Tap Drill & Thread Chart
Major diameter, pitch, and the recommended 75% engagement tap drill, for every common UNC, UNF, and metric thread — computed live, not transcribed from an old chart.
Imperial: UNC & UNF
| Size | Major dia (in) | UNC | UNC tap drill | UNF | UNF tap drill |
|---|
Metric coarse
| Size | Major dia (mm) | Pitch (mm) | Tap drill (mm) |
|---|
Fine-pitch metric threads exist for most of these sizes too; this chart covers the coarse (default) pitch.
Why the drill sizes here are calculated, not copied
Every tap drill size on this page comes from the same formula behind the Tap Drill Size Calculator: drill = major diameter − (1.299 × pitch × 75%), then rounded to the nearest real drill using the same number, letter, and fractional index as the Drill Bit Size Chart. That means every value here is internally consistent with every other calculator on this site, not hand-copied from a different source with a different rounding convention.
75% engagement, the general-purpose default
All the imperial and metric coarse tap drills shown target 75% thread engagement — the standard default for most general work, giving roughly 90% of full thread strength while tapping noticeably easier than 100% engagement. If a print specifies a different engagement percentage, use the Tap Drill Size Calculator directly instead of this chart.
Rounding direction matters
The "nearest real drill" shown may round up or down from the exact theoretical size — rounding up gives slightly less engagement than 75%, rounding down gives slightly more. If you need to know exactly how much a specific substitution shifts engagement, check it with the Thread Percentage Calculator.