Metric Hex Bolt, Nut and Washer Dimensions
Wrench flats, head height, nut thickness and washer sizes for metric bolts from M3 to M36.
Hex bolts, nuts and plain washers come in standard sizes, so a design can be laid out before any parts are bought. This table lists the main nominal dimensions for each coarse-thread size: the width across the hex flats (the wrench size), the height of the bolt head, the thickness of a standard nut, and the hole, outside diameter and thickness of a normal-series washer.
The table
| Thread | Coarse pitch (mm) | Hex across flats (mm) | Hex across flats (in) | Bolt head height k (mm) | Nut thickness m (mm) | Washer hole (mm) | Washer outside diameter (mm) | Washer thickness (mm) |
|---|---|---|---|---|---|---|---|---|
| M3 | 0.5 | 5.5 | 0.217 | 2 | 2.4 | 3.2 | 7 | 0.5 |
| M4 | 0.7 | 7 | 0.276 | 2.8 | 3.2 | 4.3 | 9 | 0.8 |
| M5 | 0.8 | 8 | 0.315 | 3.5 | 4.7 | 5.3 | 10 | 1 |
| M6 | 1 | 10 | 0.394 | 4 | 5.2 | 6.4 | 12 | 1.6 |
| M8 | 1.25 | 13 | 0.512 | 5.3 | 6.8 | 8.4 | 16 | 1.6 |
| M10 | 1.5 | 16 | 0.63 | 6.4 | 8.4 | 10.5 | 20 | 2 |
| M12 | 1.75 | 18 | 0.709 | 7.5 | 10.8 | 13 | 24 | 2.5 |
| M14 | 2 | 21 | 0.827 | 8.8 | 12.8 | 15 | 28 | 2.5 |
| M16 | 2 | 24 | 0.945 | 10 | 14.8 | 17 | 30 | 3 |
| M20 | 2.5 | 30 | 1.181 | 12.5 | 18 | 21 | 37 | 3 |
| M24 | 3 | 36 | 1.417 | 15 | 21.5 | 25 | 44 | 4 |
| M30 | 3.5 | 46 | 1.811 | 18.7 | 25.6 | 31 | 56 | 4 |
| M36 | 4 | 55 | 2.165 | 22.5 | 31 | 37 | 66 | 5 |
Source and notes: ISO 4017 (hex head bolts, product grade A and B), ISO 4032 (hex nuts, style 1) and ISO 7089 (plain washers, normal series). Dimensions are nominal. DIN 931 and DIN 933 bolts and DIN 934 nuts use larger flats for M10 (17 mm), M12 (19 mm), M14 (22 mm) and M22; the ISO flats are listed.
How to use this table
- Find the thread size in the first column.
- Use the across flats value for the wrench, and for the space needed around the bolt head or nut.
- Add the head height, nut thickness and washer thickness when working out the bolt length and the grip.
- Check the washer outside diameter against the room available on the surface.
Worked example: an M12 bolt through two 10 mm plates, with a washer under the nut
- Plates 20 mm + washer 2.5 mm + nut 10.8 mm = 33.3 mm.
- The bolt should stick out at least two pitches beyond the nut: 2 × 1.75 = 3.5 mm, so the minimum length is 36.8 mm.
- Standard lengths run in steps of 5 mm in this range (35, 40, 45 mm), so choose M12 × 40. The wrench size is 18 mm.
Notes and cautions
- ISO and DIN flats differ for M10 (16 and 17 mm), M12 (18 and 19 mm) and M14 (21 and 22 mm). The table gives the ISO figures; a 17 mm wrench is a loose fit on an ISO M10 head, so keep both sizes in the toolbox.
- Washers come in other series (light, heavy, large-diameter “fender”). The normal series is listed here.
- Nut thickness is for style 1 hex nuts; thin (jam) nuts and heavy nuts are different.
Related tools and guides
- Hex Bolt Wrench Size Chart
- Metric Bolt Clearance Hole Sizes
- Metric Thread Table (Coarse)
- Bolted and Welded Joints (guide)
- All reference tables
FAQ
What size wrench for an M10 bolt?
16 mm for ISO hardware (the standard now), 17 mm for older DIN hardware.
How long should a bolt stick out past the nut?
At least two full threads (two pitches) past the nut face, so the nut is fully engaged. More is fine, but a long tail can get in the way.
What is a plain washer for?
It spreads the load under the head or nut so it does not dig into the surface, and it protects the surface when the nut is turned.
Sources
- ISO 4017 (hex head bolts), ISO 4032 (hex nuts, style 1), ISO 7089 (plain washers, normal series); DIN 931, 933 and 934 for the older flats.
- ISO 4017 (hex head bolts, product grade A and B), ISO 4032 (hex nuts, style 1) and ISO 7089 (plain washers, normal series). Dimensions are nominal. DIN 931 and DIN 933 bolts and DIN 934 nuts use larger flats for M10 (17 mm), M12 (19 mm), M14 (22 mm) and M22; the ISO flats are listed.