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Wrench Size Chart for Bolts and Nuts

Guide · Updated October 28, 2026

The wrench size is the width across the flats of the hex. For inch fasteners it comes from ASME B18.2.1 (bolts and cap screws) and B18.2.2 (nuts). For most sizes the screw and nut use the same wrench, but at 7/16 and 9/16 they do not. Heavy hex bolts and nuts are 1/8 in larger across the flats than standard hex. Metric hex sizes changed when ISO replaced DIN for M10, M12, M14 and M22, so two wrenches can be right for the same metric bolt.

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Step by step

Identify the fastener: Measure the thread diameter and note whether it is a cap screw, nut or heavy hex.
Read the wrench size: Use the inch or metric chart below.
Check for the exceptions: 7/16 and 9/16 nuts are bigger than the screws; M10 and M12 can be ISO or DIN.
Choose a six-point socket: A six-point grips the flats and is less likely to round a tight or worn head.
Torque to spec: Use a torque wrench and the right torque for the grade and lubrication.

Inch hex cap screws and nuts

1/4: screw 7/16, nut 7/16, heavy hex nut 1/2
5/16: 1/2, 1/2, heavy nut 9/16
3/8: 9/16, 9/16, heavy nut 11/16
7/16: screw 5/8, nut 11/16, heavy nut 3/4
1/2: 3/4, 3/4, heavy hex 7/8
9/16: screw 13/16, nut 7/8, heavy hex 15/16
5/8: 15/16, 15/16, heavy hex 1-1/16
3/4: 1-1/8, 1-1/8, heavy hex 1-1/4
7/8: 1-5/16, 1-5/16, heavy hex 1-7/16
1: 1-1/2, 1-1/2, heavy hex 1-5/8
1-1/4: 1-7/8, 1-7/8, heavy hex 2
1-1/2: 2-1/4, 2-1/4, heavy hex 2-3/8

From 1/2 in up, standard hex across flats is 1.5 × D. Heavy hex is 1.5 × D + 1/8 in. Structural A325 and A490 bolts use heavy hex heads.

Metric hex bolts and nuts (ISO vs DIN)

M3: 5.5 mm
M4: 7 mm
M5: 8 mm
M6: 10 mm
M8: 13 mm
M10: 16 mm ISO, 17 mm DIN
M12: 18 mm ISO, 19 mm DIN
M14: 21 mm ISO, 22 mm DIN
M16: 24 mm
M20: 30 mm
M22: 34 mm ISO, 32 mm DIN
M24: 36 mm
M30: 46 mm

Automotive and Japanese fasteners sometimes use smaller flange heads: an M6 small series flange bolt has an 8 mm hex, and many M8 flange bolts take 12 mm.

Hex keys for socket screws

Socket head cap screws: #10 5/32, 1/4 3/16, 5/16 1/4, 3/8 5/16, 1/2 3/8 in; M5 4, M6 5, M8 6, M10 8, M12 10 mm. Set screws take smaller keys, for example 1/8 in for a 1/4 set screw and 3 mm for an M6 set screw.

Worked example: across corners and socket clearance

A 1/2 in hex cap screw is 3/4 in across flats. Across corners = 1.1547 × 0.750 = 0.866 in, which matches the B18.2.1 maximum. A socket wall adds about 1/8 in on each side, so leave about 1.125 in clearance around the head in a counterbore or next to a wall.

Worked example: mixed fasteners

You pull an M10 bolt and the 16 mm wrench is loose: it is probably a DIN 933 bolt with a 17 mm head. An inch 5/8 wrench (15.9 mm) is close to 16 mm and slips on a 16 mm head. For a 7/16 bolt, bring both 5/8 and 11/16 wrenches, because the nut is 1/16 bigger.

Common problems

Using a 12-point socket on a rusted head, using an inch wrench that is close to a metric size, ignoring the 7/16 and 9/16 nut exception, and rounding heads with open-end wrenches. Mic the flats if in doubt.

FAQ

What wrench fits a 3/8 bolt?

9/16 in for both the hex cap screw and the hex nut.

What wrench fits an M10 bolt?

16 mm for ISO, 17 mm for older DIN bolts.

What wrench fits a 1/2 inch bolt?

3/4 in; heavy hex takes 7/8 in.

How do I check a wrench opening?

Check the gage itself before trusting a measurement: gage blocks set mics, calipers and indicators; build any stack free at gageblockcalc.com.

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General shop information only — follow your drawing, tool maker's data and your quality system.