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Tap Drill Chart Explained
Guide · Updated October 27, 2026
Tap drill charts look like magic numbers, but most sizes come from one simple rule. Once you know it, you can check any chart and choose a drill for the thread percentage you need.
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Step by step
- Metric rule: Tap drill ≈ major diameter − pitch. For M6×1, that's 5.0 mm.
- Inch rule: Tap drill ≈ major − 1/TPI. For 1/4-20, 0.250 − 0.050 = 0.200 in, so use a #7 (0.201 in).
- Choose the thread percentage: About 75% thread is standard. Use 60–65% for tough materials or deep holes.
- Adjust for the material: Go slightly bigger in stainless and titanium to save taps.
- Check the hole: Check the drilled hole with a pin gage or calipers before you tap.
Common sizes
- #10-32 → #21 (0.159 in)
- 1/4-20 → #7 (0.201 in)
- 5/16-18 → F (0.257 in)
- 3/8-16 → 5/16 in (0.3125 in)
- M5×0.8 → 4.2 mm
- M6×1 → 5.0 mm
- M8×1.25 → 6.8 mm
- M10×1.5 → 8.5 mm
Related
Related: drilling speeds and feeds, pin gages.
FAQ
How do you calculate a tap drill size?
Metric: major minus pitch. Inch: major minus 1/TPI. Both give about 75% thread.
What is the tap drill for 1/4-20?
A #7 drill (0.201 in).
What is the tap drill for M8?
6.8 mm for M8×1.25 coarse thread.
Is 100% thread stronger?
Barely, and it greatly raises tap breakage. 75% is the standard compromise.
How do I check a tap drill hole?
Use pin gages. Check those against gage blocks, with stacks built free at gageblockcalc.com.
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