaced square-drive and slotted heads wherever a tool must not cam out - bicycles, furniture, machinery and electronics all standardize on them. The naming also runs the other way in some markets: unbranded catalogs say hex key or hex wrench, British usage leans Allen key, and security variants add a pin in the center (which needs a keyed-out version of the same sizes). Whatever the label, the size logic in the table stays identical.