Tap drill size calculator
Pick the thread, set the engagement, get the drill that is actually in your index.
The formula
For both Unified and ISO metric threads:
tap drill Ø = major Ø − (engagement ÷ 100) × 1.299 × pitch
Two checks confirm the constant is 1.299 and not the 1.0825 that sometimes circulates. A 1/4-20 UNC at 75% gives 0.2013", and the standard chart answer is a #7 drill at 0.2010". An M6 × 1.0 at 75% gives 5.026 mm, and the chart answer is 5.0 mm. Both land on the number a real shop uses.
How much engagement do you actually want?
75% is the shop default and it is usually the wrong answer for hard material. Thread strength climbs slowly above 60% while tapping torque climbs fast, so the tap is doing dramatically more work for a few percent more holding power. In stainless, tool steel or anything gummy, dropping to 55 to 65% is the difference between a tapped hole and a broken tap in a nearly finished part.
| Engagement | Use it for |
|---|---|
| 50–60% | Tough or gummy material, deep holes, small taps |
| 65–72% | Stainless, alloy steel, blind holes |
| 75% | General shop work in mild steel and aluminum |
| 80%+ | Soft material, thin sections, high torque and tap risk |