Allen Key Size Chart: What Size Key for Each Bolt?

How the Allen key size chart works

A metric bolt is named by its shank diameter: an M8 bolt has an 8 mm shank. The Allen key does not fit the shank; it fits the hex socket inside the bolt head, and the socket is smaller than the shank. The Allen key size is the distance across the flats of that hex socket, measured from one flat side to the opposite flat side. For an M8 socket head cap screw the socket is 6 mm across the flats, so the key is 6 mm. The inch figures are the millimetres divided by 25.4, rounded to 2 decimals: 6 ÷ 25.4 = 0.24 in.

At the default — M8 — the key is 6 mm, which is 6 ÷ 25.4 = 0.24 in across the flats. That is row 4 of the chart. The reverse lookup works the other way: a 6-mm key fits an M8 bolt, which is 6 ÷ 25.4 = 0.24 in.

The worked example, line by line

Setup: M8 bolt; reverse key 6 mm.

Line 1 — the bolt socket. An M8 bolt has an 8 mm shank and a 6 mm hex socket in its head, measured across the flats.

Line 2 — the key in millimetres. The key matches the socket, so it is 6 mm.

Line 3 — the key in inches. 6 ÷ 25.4 = 0.24 in.

Line 4 — the chart row. M8 is row 4 of the 7-row chart.

Line 5 — the reverse lookup. A 6-mm key fits an M8 bolt, which is 6 ÷ 25.4 = 0.24 in.

Line 6 — the second reverse. A 10-mm key fits an M12 bolt, which is 10 ÷ 25.4 = 0.39 in.

Every branch, checked

M4: the key is 3 mm, which is 3 ÷ 25.4 = 0.12 in. The smallest bolt in the main chart takes the smallest key.

M5: the key is 4 mm, which is 4 ÷ 25.4 = 0.16 in. A 4-mm key is the one most bike tool kits reach for first, and it fits the M5 bolt.

M6: the key is 5 mm, which is 5 ÷ 25.4 = 0.20 in. Check the head before you buy: an M6 button-head screw takes a 4 mm key instead.

M10: the key is 8 mm, which is 8 ÷ 25.4 = 0.31 in. At this size the key is a long-arm key more often than not, because the bolt is tight.

M12: the key is 10 mm, which is 10 ÷ 25.4 = 0.39 in. A 10-mm key is the one that fits the M12 bolt, and it is also the key that turns most engine covers.

M16: the key is 14 mm, which is 14 ÷ 25.4 = 0.55 in. The largest bolt in the main chart takes the largest key in it.

M3: the key is 2.5 mm, which is 2.5 ÷ 25.4 = 0.10 in. The smallest key in the full ISO list, for the smallest socket head cap screw.

M14: the key is 12 mm, which is 12 ÷ 25.4 = 0.47 in. The M14 sits between the M12 and the M16: its 12 mm socket is 2 mm wider than the M12 socket and 2 mm narrower than the M16 socket.

M20: the key is 17 mm, which is 17 ÷ 25.4 = 0.67 in. The largest key in the full ISO list, for heavy machinery bolts.

Reverse — the 6-mm key: a 6-mm key fits an M8 bolt, a 10-mm key fits an M12 bolt, a 5-mm key fits an M6 bolt, and a 3-mm key fits an M4 bolt. Work backwards from the key you already own when the bolt head is out of reach.

What the defaults assume

The chart is the ISO 4762 / DIN 912 hex socket standard: the key size is the distance across the flats of the hex socket in the bolt head, and the inch figures are the millimetres divided by 25.4, rounded to 2 decimals. It assumes a standard ISO socket head cap screw. Button-head and flat-head screws take a smaller key than cap screws of the same size — an M6 button-head screw takes a 4 mm key, not the 5 mm in the chart — so check the head before buying. Some older DIN and JIS fasteners differ by 1 mm, so test-fit before torquing. It is explicitly NOT a torque chart: the torque for a bolt depends on the grade, the thread and the lubrication, not on the key size. It is explicitly NOT an SAE-size chart: inch bolts use inch keys, and a 0.24 in key is not a 1/4 in key. Rounding convention: inches are rounded to 2 decimals, and all figures are computed from the millimetre sizes, rounded only for display.

Output · formula · meaning

Who uses this

• The home mechanic changing a brake caliper: they learn the M8 caliper bolts take a 6 mm Allen key — so they buy the 6 mm before they jack the car up, not after.
• The DIYer assembling flat-pack furniture: they learn the M6 bolts in the kit take a 5 mm Allen key — so they grab the one key, not the whole set.
• The cyclist adjusting a seat post: they learn the M5 clamp bolts take a 4 mm Allen key — so they know the 4 mm is the one that lives in the saddle bag.
• The hobbyist sorting a mixed bolt bin: they learn a 10-mm key fits an M12 bolt — so they sort the bin by key, not by shank, and the chart does the work.

Accuracy & limitations

The chart is exact for the ISO 4762 / DIN 912 standard: the key sizes are the ISO hex socket sizes, and the inch figures are the millimetres divided by 25.4, rounded to 2 decimals. What the chart cannot do is measure your bolt for you: button-head and flat-head screws take a smaller key than cap screws of the same size, so check the bolt head before buying — an M6 button-head screw takes a 4 mm key, not the 5 mm in the chart. Some older DIN and JIS fasteners differ by 1 mm, so test-fit before torquing. It does not cover Torx heads, Phillips heads, or hex-head bolts, which use sockets or screwdrivers, not Allen keys. It is not a torque chart and not an SAE chart: a 0.24 in key is not a 1/4 in key, and the torque for a bolt depends on the grade and the thread, not on the key. For the socket for the same bolt heads, see socket size chart; for the bolt itself, see bolt size chart; for the wrench, see wrench size chart.