Bolt and Nut Wrench Size Chart
The wrench size is the across-flats width of the hex head. An M10 ISO bolt takes a 16 mm wrench (17 mm on older DIN bolts), M12 takes 18 mm (19 mm DIN), and a 1/2 in hex cap screw or nut takes a 3/4 in wrench, or 7/8 in for heavy hex. Find the bolt below for its head, nut and heavy hex sizes.
Wrench sizes by bolt size
Showing 36 of 36 rows
| Size | Bolt head | Hex nut | Heavy hex / DIN | Head in other unit | Other-system wrench that fits |
|---|---|---|---|---|---|
| M3 | 5.5 mm | 5.5 mm | — | 0.217 in | 1/4 in (+0.85 mm) |
| M4 | 7 mm | 7 mm | — | 0.276 in | 5/16 in (+0.94 mm) |
| M5 | 8 mm | 8 mm | — | 0.315 in | 11/32 in (+0.73 mm) |
| M6 | 10 mm | 10 mm | — | 0.394 in | none within 1 mm |
| M8 | 13 mm | 13 mm | — | 0.512 in | none within 1 mm |
| M10 | 16 mm | 16 mm | 17 mm (DIN) | 0.630 in | none within 1 mm |
| M12 | 18 mm | 18 mm | 19 mm (DIN) | 0.709 in | none within 1 mm |
| M14 | 21 mm | 21 mm | 22 mm (DIN) | 0.827 in | none within 1 mm |
| M16 | 24 mm | 24 mm | — | 0.945 in | none within 1 mm |
| M18 | 27 mm | 27 mm | — | 1.063 in | none within 1 mm |
| M20 | 30 mm | 30 mm | — | 1.181 in | 1-3/16 in (+0.16 mm) |
| M22 | 34 mm | 34 mm | 32 mm (DIN) | 1.339 in | 1-3/8 in (+0.92 mm) |
| M24 | 36 mm | 36 mm | — | 1.417 in | 1-7/16 in (+0.51 mm) |
| M27 | 41 mm | 41 mm | — | 1.614 in | 1-5/8 in (+0.27 mm) |
| M30 | 46 mm | 46 mm | — | 1.811 in | 1-13/16 in (+0.04 mm) |
| M33 | 50 mm | 50 mm | — | 1.969 in | 2 in (+0.80 mm) |
| M36 | 55 mm | 55 mm | — | 2.165 in | 2-3/16 in (+0.56 mm) |
| M42 | 65 mm | 65 mm | — | 2.559 in | 2-9/16 in (+0.09 mm) |
| M48 | 75 mm | 75 mm | — | 2.953 in | none within 1 mm |
| #6 | — | 5/16 in | — | 7.94 mm | 8 mm (+0.06 mm) |
| #8 | — | 11/32 in | — | 8.73 mm | 9 mm (+0.27 mm) |
| #10 | — | 3/8 in | — | 9.52 mm | 10 mm (+0.48 mm) |
| 1/4 | 7/16 in | 7/16 in | 1/2 in | 11.11 mm | 12 mm (+0.89 mm) |
| 5/16 | 1/2 in | 1/2 in | 9/16 in | 12.70 mm | 13 mm (+0.30 mm) |
| 3/8 | 9/16 in | 9/16 in | 11/16 in | 14.29 mm | 15 mm (+0.71 mm) |
| 7/16 | 5/8 in | 11/16 in | 3/4 in | 15.88 mm | 16 mm (+0.13 mm) |
| 1/2 | 3/4 in | 3/4 in | 7/8 in | 19.05 mm | 20 mm (+0.95 mm) |
| 9/16 | 13/16 in | 7/8 in | 15/16 in | 20.64 mm | 21 mm (+0.36 mm) |
| 5/8 | 15/16 in | 15/16 in | 1-1/16 in | 23.81 mm | 24 mm (+0.19 mm) |
| 3/4 | 1-1/8 in | 1-1/8 in | 1-1/4 in | 28.57 mm | none within 1 mm |
| 7/8 | 1-5/16 in | 1-5/16 in | 1-7/16 in | 33.34 mm | 34 mm (+0.66 mm) |
| 1 | 1-1/2 in | 1-1/2 in | 1-5/8 in | 38.10 mm | none within 1 mm |
| 1-1/8 | 1-11/16 in | 1-11/16 in | 1-13/16 in | 42.86 mm | none within 1 mm |
| 1-1/4 | 1-7/8 in | 1-7/8 in | 2 in | 47.63 mm | none within 1 mm |
| 1-3/8 | 2-1/16 in | 2-1/16 in | 2-3/16 in | 52.39 mm | none within 1 mm |
| 1-1/2 | 2-1/4 in | 2-1/4 in | 2-3/8 in | 57.15 mm | none within 1 mm |
How to read the chart
Pick Metric or Inch, then find the thread size. "Bolt head" is the hex bolt or cap screw head, "Hex nut" the standard nut, and the fourth column the heavy hex size (inch) or the older DIN size where it differs from ISO (metric).
