For any metric fine thread, the tap drill is the nominal diameter minus the pitch. M8 x 0.75 takes a 7.25 mm drill, M8 x 1 takes 7 mm, M10 x 1.25 takes 8.75 mm and M12 x 1.5 takes 10.5 mm. That rule gives about 77% thread, the standard figure on tap drill charts.
Fine-pitch metric threads turn up on bicycle parts, automotive fittings, optics and hydraulic adaptors, and they are easy to confuse with the coarse thread of the same diameter. Check the pitch with a thread gauge or by measuring across 10 threads before you drill. The chart below covers fine pitches from M2 to M30; for coarse threads and inch sizes, use the tap drill size calculator.
Metric fine thread tap drill chart
| Thread | Tap drill | In inches | Thread |
|---|---|---|---|
| M2 x 0.25 | 1.75 mm | 0.0689 in | 77% |
| M2.2 x 0.25 | 1.95 mm | 0.0768 in | 77% |
| M2.3 x 0.25 | 2.05 mm | 0.0807 in | 77% |
| M2.5 x 0.35 | 2.15 mm | 0.0846 in | 77% |
| M2.6 x 0.35 | 2.25 mm | 0.0886 in | 77% |
| M3 x 0.35 | 2.65 mm | 0.1043 in | 77% |
| M3.5 x 0.35 | 3.15 mm | 0.1240 in | 77% |
| M4 x 0.35 | 3.65 mm | 0.1437 in | 77% |
| M4 x 0.5 | 3.5 mm | 0.1378 in | 77% |
| M5 x 0.35 | 4.65 mm | 0.1831 in | 77% |
| M5 x 0.5 | 4.5 mm | 0.1772 in | 77% |
| M5 x 0.75 | 4.25 mm | 0.1673 in | 77% |
| M5.5 x 0.5 | 5 mm | 0.1969 in | 77% |
| M6 x 0.5 | 5.5 mm | 0.2165 in | 77% |
| M6 x 0.75 | 5.25 mm | 0.2067 in | 77% |
| M7 x 0.75 | 6.25 mm | 0.2461 in | 77% |
| M8 x 0.5 | 7.5 mm | 0.2953 in | 77% |
| M8 x 0.75 | 7.25 mm | 0.2854 in | 77% |
| M8 x 1 | 7 mm | 0.2756 in | 77% |
| M9 x 0.75 | 8.25 mm | 0.3248 in | 77% |
| M9 x 1 | 8 mm | 0.3150 in | 77% |
| M10 x 0.5 | 9.5 mm | 0.3740 in | 77% |
| M10 x 0.75 | 9.25 mm | 0.3642 in | 77% |
| M10 x 1 | 9 mm | 0.3543 in | 77% |
| M10 x 1.25 | 8.75 mm | 0.3445 in | 77% |
| M11 x 0.75 | 10.25 mm | 0.4035 in | 77% |
| M11 x 1 | 10 mm | 0.3937 in | 77% |
| M11 x 1.25 | 9.75 mm | 0.3839 in | 77% |
| M12 x 0.5 | 11.5 mm | 0.4528 in | 77% |
| M12 x 0.75 | 11.25 mm | 0.4429 in | 77% |
| M12 x 1 | 11 mm | 0.4331 in | 77% |
| M12 x 1.25 | 10.75 mm | 0.4232 in | 77% |
| M12 x 1.5 | 10.5 mm | 0.4134 in | 77% |
| M14 x 1 | 13 mm | 0.5118 in | 77% |
| M14 x 1.25 | 12.75 mm | 0.5020 in | 77% |
| M14 x 1.5 | 12.5 mm | 0.4921 in | 77% |
| M15 x 0.75 | 14.25 mm | 0.5610 in | 77% |
| M15 x 1 | 14 mm | 0.5512 in | 77% |
| M15 x 1.5 | 13.5 mm | 0.5315 in | 77% |
| M16 x 0.5 | 15.5 mm | 0.6102 in | 77% |
| M16 x 0.75 | 15.25 mm | 0.6004 in | 77% |
| M16 x 1 | 15 mm | 0.5906 in | 77% |
| M16 x 1.25 | 14.75 mm | 0.5807 in | 77% |
| M16 x 1.5 | 14.5 mm | 0.5709 in | 77% |
| M17 x 1 | 16 mm | 0.6299 in | 77% |
| M17 x 1.5 | 15.5 mm | 0.6102 in | 77% |
| M18 x 0.75 | 17.25 mm | 0.6791 in | 77% |
| M18 x 1 | 17 mm | 0.6693 in | 77% |
| M18 x 1.25 | 16.75 mm | 0.6594 in | 77% |
| M18 x 1.5 | 16.5 mm | 0.6496 in | 77% |
| M18 x 2 | 16 mm | 0.6299 in | 77% |
| M19 x 1 | 18 mm | 0.7087 in | 77% |
| M20 x 1.5 | 18.5 mm | 0.7283 in | 77% |
| M20 x 2 | 18 mm | 0.7087 in | 77% |
| M22 x 1 | 21 mm | 0.8268 in | 77% |
