This guide gives the main metric hex bolt dimensions from M5 to M36: head height, across corners, bearing face, thread length and clearance holes. The values, in mm, follow ISO 4014 (partly threaded) and ISO 4017 (fully threaded). For spanner sizes, see the spanner size chart.
| Thread | Head height k | Across corners e, grade A | Across corners e, grade B | Bearing face dw, grade A | Bearing face dw, grade B |
|---|---|---|---|---|---|
| M5 | 3.5 | 8.79 | 8.63 | 7.20 | – |
| M6 | 4 | 11.05 | 10.89 | 8.88 | 8.74 |
| M8 | 5.3 | 14.38 | 14.20 | 11.63 | 11.47 |
| M10 | 6.4 | 17.77 | 17.59 | 14.63 | 14.47 |
| M12 | 7.5 | 20.03 | 19.85 | 16.63 | 16.47 |
| M14 | 8.8 | 23.36 | 22.78 | 19.64 | 19.15 |
| M16 | 10 | 26.75 | 26.17 | 22.49 | 22.00 |
| M18 | 11.5 | 30.14 | 29.56 | 25.34 | 24.85 |
| M20 | 12.5 | 33.53 | 32.95 | 28.19 | 27.70 |
| M22 | 14 | 37.72 | 37.29 | 31.71 | 31.35 |
| M24 | 15 | 39.98 | 39.55 | 33.61 | 33.25 |
| M27 | 17 | – | 45.20 | – | 38.00 |
| M30 | 18.7 | – | 50.85 | – | 42.75 |
| M33 | 21 | – | 55.37 | – | 46.55 |
| M36 | 22.5 | – | 60.79 | – | 51.11 |
How to read the chart: k is the nominal head height. e and dw are minimum values. Grade A covers M5 to M24 bolts up to 10 times the diameter or 150 mm long, whichever is shorter. Other bolts are grade B, so grade A shows a dash from M27. The 2022 editions raised the M5 bearing face: grade A from 6.88 to 7.20 mm. The new M5 grade B value is not given here.
Head height tolerance: grade A allows plus or minus 0.15 mm on M5 to M8, 0.18 mm on M10 to M16 and 0.215 mm on M18 to M24. So an M12 head is 7.32 to 7.68 mm high. Grade B allows 0.24 to 0.42 mm either way.
Across corners: a sharp hexagon measures 1.1547 times its across-flats size, so 20.78 mm for an 18 mm M12 head. Real heads may be slightly small across flats, with rounded corners, so the standard gives a minimum: 20.03 mm for M12 grade A.
A half thread bolt to ISO 4014 or DIN 931 has a thread length b that depends on the diameter d and the bolt length:
| Thread | Length up to 125 mm | Over 125 up to 200 mm | Over 200 mm |
|---|---|---|---|
| M5 | 16 | 22 | 35 |
| M6 | 18 | 24 | 37 |
| M8 | 22 | 28 | 41 |
| M10 | 26 | 32 | 45 |
| M12 | 30 | 36 | 49 |
| M14 | 34 | 40 | 53 |
| M16 | 38 | 44 | 57 |
| M18 | 42 | 48 | 61 |
| M20 | 46 | 52 | 65 |
| M22 | 50 | 56 | 69 |
| M24 | 54 | 60 | 73 |
| M27 | 60 | 66 | 79 |
| M30 | 66 | 72 | 85 |
| M33 | 72 | 78 | 91 |
| M36 | 78 | 84 | 97 |
How to read the chart: on an M12 x 100 bolt b is 30 mm, so the grip length, from under the head to the first full thread, is at most 100 minus 30 = 70 mm. Up to 2 pitches of thread at the tip may be incomplete. ISO 4014 standard lengths stop at 10 times the diameter, such as 50 mm for M5 and 120 mm for M12. Longer bolts use the same rule.