Metric bolts changed size at M10, M12, M14 and M22 when ISO replaced DIN. Both are still on the market, so check the head before you reach for a wrench.
The last column is the smallest wrench from the other system that is not smaller than the head, with the play it leaves. Play of a few tenths of a millimeter is common; more than that and the wrench is likely to round the corners. Where no wrench fits within 1 mm, the column says so: use the matching system.
Tightening to a torque value? Convert units with the torque conversion chart and work out preload with the bolt torque calculator.
Where the sizes come from
Across-flats sizes are the basic values in ISO 4014 and ISO 4017 (hex head bolts and screws), ISO 4032 (hex nuts), DIN 931, 933 and 934 (the older metric sizes), ASME B18.2.1 (hex cap screws and heavy hex bolts), ASME B18.2.2 (hex and heavy hex nuts) and ASME B18.6.3 (machine screw nuts, #6 to #10).
The inch or millimeter equivalent is the across-flats size × or ÷ 25.4. The cross-system wrench is the smallest size in a common wrench set (1/16 in steps to 3 in plus 11/32, 19/32 and 25/32 in; whole millimeters plus 5.5 mm) that is at least as wide as the head, if one leaves no more than 1 mm of play.
Example: Which metric wrench fits a 1/2 in hex cap screw?
- A 1/2 in hex cap screw has a 3/4 in across-flats head (ASME B18.2.1).
- 3/4 in × 25.4 = 19.05 mm.
- 19 mm is smaller than 19.05 mm, so the smallest metric wrench that fits is 20 mm, leaving 0.95 mm of play.
Result: Use a 3/4 in wrench. A 19 mm wrench is nominally 0.05 mm too small, and 20 mm leaves almost 1 mm of play.
Common mistakes
- Assuming every M10 head is 17 mm. ISO M10 bolts are 16 mm; 17 mm is the older DIN size. The same split applies at M12, M14 and M22.
- Using a metric wrench on an inch head "because it is close". Check the play in the last column. Too much play rounds the corners, especially at high torque.
- Taking the nut size from the bolt head. Inch hex nuts differ from cap screw heads at 7/16 and 9/16 in, and heavy hex nuts are bigger again.
- Confusing wrench size with thread size. An M10 bolt has a 10 mm thread but a 16 mm head; a 1/2 in bolt has a 3/4 in head.
- Calling out a socket without the drive. On work instructions, state the socket size, the drive (1/4, 3/8, 1/2 in) and the torque, not just the bolt size.
Printable chart
The PDF prints on US Letter or A4. The images are made from the same values as the table above: a full chart, and a 2:3 portrait version for pinning.
Frequently asked questions
What size wrench fits an M8 bolt?
13 mm, for both ISO and DIN M8 bolts and nuts.
What size wrench for an M10 bolt: 16 or 17 mm?
16 mm for ISO 4014/4017 bolts and ISO 4032 nuts; 17 mm for DIN 931/933 bolts and DIN 934 nuts. Both are still sold.
What size socket for a 1/2 inch bolt?
3/4 in for a standard hex cap screw or hex nut, 7/8 in for a heavy hex bolt or nut.
Is a 19 mm socket the same as 3/4 inch?
Close but not the same: 3/4 in is 19.05 mm. A 19 mm socket is nominally slightly small for a 3/4 in hex.
What wrench fits a 3/8 inch bolt?
9/16 in for both the hex cap screw head and the hex nut; 11/16 in for a heavy hex nut.
See also
More reference charts
Sources
- ISO 4014 and ISO 4017, Hexagon head bolts and screws, ISO
- ISO 4032, Hexagon regular nuts (style 1), ISO
- DIN 931, DIN 933 and DIN 934 (withdrawn; superseded by the ISO standards), DIN
- ASME B18.2.1, Square, Hex, Heavy Hex, and Askew Head Bolts and Hex, Heavy Hex, Hex Flange, Lobed Head, and Lag Screws, ASME
- ASME B18.2.2, Nuts for General Applications, ASME
- ASME B18.6.3, Machine Screws, Tapping Screws, and Metallic Drive Screws, ASME
COPLAIN AI PLATFORM
Generate yours automatically from your existing docs
Coplain writes assembly job aids that name the right socket, drive and torque for every fastener, so nobody has to hunt for a wrench size on the line.
Generate yours automatically with Coplain →Related Tools & Templates