Why the drill size matters
Before tapping a thread, you drill a hole slightly smaller than the screw. The tap then cuts the thread into the wall of the hole. Too large a hole leaves shallow threads that strip; too small a hole makes tapping hard and often breaks the tap.
The formula
The percentage of thread describes how deep the thread is compared with a full theoretical thread. The classic formula from shop references is:
Tap drill = major diameter − 1.299 × pitch × (% thread ÷ 100)
where the pitch is 1 ÷ threads per inch for inch threads, or the pitch in millimeters for metric threads. At 75% thread, the factor 1.299 × 0.75 is very close to 1, which gives the well-known quick rule:
Tap drill ≈ diameter − pitch
Example
For a 1/4-20 thread at 75%: 0.25 − 1.299 × (1 ÷ 20) × 0.75 ≈ 0.201 in, exactly a #7 drill. For M6 × 1: 6 − 1 = 5.0 mm.
Common tap drills
| Thread | Tap drill |
|---|---|
| 6-32 | #36 (0.1065 in) |
| 8-32 | #29 (0.136 in) |
| 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) |
| 1/2-13 | 27/64 in (0.4219 in) |
| M5 × 0.8 | 4.2 mm |
| M8 × 1.25 | 6.8 mm |
| M10 × 1.5 | 8.5 mm |
How much thread do you need?
Tests show that a 75% thread is only about 5% weaker than a full thread, but takes roughly three times less torque to tap. So:
- 75% is the standard choice for most steels, aluminum and brass.
- 60% to 65% suits hard or tough materials such as stainless steel and tool steel, and deep holes.
- Up to 80% to 85% can help in thin sheet, where few threads engage.
Tips
- Thread-forming (roll) taps do not cut chips; they need a larger hole, about the pitch diameter. Check the tap maker’s chart.
- Use cutting fluid, and back the tap off every half to one turn to break chips with a hand tap.
- For blind holes, drill deeper than the thread length so the tap does not bottom out on chips.
- Chamfer the hole slightly before tapping to help the tap start straight.
Thread pitch conversion
To compare inch and metric threads, pitch in mm = 25.4 ÷ TPI. A 20 TPI thread has a 1.27 mm pitch, close to but not interchangeable with a 1.25 mm metric pitch.