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Socket Size Chart: What Size Socket for Each Bolt?

An M12 bolt takes an 18 mm socket, which is 0.71 in across the flats. The socket size is the distance across the flats of the hex head, not the bolt diameter: an M12 bolt has an 18 mm head. Enter any metric bolt size to find its socket, or enter a socket size to find the bolt it fits.

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Socket size — metric bolt to socket lookup

The socket that fits a bolt is the size of the bolt head, not the bolt itself. A metric bolt is named by its shank diameter: an M12 bolt has a 12 mm shank. The hex head on top of it is wider, and the socket fits that head: an M12 bolt has an 18 mm head, so it takes an 18 mm socket. This chart lists the ISO across-flats head sizes for metric hex bolts from M6 to M20, in millimetres and in inches, and it works backwards too: enter a socket size to find the bolt it fits.

Bolt size (M number)

The metric bolt size, as the M number on the bolt head. The chart covers M6, M8, M10, M12, M14, M16 and M20. The default is M12.

Reverse: socket size (mm)

A socket size in millimetres, to work out which bolt it fits. The chart covers 10, 13, 16, 18, 21, 24 and 30 mm. The default is 13 mm, which fits an M8 bolt.

Socket size for your bolt18 mm socket (0.71 in)An M12 bolt takes an 18 mm socket, which is 0.71 in across the flats. That is row 4 of the chart. A 13-mm socket fits an M8 bolt (0.51 in).

Bolt: M12

Socket: 18 mm

Socket in inches: 0.71 in

Chart row: Row 4 of 7

M6: M6 → 10 mm (0.39 in)

M8: M8 → 13 mm (0.51 in)

M10: M10 → 16 mm (0.63 in)

M16: M16 → 24 mm (0.94 in)

M14: M14 → 21 mm (0.83 in)

Reverse, 13-mm socket: A 13-mm socket fits an M8 bolt (0.51 in).

Reverse, 24-mm socket: A 24-mm socket fits an M16 bolt (0.94 in).

An M12 bolt takes an 18 mm socket. That is 0.71 in across the flats, which is row 4 of the chart. A 13-mm socket fits an M8 bolt (0.51 in).

M6 → 10 mm socket (0.39 in).

M8 → 13 mm socket (0.51 in).

M10 → 16 mm socket (0.63 in).

M16 → 24 mm socket (0.94 in).

M14 → 21 mm socket (0.83 in).

Reverse: A 13-mm socket fits an M8 bolt (0.51 in). A 24-mm socket fits an M16 bolt (0.94 in).

How the socket size chart works

A metric bolt is named by its shank diameter: an M12 bolt has a 12 mm shank. The socket does not fit the shank; it fits the hex head on top of it, and the head is wider than the shank. The socket size is the distance across the flats of that hex head, measured from one flat side to the opposite flat side. For an M12 bolt the head is 18 mm across the flats, so the socket is 18 mm. The inch figures are the millimetres divided by 25.4, rounded to 2 decimals: 18 ÷ 25.4 = 0.71 in.

At the default — M12 — the socket is 18 mm, which is 18 ÷ 25.4 = 0.71 in across the flats. That is row 4 of the chart. The reverse lookup works the other way: a 13-mm socket fits an M8 bolt, which is 13 ÷ 25.4 = 0.51 in.

Bolt	Socket (mm)	Socket (in)
M6	10 mm	0.39 in
M8	13 mm	0.51 in
M10	16 mm	0.63 in
M12	18 mm	0.71 in
M14	21 mm	0.83 in
M16	24 mm	0.94 in
M20	30 mm	1.18 in

The worked example, line by line

Setup: M12 bolt; reverse socket 13 mm.

Line 1 — the bolt head. An M12 bolt has a 12 mm shank and an 18 mm hex head, measured across the flats.

Line 2 — the socket in millimetres. The socket matches the head, so it is 18 mm.

Line 3 — the socket in inches. 18 ÷ 25.4 = 0.71 in.

Line 4 — the chart row. M12 is row 4 of the 7-row chart.

Line 5 — the reverse lookup. A 13-mm socket fits an M8 bolt, which is 13 ÷ 25.4 = 0.51 in.

Line 6 — the second reverse. A 24-mm socket fits an M16 bolt, which is 24 ÷ 25.4 = 0.94 in.

Every branch, checked

M6: the socket is 10 mm, which is 10 ÷ 25.4 = 0.39 in. The smallest bolt in the chart takes the smallest socket.

M8: the socket is 13 mm, which is 13 ÷ 25.4 = 0.51 in. A 13-mm socket is the one most toolboxes reach for first, and it fits the M8 bolt.

M10: the socket is 16 mm, which is 16 ÷ 25.4 = 0.63 in. Check the head before you buy: some older DIN and JIS M10 fasteners use a 15 mm head instead.

M16: the socket is 24 mm, which is 24 ÷ 25.4 = 0.94 in. At this size the socket is a deep socket more often than not, because the bolt is long.

M14: the socket is 21 mm, which is 21 ÷ 25.4 = 0.83 in. The M14 sits between the M12 and the M16: its 21 mm head is 3 mm wider than the M12 head and 3 mm narrower than the M16 head.

M20: the socket is 30 mm, which is 30 ÷ 25.4 = 1.18 in. The largest bolt in the chart takes the largest socket, and the inch figure passes 1 in for the first time.

Reverse — the 13-mm socket: a 13-mm socket fits an M8 bolt, and a 24-mm socket fits an M16 bolt. Work backwards from the socket you already own when the bolt head is out of reach.

