How to use the tap drill hole size calculator
This page shows the hole diameter to drill before tapping. Pick inch (UNC / UNF) or metric threads, enter the nominal thread diameter and the threads per inch or the pitch, and the theoretical hole size appears at once; lower the engagement figure and the correspondingly larger hole appears as well. The difference between nominal diameter and hole size is shown too, which gives a feel for how deeply the thread bites.
The relation applied is the widely used machinist formula for 60 degree threads: hole size = nominal diameter - 0.01299 x engagement (%) x pitch. For unified inch threads the pitch is 1 / TPI, so the hole is D - 0.01299 x engagement (%) / TPI; a 1/4-20 tap at 75% gives 0.2013 in (5.11 mm). At 100% engagement (the full basic thread height) the hole is D - 1.299 x pitch, and at 75% it is D - 0.974 x pitch. The familiar D - pitch drill corresponds to about 77% engagement. Inch results show millimeters in parentheses and metric results show inches. This is the approximate shop relation and can differ from the dimension tables in the original standard. Values are as of October 2026, so check the specification of the tooling you use.
Tables of standard drill diameters, the internal minor diameter and tapping conditions by tap type were not confirmed in an authoritative source, so none of them are given or covered. The output is a theoretical hole size, so in practice you pick from the drills you hold and adjust for material and tap type. This calculator is for reference and does not replace design; for critical joints check the tooling specification and the drawing tolerances.
Frequently asked questions
Tables of standard drill diameters were not confirmed in an authoritative source, so they are not covered. This page works out the theoretical hole size only and you pick the real drill from the sizes you hold.
The hole gets larger, tapping gets lighter and the thread bites less deeply. The common D - pitch drill gives about 77% engagement, while 100% needs D - 1.299 x pitch (pitch = 1 / TPI for inch threads).
Threads per inch or pitch is entered directly, so UNF and metric fine threads follow the same relation as UNC and metric coarse. Internal minor diameter tables and pipe threads such as NPT are not covered.