Metric bolt dimensions
The M number is the thread diameter. Everything else follows from it. Watch the head size column: ISO 4014 and the older DIN 931 disagree at M10, M12 and M14, which is why a 16mm spanner fits some M10 bolts and a 17mm fits others.
Socket cap screws to ISO 4762 use their own head and key sizes, listed in the last column.
| Size | Coarse pitch (mm) | Head A/F ISO | Head A/F DIN | Nut A/F | Cap screw hex key |
|---|---|---|---|---|---|
| M3 | 0.50 | 5.5 | 5.5 | 5.5 | 2.5 |
| M4 | 0.70 | 7 | 7 | 7 | 3 |
| M5 | 0.80 | 8 | 8 | 8 | 4 |
| M6 | 1.00 | 10 | 10 | 10 | 5 |
| M8 | 1.25 | 13 | 13 | 13 | 6 |
| M10 | 1.50 | 16 | 17 | 17 | 8 |
| M12 | 1.75 | 18 | 19 | 19 | 10 |
| M14 | 2.00 | 21 | 22 | 22 | 12 |
| M16 | 2.00 | 24 | 24 | 24 | 14 |
| M20 | 2.50 | 30 | 30 | 30 | 17 |
| M24 | 3.00 | 36 | 36 | 36 | 19 |
Property class and metric bolt torque
The number stamped on a metric bolt head is its property class. Read it as two parts: the first figure times 100 is tensile strength in MPa, and the two multiplied by 10 give yield strength. So 8.8 is 800 MPa tensile and 640 MPa yield.
Torque figures below are dry, plain finish, calculated with a nut factor of 0.20. Reduce by about 25 percent for oiled threads.
| Size | Class 8.8 | Class 10.9 | Class 12.9 |
|---|---|---|---|
| M6 | 10 | 14 | 17 |
| M8 | 25 | 35 | 41 |
| M10 | 49 | 69 | 83 |
| M12 | 85 | 120 | 145 |
| M14 | 135 | 190 | 230 |
| M16 | 210 | 295 | 355 |
| M20 | 410 | 580 | 690 |
| M24 | 710 | 1,000 | 1,200 |
Hex key sizes
Socket cap screws take a hex key roughly 0.75 times the thread diameter, which gives the clean progression in the table above: M5 takes 4mm, M6 takes 5mm, M8 takes 6mm, M10 takes 8mm.
Button head and countersunk socket screws use a smaller key than a cap head of the same thread, because the head is shallower. An M6 button head takes a 4mm key, not 5mm.
| Size | Cap head | Button head | Countersunk |
|---|---|---|---|
| M3 | 2.5 | 2 | 2 |
| M4 | 3 | 2.5 | 2.5 |
| M5 | 4 | 3 | 3 |
| M6 | 5 | 4 | 4 |
| M8 | 6 | 5 | 5 |
| M10 | 8 | 6 | 6 |
| M12 | 10 | 8 | 8 |
How to measure metric bolts
Measure the outside diameter of the thread with a caliper. That figure, rounded to the nearest whole millimetre, is the M number. Do not measure the head.
Then check pitch with a thread gauge, or measure across ten thread crests and divide by ten. Length is measured from under the head on a standard hex or cap screw, and from the top of the head on a countersunk one.
