Tap Drill Calculator
Enter the thread major diameter and pitch to get the tap drill at 75% thread, adjustable for other engagement percentages, plus the nearest real drill bit.
Last updated: 2026-09-15
How the calculation works
- A 100% thread would fill the full V; tap drills cut a partial thread. The standard 75% engagement comes from drill = major − pitch.
- Higher engagement percentages need a smaller drill: the calculator adjusts the hole linearly to your target.
- The nearest larger standard drill bit is suggested since drills come in discrete sizes.
Formula
Tap drill (75%) = D − P Other engagement: drill = D − P × (target% / 75)
| Symbol | Meaning | Unit |
|---|---|---|
D | Major diameter | mm |
P | Thread pitch | mm |
% | Thread engagement percentage | — |
Worked example
M6×1 at 75%: drill = 6 − 1 = 5.0 mm — the classic answer. At 65% for stainless: hole = 6 − 0.65 = 5.35 mm, so a 5.4 mm bit reduces tap breakage risk noticeably.
Interpreting the result
75% thread delivers nearly full strength (over 90% of a 100% thread) with far less tapping torque — the standard compromise. In tough alloys, dropping to 65–70% trades a few percent of strength for much lower tap breakage risk. Softer aluminum benefits from 80–85% engagement.
Assumptions
- ISO metric threads with uniform 60° profile.
- Drill produces a round, correctly sized hole (sharp bit, correct speed and coolant).
Limitations
- Unified inch threads use drill = major − 1/TPI for 75% — convert to mm or use an inch chart.
- Cast iron and gummy stainless sometimes need special tap geometry beyond drill size.
- Does not cover pipe threads (NPT/BSP), which taper and use different charts.
Frequently asked questions
What drill for M8 threads?
M8×1.25 coarse: 6.75 mm (8 − 1.25). The nearest common bit is 6.8 mm.
Why not drill 100% thread?
A full-V thread adds almost no strength but multiplies tapping torque and chip congestion — taps break. 75% is the engineering sweet spot.