Tool
Tap drill chart
Pick a thread and get the tap drill size. The rule behind it is simple: drill diameter = major diameter − pitch, which leaves about 75% thread engagement.
Tap drill
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Clearance drill
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Thread pitch
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Metric coarse threads
| Thread | Pitch (mm) | Tap drill (mm) | Clearance hole (mm) |
|---|---|---|---|
| M1.6 | 0.35 | 1.25 | 2.2 |
| M2 | 0.40 | 1.60 | 2.6 |
| M2.5 | 0.45 | 2.05 | 3.1 |
| M3 | 0.50 | 2.50 | 3.6 |
| M4 | 0.70 | 3.30 | 4.6 |
| M5 | 0.80 | 4.20 | 5.6 |
| M6 | 1.00 | 5.00 | 6.6 |
| M8 | 1.25 | 6.75 | 8.6 |
| M10 | 1.50 | 8.50 | 10.6 |
| M12 | 1.75 | 10.25 | 12.6 |
| M14 | 2.00 | 12.00 | 14.6 |
| M16 | 2.00 | 14.00 | 16.6 |
| M18 | 2.50 | 15.50 | 18.6 |
| M20 | 2.50 | 17.50 | 20.6 |
| M22 | 2.50 | 19.50 | 22.6 |
| M24 | 3.00 | 21.00 | 24.6 |
| M27 | 3.00 | 24.00 | 27.6 |
| M30 | 3.50 | 26.50 | 30.6 |
UNC and UNF
| Thread | Major (in) | TPI | Tap drill (in) |
|---|---|---|---|
| #4-40 | 0.1120 | 40 | 0.0870 |
| #6-32 | 0.1380 | 32 | 0.1068 |
| #8-32 | 0.1640 | 32 | 0.1328 |
| #10-24 | 0.1900 | 24 | 0.1483 |
| #10-32 | 0.1900 | 32 | 0.1588 |
| 1/4-20 | 0.2500 | 20 | 0.2000 |
| 1/4-28 | 0.2500 | 28 | 0.2143 |
| 5/16-18 | 0.3125 | 18 | 0.2569 |
| 5/16-24 | 0.3125 | 24 | 0.2708 |
| 3/8-16 | 0.3750 | 16 | 0.3125 |
| 3/8-24 | 0.3750 | 24 | 0.3333 |
| 7/16-14 | 0.4375 | 14 | 0.3661 |
| 1/2-13 | 0.5000 | 13 | 0.4231 |
| 1/2-20 | 0.5000 | 20 | 0.4500 |
| 9/16-12 | 0.5625 | 12 | 0.4792 |
| 5/8-11 | 0.6250 | 11 | 0.5341 |
| 3/4-10 | 0.7500 | 10 | 0.6500 |
| 1-8 | 1.0000 | 8 | 0.8750 |
How the number is calculated
Tap drill = major diameter − pitch. For M6 × 1.0 that is 6.0 − 1.0 = 5.0 mm. For a 1/4-20 thread, the pitch is 1/20 = 0.050 in, so the drill is 0.250 − 0.050 = 0.200 in — which is the #7 drill, not 5 mm.
The engagement percentage is a trade: 75% is the standard because it gives nearly the full strength of the thread while keeping tapping torque manageable. Above 85% the tap breaks far more often, and the strength gain is small. Below 60% the thread starts to pull out under load.
The clearance hole is a different number — it is the hole the bolt body passes through, and it is one size larger so the parts can be assembled without forcing. For metric threads, about +0.6 mm on the major diameter is a working rule (M6 → 6.6 mm).
Questions
What is a tap drill?
The hole you drill before tapping a thread. It is smaller than the thread's major diameter by roughly the pitch, which leaves about 75% thread engagement — strong enough, and far easier to tap than a full 100% thread.
How do I calculate a tap drill size for a metric thread?
Subtract the pitch from the major diameter. M6 × 1.0 gives 5.0 mm, M8 × 1.25 gives 6.75 mm (5 mm shank: 6.8 mm drills are the stock size), M10 × 1.5 gives 8.5 mm.
Why do my tapped holes strip?
Usually one of three things: the drill was too large (less than 60% engagement), the tap was run without cutting oil, or the thread was cut in a thin wall with no material around it. The drill size is the first thing to check.
Is 100% thread engagement better?
No. Going from 75% to 100% engagement roughly doubles the tapping torque for about 10% more strength, and the tap breaks more often. 65–75% is the standard trade.
What is the difference between a tap drill and a clearance drill?
A tap drill is the hole you thread; a clearance drill is the hole the bolt passes through afterwards. Clearance holes are one size up — M6 typically gets a 6.6 mm clearance hole.
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