Metric thread sizes
13 metric thread sizes with every number needed to drill, tap, clear and tighten them.
The one rule worth memorising
Tapping drill = major diameter − pitch. M6 coarse has a 1.0 mm pitch, so it is drilled 5.0 mm. M8 coarse is 1.25, so 6.75 — sold as a 6.8 mm drill, because 6.75 is not a size anyone stocks. That subtraction reproduces every published tapping-drill chart, which is worth knowing precisely because it means you never need the chart.
The clearance hole is the other number, and it is the one people get the wrong way round: the tapping drill is for the part being threaded, the clearance hole is for the part the bolt passes through. Drilling a clearance hole and then trying to tap it produces a hole with no thread left in it.
The full chart
| Thread | Pitch | Tapping drill | Clearance | Hex key | Spanner |
|---|---|---|---|---|---|
| M2 | 0.4 mm | 1.6 mm | 2.4 mm | 1.5 mm | 4 mm |
| M2.5 | 0.45 mm | 2.05 mm | 2.9 mm | 2 mm | 5 mm |
| M3 | 0.5 mm | 2.5 mm | 3.4 mm | 2.5 mm | 5.5 mm |
| M4 | 0.7 mm | 3.3 mm | 4.5 mm | 3 mm | 7 mm |
| M5 | 0.8 mm | 4.2 mm | 5.5 mm | 4 mm | 8 mm |
| M6 | 1 mm / 0.75 | 5 mm | 6.6 mm | 5 mm | 10 mm |
| M8 | 1.25 mm / 1 | 6.8 mm | 9 mm | 6 mm | 13 mm |
| M10 | 1.5 mm / 1.25 | 8.5 mm | 11 mm | 8 mm | 17 / 16 mm |
| M12 | 1.75 mm / 1.5 | 10.2 mm | 13.5 mm | 10 mm | 19 / 18 mm |
| M14 | 2 mm / 1.5 | 12 mm | 15.5 mm | 12 mm | 22 / 21 mm |
| M16 | 2 mm / 1.5 | 14 mm | 17.5 mm | 14 mm | 24 mm |
| M20 | 2.5 mm / 1.5 | 17.5 mm | 22 mm | 17 mm | 30 mm |
| M24 | 3 mm / 2 | 21 mm | 26 mm | 19 mm | 36 mm |
Why an M10 bolt needs two different spanners
It does not — but your socket set behaves as though it does. DIN 933 specifies 17 mm across the flats for an M10 hex head and ISO 4017 specifies 16. Both are M10 bolts with identical threads; only the head size differs. The same split applies at M12 and M14. This is why a set that covers 8, 10, 13, 17 and 19 will occasionally meet a bolt it cannot grip, and why metric sets sold today usually include both 16 and 17.
Coarse and fine
Every size here has a coarse pitch, which is the default and what "M6" means without further qualification. The fine series has a smaller pitch — M8×1.0 against the coarse M8×1.25 — which gives more threads per unit length, a shallower thread, and better resistance to vibrating loose. It is also more easily cross-threaded and strips more readily in soft material. Fine threads belong in steel and in adjustments; coarse threads belong everywhere else.
How deep to tap
The usual rule is 1.5 times the diameter in steel and 2 times in aluminium — deeper in softer material because the load is spread over more threads. Beyond about 2.5 diameters the extra threads carry almost nothing: the first few take most of the load, which is why a longer bolt in a deeper hole is not stronger past a point.