| M22 x 1.5 | 20.5 mm | 0.8071 in | 77% |
| M22 x 2 | 20 mm | 0.7874 in | 77% |
| M24 x 1 | 23 mm | 0.9055 in | 77% |
| M24 x 1.5 | 22.5 mm | 0.8858 in | 77% |
| M24 x 2 | 22 mm | 0.8661 in | 77% |
| M25 x 1 | 24 mm | 0.9449 in | 77% |
| M25 x 1.5 | 23.5 mm | 0.9252 in | 77% |
| M25 x 2 | 23 mm | 0.9055 in | 77% |
| M27 x 1 | 26 mm | 1.0236 in | 77% |
| M27 x 1.5 | 25.5 mm | 1.0039 in | 77% |
| M27 x 2 | 25 mm | 0.9843 in | 77% |
| M28 x 1 | 27 mm | 1.0630 in | 77% |
| M28 x 1.5 | 26.5 mm | 1.0433 in | 77% |
| M28 x 2 | 26 mm | 1.0236 in | 77% |
| M30 x 1 | 29 mm | 1.1417 in | 77% |
| M30 x 1.5 | 28.5 mm | 1.1220 in | 77% |
| M30 x 2 | 28 mm | 1.1024 in | 77% |
| M30 x 3 | 27 mm | 1.0630 in | 77% |
Coarse threads for comparison
The coarse (standard) pitch of each diameter needs a smaller drill than its fine threads, because a coarse thread is deeper:
| Coarse thread | Tap drill |
|---|---|
| M3 x 0.5 | 2.5 mm |
| M4 x 0.7 | 3.3 mm |
| M5 x 0.8 | 4.2 mm |
| M6 x 1 | 5 mm |
| M8 x 1.25 | 6.75 mm |
| M10 x 1.5 | 8.5 mm |
| M12 x 1.75 | 10.25 mm |
| M16 x 2 | 14 mm |
| M20 x 2.5 | 17.5 mm |
| M24 x 3 | 21 mm |
How to tell a fine thread from a coarse one
- Thread gauge: the quickest way. Try leaves until one sits in the thread with no light showing.
- Measure 10 threads: lay a rule along the bolt, measure the length of 10 crests and divide by 10. 12.5 mm over 10 threads on an M8 is 1.25 mm pitch (coarse); 10 mm is 1 mm pitch (fine).
- Try a known nut: a coarse nut will not start on a fine bolt of the same diameter.
- Read the callout: a metric thread with no pitch given (M8) is coarse. A fine thread is always written with its pitch (M8 x 1).
Tapping fine threads
- Fine threads are shallower, so they are stronger in thin walls and adjust more precisely, but they strip more easily in soft material and cross-thread more easily.
- Use cutting fluid and back the tap off often; fine taps clog quickly.
- For how deep to drill a blind hole, use the tapped hole depth calculator; for the hole a bolt passes through, the clearance hole size calculator (clearance does not depend on pitch).
Frequently asked questions
What size drill for an M8 x 0.75 tap?
7.25 mm (8 − 0.75). The nearest inch drills are 9/32 in or a letter K (both 7.14 mm, about 88% thread and a tight tap) and a letter L (7.37 mm, about 65% thread).
What size drill for an M8 x 1 tap?
7 mm (8 − 1). For the coarse M8 x 1.25 thread it is 6.75 mm; 6.8 mm drills are also sold for it.
What drill for M10 x 1.25?
8.75 mm (10 − 1.25). The coarse M10 x 1.5 uses 8.5 mm.
What drill for M12 x 1.25 and M12 x 1.5?
M12 x 1.25 uses 10.75 mm and M12 x 1.5 uses 10.5 mm. The coarse M12 x 1.75 uses 10.2 mm.
How do I work out a metric tap drill?
Subtract the pitch from the nominal diameter: drill = D − P. It gives about 77% thread for any metric thread, coarse or fine.
Source: Optimas, “Recommended Tapping Drill and Clearance Hole Sizes” (ISO metric coarse and fine threads), checked row by row against drill = D − P. Inch equivalents are exact conversions (1 in = 25.4 mm).