Tap Drill Chart

Thread engagement strength is a sharply diminishing curve: the first 60 to 75 percent of a thread's height carries nearly all the load, and the last quarter adds only a few percent of strength while multiplying tapping torque dramatically. At 75 percent you get essentially full strength with a tap that still cuts freely - the industry compromise since taps were standardized. Below that: 60 to 70 percent is the deliberate choice for stainless, titanium and other work-hardening alloys, where the torque saving prevents broken taps and the strength loss is single-digit percent; aerospace shops routinely spec it. Above it: chasing 85 or 100 percent thread buys nothing measurable in a bolted joint and overheats taps - most tap failures trace to holes drilled too small in pursuit of strength that was already there. Two refinements worth knowing. Fine threads need less correction than the raw percentage suggests: a UNF thread's shallower pitch means the ideal drill lands closer to a standard size anyway. And the percentage math assumes cutting taps at all - roll-forming taps (the ones with no flutes) displace metal instead of cutting it and need a larger hole per their own charts, so identify your tap type before trusting any chart, including this one.

The formula is the classic approximation for Unified threads: ideal tap drill equals major diameter minus the pitch, where pitch is one divided by threads per inch. For 1/4-20: 0.250 minus 0.050 = 0.200 inch. It is an approximation of the full thread geometry - the exact minor diameter depends on the thread form's truncation - but across the whole UNC/UNF range it lands within a few thousandths of the exact value, which is well inside the tolerance that matters when the answer gets snapped to a real drill anyway. The snapping rule used here: pick the nearest standard drill across all three American series (number, letter, fractional), and when two sizes tie, take the larger drill - erring toward an easier tap and a marginally shallower thread, which is the failure mode you can recover from. The validation is the strongest part: run the chart and it reproduces the published machinist pairings exactly - #7 for 1/4-20, #21 for 10-32, #25 for 10-24, #29 for 8-32, #36 for 6-32, F for 5/16-18, Q for 3/8-24, 27/64 for 1/2-13 - nineteen out of nineteen. The chart is computation, but it is computation with a known-good answer key.

Four cases with their own answers. Metric threads: the same formula works in millimeters (drill equals major minus pitch, so an M6 x 1 wants a 5 mm drill) but the snapping happens in the metric drill series - use a metric chart, because an American number-drill answer for an M-thread will be a frustrating near-miss. Pipe threads: NPT taps have their own taper geometry and their own chart - a 1/8-27 NPT tap wants a letter Q drill, which no formula from bolt diameters will give you, because pipe threads are specified by bore, not bolt size. Roll-forming taps: as above, they need larger holes than cutting taps - several percent larger by diameter - and ship with their own recommendations; using a cutting-tap chart with a forming tap produces undersized holes and broken taps of a different kind. And deep or blind holes: below roughly three diameters of depth, chip packing changes the calculus - switch to spiral-point or spiral-flute taps, peck drill, and sometimes step up a drill size to survive the depth. The honest scope of this chart: straight-flute cutting taps in through or shallow blind holes, Unified threads, common sizes - which is the overwhelming majority of bench work, and the portion where a single reference table actually settles the question.