The right hole for a tapped thread
Drill the tapping hole too small and the tap binds and breaks; too large and the threads are shallow and strip. The tap drill size is the sweet spot, set by the thread major diameter, its pitch, and how much thread engagement you want. This calculator returns that size for metric or imperial threads.
The tap drill formula
Tap drill = major diameter – (percent thread / 100) x 1.299 x pitch. For metric threads the pitch is in millimetres; for imperial threads the pitch is 1 divided by the threads per inch. The 1.299 constant comes from the 60-degree thread form.
How much thread engagement?
Full theoretical thread is rarely worth it — going from 60 to 75 percent engagement nearly doubles the tapping torque for only a few percent more strength. Most shops target around 75 percent for general work, dropping to 50 to 65 percent in hard or gummy materials to save taps.
| Engagement | Strength | Tapping effort |
|---|---|---|
| 50% | About 90% of a 75% thread | Lowest |
| 65% | Very close to full | Moderate |
| 75% | Standard reference | High |
Worked examples
1/4-20 UNC at 75 percent: pitch = 1/20 = 0.05 in, tap drill = 0.25 – 0.75 x 1.299 x 0.05 = 0.201 in, which is a number 7 drill — the textbook answer. M6 x 1.0 at 75 percent: 6 – 0.75 x 1.299 x 1 = 5.03 mm, so a 5.0 mm drill, exactly the standard recommendation.
Drill it at the right speed
Once you have the size, set your spindle with the drill speed calculator so the tapping hole comes out clean and round.
FAQ
Does this give a standard drill size?
It gives the exact decimal size. Pick the nearest standard drill at or just above that value — being a hair larger eases tapping at a small cost in thread strength.
Metric pitch or TPI?
Metric threads are specified by pitch in millimetres (the distance between threads). Imperial threads use threads per inch. Switch the thread system and the calculator asks for the right one.