What size Allen key fits an M8 socket head cap screw?

A 6 mm hex key (DIN 912 / ISO 4762).

What hex key goes with each metric socket head cap screw?

M2 – 1.5 mm, M2.5 – 2 mm, M3 – 2.5 mm, M4 – 3 mm, M5 – 4 mm, M6 – 5 mm, M8 – 6 mm, M10 – 8 mm, M12 – 10 mm, M14 – 12 mm, M16 – 14 mm, M20 – 17 mm, M24 – 19 mm, M30 – 22 mm, M36 – 27 mm, M42 – 32 mm, M48 – 36 mm.

How tall is the head on an M10 socket head cap screw?

10 mm — the head height equals the screw size.

How long is the thread on an M12 socket head cap screw?

36 mm (reference); shorter screws are threaded close to the head.

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Metric Socket Head Cap Screw Dimensions

Hex key (Allen wrench) size, head height and thread length for metric socket head cap screws, M2 to M48. DIN 912 / ISO 4762.

A metric socket head cap screw is sized by its thread — M10 is 10 mm across the thread — and measured from under the head to the end, in millimetres. The head is as tall as the screw is wide: an M10 has a 10 mm head. It takes a hex key (Allen wrench) in the socket.

Letters match the table columns: s hex key (socket, dashed), k head height, b thread length, L length.

Metric socket head cap screws — DIN 912 / ISO 4762 (mm)
Size	Hex key s	Head height k max	Thread length b (ref)
M2	1.5 mm	2	16
M2.5	2 mm	2.5	17
M3	2.5 mm	3	18
M4	3 mm	4	20
M5	4 mm	5	22
M6	5 mm	6	24
M8	6 mm	8	28
M10	8 mm	10	32
M12	10 mm	12	36
M14	12 mm	14	40
M16	14 mm	16	44
M20	17 mm	20	52
M24	19 mm	24	60
M30	22 mm	30	72
M36	27 mm	36	84
M42	32 mm	42	96
M48	36 mm	48	108

Thread length is a reference for longer screws; a screw shorter than about b is threaded close to the head. Strength by class (8.8, 10.9, 12.9): ISO 898-1.

Low head socket cap screws — DIN 7984 (mm)
Size	Hex key s	Head dia dk max	Head height k max	Thread length b (ref)
M3	2 mm	5.5	2	12
M4	2.5 mm	7	2.8	14
M5	3 mm	8.5	—	16
M6	4 mm	10	4	18
M8	5 mm	13	5	22
M10	7 mm	16	6	26
M12	8 mm	18	7	30
M14	10 mm	21	8	34
M16	12 mm	24	9	38
M20	14 mm	30	11	46

A lower head than DIN 912, for tight spots — and a smaller hex key at the same size. A dash: our sources differ — give us a call.

Low head socket cap screws with pilot recess — DIN 6912 (mm)
Size	Hex key s	Head dia dk max	Head height k max	Thread length b (ref)
M4	3 mm	7	2.8	14
M5	4 mm	8.5	3.5	16
M6	5 mm	10	4	18
M8	6 mm	13	5	22
M10	8 mm	16	6.5	26
M12	10 mm	18	7.5	30
M14	12 mm	21	8.5	34
M16	14 mm	24	10	38

Also a low head, but with a pilot recess and a key one size bigger than DIN 7984 at most sizes — the two are not interchangeable on the key.

Good to know

Metric hex keys are sized in millimetres. Inch socket head screws are on the socket head cap screw chart; metric thread pitches on the metric thread pitch chart.

Quick answers

What size Allen key fits an M8 socket head cap screw?: A 6 mm hex key (DIN 912 / ISO 4762).
What hex key goes with each metric socket head cap screw?: M2 – 1.5 mm, M2.5 – 2 mm, M3 – 2.5 mm, M4 – 3 mm, M5 – 4 mm, M6 – 5 mm, M8 – 6 mm, M10 – 8 mm, M12 – 10 mm, M14 – 12 mm, M16 – 14 mm, M20 – 17 mm, M24 – 19 mm, M30 – 22 mm, M36 – 27 mm, M42 – 32 mm, M48 – 36 mm.
How tall is the head on an M10 socket head cap screw?: 10 mm — the head height equals the screw size.
How long is the thread on an M12 socket head cap screw?: 36 mm (reference); shorter screws are threaded close to the head.

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