Tap Drill Chart

How to use this chart

The tap drill column lists the starting-hole diameter to drill before cutting internal threads with a tap at the listed thread size. Drilling this size first, then tapping, leaves the tap enough material to cut a full thread profile without removing so much that the resulting thread is undersized or the tap binds.

Coarse and fine threads at the same nominal diameter call for different tap drills because they use different pitch. A common workshop approximation is major diameter minus pitch, so a fine-pitch thread — which has a smaller pitch than its coarse counterpart at the same size — needs a slightly larger tap drill than the coarse version. This chart lists coarse-pitch values only; fine-pitch threads need their own tap drill, not the coarse figure shown here.

These are general workshop reference figures, not a universal recommendation. Material hardness, thread-forming (cold-forming) taps versus cutting taps, hole depth, and the thread engagement required for a given application can all call for a different tap drill than the one shown here. Confirm against your tap manufacturer's data before tapping a critical part.

For thread sizes where BoltLab has cross-verified tap-drill data against a cited standard, the Tapping Atlas labels each value's verification status (verified or source-bound) by thread designation, rather than presenting a single unqualified figure — check there when that distinction matters for your work.