What the defaults assume

The chart is the ISO metric hex-head across-flats standard: the socket size is the distance across the flats of the hex head, and the inch figures are the millimetres divided by 25.4, rounded to 2 decimals. It assumes a standard ISO hex bolt or nut. Some older DIN and JIS fasteners differ — some M10 use a 15 mm head, and some older M12 use a 17 mm head — so check the bolt head before buying. It is explicitly NOT a torque-spec chart: the torque for a bolt depends on the grade, the thread and the lubrication, not on the socket size. It is explicitly NOT an SAE-size chart: inch bolts use inch sockets, and a 0.71 in socket is not a 3/4 in socket. Rounding convention: inches are rounded to 2 decimals, and all figures are computed from the millimetre sizes, rounded only for display.

Output · formula · meaning

Output	Formula	What it means
Socket (mm)	ISO across-flats lookup	18 mm for an M12 bolt.
Socket (in)	mm ÷ 25.4	0.71 in for an M12 bolt.
M6	same chart	M6 → 10 mm (0.39 in).
M8	same chart	M8 → 13 mm (0.51 in).
M10	same chart	M10 → 16 mm (0.63 in).
M16	same chart	M16 → 24 mm (0.94 in).
M14	same chart	M14 → 21 mm (0.83 in).
Reverse: 13-mm socket	socket → bolt lookup	A 13-mm socket fits an M8 bolt (0.51 in).
Reverse: 24-mm socket	socket → bolt lookup	A 24-mm socket fits an M16 bolt (0.94 in).

Who uses this

The home mechanic changing a brake caliper: they learn the M12 caliper bolt takes an 18 mm socket — so they buy the 18 mm before they jack the car up, not after.
The DIYer assembling flat-pack furniture: they learn the M8 bolts in the kit take a 13 mm socket — so they grab the one socket, not the whole set.
The renter hanging a wall bracket: they learn the M6 lag bolts take a 10 mm socket — so they know the 10 mm is the one that always goes missing.
The hobbyist sorting a mixed bolt bin: they learn a 24-mm socket fits an M16 bolt — so they sort the bin by socket, not by shank, and the chart does the work.

Accuracy & limitations

The chart is exact for the ISO standard: the socket sizes are the ISO across-flats head 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: some older DIN and JIS fasteners differ, so check the bolt head before buying — an M10 with a 15 mm head needs a 15 mm socket, not the 16 mm in the chart. It does not cover hex socket (Allen) heads, Torx heads, or carriage bolts, which use different sizes. It is not a torque chart and not an SAE chart: a 0.71 in socket is not a 3/4 in socket, and the torque for a bolt depends on the grade and the thread, not on the socket. For the bolt itself, see bolt size chart; for the torque, see bolt torque chart; for the wrench, see wrench size chart.

Cite this reference tableBright Side Kit. “Socket Size Chart: What Size Socket for Each Bolt?” brightoolbox.com, 2026. Method: ISO metric hex-head across-flats standard; socket mm is the across-flats head size for the bolt; socket in = mm ÷ 25.4, rounded to 2 decimals. Rows: M6 → 10 mm (0.39 in); M8 → 13 mm (0.51 in); M10 → 16 mm (0.63 in); M12 → 18 mm (0.71 in); M14 → 21 mm (0.83 in); M16 → 24 mm (0.94 in); M20 → 30 mm (1.18 in). Reverse: 13-mm socket → M8 bolt; 24-mm socket → M16 bolt. Note: some older DIN and JIS fasteners differ (some M10 use 15 mm) — check the bolt head before buying. This is not a torque-spec chart and not an SAE-size chart.

Bright Side Kit. "Socket Size Chart: What Size Socket for Each Bolt?" https://brightoolbox.com/socket-size-chart/ Accessed October 24, 2026.

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Frequently asked questions

What size socket do I need for an M12 bolt?

18 mm, which is 0.71 in. An M12 bolt has an 18 mm hex head, so it takes an 18 mm socket. That is row 4 of the chart.

What size socket do I need for an M10 bolt?

16 mm, which is 0.63 in. An M10 bolt has a 16 mm hex head. Check the head before you buy: some older DIN and JIS M10 fasteners use a 15 mm head instead.

What size socket do I need for an M8 bolt?

13 mm, which is 0.51 in. An M8 bolt has a 13 mm hex head, so it takes a 13 mm socket. That is row 2 of the chart.

What size socket do I need for an M6 bolt?

10 mm, which is 0.39 in. An M6 bolt has a 10 mm hex head, so it takes a 10 mm socket. That is the smallest socket in the chart.

What size socket do I need for an M20 bolt?

30 mm, which is 1.18 in. An M20 bolt has a 30 mm hex head, so it takes a 30 mm socket. That is the largest socket in the chart, and the first one over 1 in.

I have a 13-mm socket — what bolt does it fit?

An M8 bolt. A 13-mm socket fits an M8 bolt, which is 0.51 in across the flats. A 24-mm socket fits an M16 bolt.

Is the socket size the same as the bolt size?

No. The bolt size is the shank diameter and the socket size is the head size, and the head is wider than the shank: an M12 bolt has a 12 mm shank and an 18 mm head. The socket matches the head, not the shank.

Are these sizes the same for old bolts?

Mostly, but not always. The chart is the ISO standard, and most modern bolts follow it. Some older DIN and JIS fasteners differ — some M10 use a 15 mm head and some older M12 use a 17 mm head — so check the bolt head before buying.