Full thread bolts (ISO 4017) are threaded to within 3 pitches of the head: 4.5 mm on M10. Pitches are in the thread pitch chart.
| Thread | Fine (close) | Medium (normal) | Coarse (loose) |
|---|---|---|---|
| M5 | 5.3 | 5.5 | 5.8 |
| M6 | 6.4 | 6.6 | 7 |
| M8 | 8.4 | 9 | 10 |
| M10 | 10.5 | 11 | 12 |
| M12 | 13 | 13.5 | 14.5 |
| M14 | 15 | 15.5 | 16.5 |
| M16 | 17 | 17.5 | 18.5 |
| M18 | 19 | 20 | 21 |
| M20 | 21 | 22 | 24 |
| M22 | 23 | 24 | 26 |
| M24 | 25 | 26 | 28 |
| M27 | 28 | 30 | 32 |
| M30 | 31 | 33 | 35 |
| M33 | 34 | 36 | 38 |
| M36 | 37 | 39 | 42 |
How to read the chart: hole diameters from ISO 273, adopted in India as IS 1821.
The fillet under the head can be wider than a fine or medium hole: up to 13.7 mm on M12, against a 13.5 mm medium hole. If the head does not sit flat, chamfer the hole edge slightly, as ISO 273 recommends. Structural steel codes set their own hole sizes, so check the drawing.
The bearing face is the ring under the head that presses on the part, and dw is its smallest allowed diameter. ISO heads have a thin raised washer face here: 0.15 to 0.5 mm high on M5 and M6, 0.15 to 0.6 mm on M8 to M14 and 0.2 to 0.8 mm on M16 to M36.
Bolt length is measured from under the head to the end. Grade A allows plus or minus 0.42 mm at 20 to 30 mm long, rising to 0.8 mm at 130 to 150 mm. Grade B allows plus or minus 1.5 mm at 55 to 80 mm, 1.75 mm at 100 mm and 2.3 mm at 200 to 250 mm.
ISO 4014 and ISO 4017 replaced the older DIN 931 and DIN 933. The nominal head height, thread length and clearance holes are the same. DIN heads are 1 mm bigger across flats in M10, M12 and M14 and 2 mm smaller in M22, so the grade A minimum across corners differs:
| Thread | ISO 4014 / 4017 | DIN 931 / 933 |
|---|---|---|
| M10 | 17.77 | 18.90 |
| M12 | 20.03 | 21.10 |
| M14 | 23.36 | 24.49 |
| M22 | 37.72 | 35.72 |
Both types are still sold, so check which head you have. In India, IS 1364 (Part 1) is identical to ISO 4014.
JMIKS Fasteners supplies hex bolts in M3 to M36 and 1/4″ to 1-1/4″ BSW. Grades are SS 304, SS 316 and SS 202, mild steel 4.6 and 5.6, and high tensile 8.8, 10.9 and 12.9. Mild steel and high tensile bolts come self colour, zinc plated or hot dip galvanized, and 12.9 is normally self colour. See also hex nuts, the hex bolt weight chart, the fastener weight calculator and the bolt grade chart.
7.5 mm nominal for ISO and DIN bolts. Grade A heads are 7.32 to 7.68 mm high.
11 mm, the ISO 273 medium hole. Use 10.5 mm for a close fit or 12 mm for a loose fit. To tap an M10 thread instead, use an 8.5 mm drill.
Medium holes to ISO 273 are 17.5, 22 and 26 mm. Coarse holes are 18.5, 24 and 28 mm.
Twice the diameter plus 6 mm on bolts up to 125 mm long, so 38 mm on M16 x 100. Bolts over 125 mm up to 200 mm long have twice the diameter plus 12 mm, and longer bolts plus 25 mm.
At least 26.75 mm for grade A and 26.17 mm for grade B. Its 24 mm hexagon would measure 27.71 mm with sharp corners.
JMIKS Fasteners is a fastener wholesaler and supplier in Nava Odhav, Ahmedabad. Send the size, length, half or full thread, grade, finish and quantity on WhatsApp (+91 90333 11425) or through the enquiry form, or get our catalogue from the catalogue page. We reply within one working day.
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