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Socket size chart for inch and metric bolts

A 1/2 in hex cap screw takes a 3/4 in socket, but a 1/2 in heavy hex structural bolt takes 7/8 in. An M12 bolt takes 18 mm to ISO 4017 or 19 mm to the older DIN 933. Metric heads differ between the two standards at M10, M12, M14 and M22.

By the Torque Reference editors 2 sources cited Last reviewed Method
Contents7 sections

Key takeaways

  • From 1/2 in up, the inch hex cap screw socket is 1.5 times the bolt diameter ⁠1.
  • Heavy hex structural bolts take a socket 1/8 in larger than a hex cap screw of the same size ⁠1 ⁠2.
  • ISO vs DIN: M10 16 or 17 mm, M12 18 or 19 mm, M14 21 or 22 mm, M22 34 or 32 mm ⁠1.
  • The nut can need a different socket from the bolt head: 7/16 and 9/16 in, and the DIN/ISO nut sizes.

Socket size for inch bolts

BoltHex cap screwHeavy hex / structural
1/4 in7/16 in–
5/16 in1/2 in–
3/8 in9/16 in–
7/16 in5/8 in–
1/2 in3/4 in7/8 in
9/16 in13/16 in–
5/8 in15/16 in1-1/16 in
3/4 in1-1/8 in1-1/4 in
7/8 in1-5/16 in1-7/16 in
1 in1-1/2 in1-5/8 in

Widths across flats from ASME B18.2.1 as tabulated by Fastenal⁠1; heavy hex structural per RCSC⁠2.

Socket size for metric bolts

BoltISO 4014 / 4017DIN 931 / 933
M35.5 mm5.5 mm
M47 mm7 mm
M58 mm8 mm
M610 mm10 mm
M813 mm13 mm
M1016 mm17 mm differs
M1218 mm19 mm differs
M1421 mm22 mm differs
M1624 mm24 mm
M1827 mm27 mm
M2030 mm30 mm
M2234 mm32 mm differs
M2436 mm36 mm
M2741 mm41 mm
M3046 mm46 mm

ISO and DIN heads differ at M10, M12, M14 and M22⁠1.

Inch hex cap screws and nuts

From 1/2 in up, the width across flats of a hex cap screw is one and a half times the diameter: 3/4 in for 1/2 in, 15/16 for 5/8, 1-1/8 for 3/4, 1-1/2 for 1 in ⁠1. Smaller sizes run proportionally larger, with 7/16 in for a 1/4 in bolt.

The nut does not always match the head. A standard hex nut takes 11/16 in on a 7/16 in bolt, where the cap screw takes 5/8. On a 9/16 in bolt, the nut takes 7/8 in and the cap screw 13/16 ⁠1. In those two sizes you need two different sockets for one joint.

Heavy hex structural bolts

Heavy hex structural bolts, the ASTM F3125 A325 and A490 types, are 1/8 in larger across the flats than a hex cap screw of the same size. They take 7/8 in at 1/2 in, 1-1/16 at 5/8, 1-1/4 at 3/4, 1-7/16 at 7/8 and 1-5/8 at 1 in ⁠1 ⁠2. Heavy hex nuts follow the same widths in these sizes ⁠1. A socket that fits a hardware-store 3/4 in bolt (1-1/8 in) will not fit a 3/4 in structural bolt. Structural bolts are also tightened by RCSC methods, not table torque ⁠2. See turn-of-nut.

ISO vs DIN metric heads

Older DIN 931/933 bolts and current ISO 4014/4017 bolts differ across the flats at four sizes ⁠1:

Metric sizes where ISO and DIN hex heads differ
Bolt ISO 4014 / 4017 DIN 931 / 933
M10 16 mm 17 mm
M12 18 mm 19 mm
M14 21 mm 22 mm
M22 34 mm 32 mm

An M12 bolt can therefore need either an 18 mm or a 19 mm socket. Nuts follow the same split at M10 to M14: DIN 934 nuts are 17, 19 and 22 mm, and ISO 4032 nuts are 16, 18 and 21 mm ⁠1. Every other size from M3 to M30 is the same in both.

Do not force a near-fit inch socket onto a metric head. A 3/4 in socket is 19.05 mm, loose on a 19 mm DIN head. A loose socket rounds the corners, and the torque reading means little once it slips.

Sources

  1. 1
  2. 2

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