Tap Drill Size Calculator
Find the correct tap drill size for any inch or metric thread at your target percent of thread engagement.
Using this calculator responsibly
The Tap Drill Size calculator is an engineering aid for checking a shop-floor estimate before a setup, quote, or process review. Enter values in the units shown, confirm material properties against the applicable specification, and keep a record of the assumptions used. Calculator output does not override machine capacity, tooling limits, part requirements, or a qualified engineer's approval.
Before using a result in production, repeat the calculation with the extreme values allowed by the print or process window. Check that the selected material, thickness, geometry, and safety factor reflect the actual job. If a result approaches a press, die, fastener, or machine limit, stop and verify it with the equipment manufacturer or responsible engineer.
Inputs and results
Read every field label before entering data and avoid mixing inch and metric values. A useful result should be traceable to the job number, revision, material certification, and machine or tooling record. Treat warnings, invalid ranges, and unusual outputs as reasons to inspect the inputs rather than as values to work around.
The tap drill leaves the right amount of material for the tap to cut threads. Aiming for about 75% thread gives nearly full strength while keeping tapping torque manageable.
For inch threads, subtract roughly one pitch (1/TPI) from the major diameter; for metric, subtract the pitch.
Formula: Tap drill ≈ Major Ø − (1 / TPI) [inch]; Major Ø − Pitch [metric]
Worked example
- 1/4-20 UNC: 0.250 − 1/20 = 0.250 − 0.050 = 0.200 in → use a #7 (0.201 in) drill.
- M6 × 1.0: 6.0 − 1.0 = 5.0 mm drill.