How the calculation works

Approximate drill = major diameter − (0.75 ÷ TPI) × (thread percentage ÷ 75).

Example

A 1/4-20 thread at 75% suggests a 0.2125-inch drill, close to a #7 drill.

When to use this calculator

Use it to choose a starting drill size before checking the tap manufacturer’s chart.

Results are planning estimates. Verify critical dimensions, machine settings, material specifications, and local code requirements before work begins.