Thread Calculator

Calculate Tap Drill Size for various threads.

Thread Details

System

Major Dia (in)

Threads Per Inch (TPI)

Tap Drill Size

Enter standard thread details.

About this calculator

Overview

This calculator helps machinists find the correct drill bit size for tapping holes. It supports both Unified National (Imperial) and Metric threads.

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Pro Tips

For 75% thread in Imperial: Tap Drill = Major Dia - (0.9743 / TPI) (approx 1/TPI for rough)
Use a chart for critical aerospace applications.

Fun Facts

Formula: Imperial: Drill = Major Dia - (1 / TPI) Metric: Drill = Major Dia - Pitch