Tap drill calculator
The tap drill for an M8 × 1.25 thread at 75% engagement is 6.8 mm, from drill Ø = major Ø − (pitch × %) ÷ 76.98. Type or pick the thread below: the Make tab’s Tap drill view gives the drill for 60° metric, Unified and UNJ threads and 55° Whitworth and BSP threads from the percent of thread, checks it against the class’s internal minor diameter, and lists the nearest standard drills. ACME and trapezoidal threads get their core hole, and taper pipe threads (NPT, NPTF, Rc) the practice drill, drilled only or for a taper reamer.
For most metric sizes the 6H minor diameter sits at about 68–83 % thread, so 70–75 % is the usual target. A drill outside the class minor leaves the thread out of tolerance however strong it is.
Pick the tap for ISO metric and Unified threads: a forming (roll) tap needs a larger hole, from the tap makers’ formula with each maker’s own hole beside it, and a wire thread insert needs the oversize STI tap’s drill, with the tapped hole’s limits and the insert lengths.
How is tap drill size calculated?
A tap does not cut a full-depth thread; the drilled hole is left slightly larger than the thread’s minor diameter so only part of the thread flank is formed. The percent of thread sets how much.
For metric threads: drill Ø = major Ø − (pitch × %thread) ÷ 76.98. For inch threads: drill Ø = major Ø − (0.01299 × %thread) ÷ TPI. The constant 76.98 (and its inch equivalent 1 ÷ 0.01299) comes from the geometry of the standard 60° thread form.
What size hole for a forming tap?
A forming tap cuts no chips: it pushes the metal out of the root and up the flanks into the crest, so the hole is drilled larger than for a cutting tap. The makers’ formula is hole Ø = major Ø − 0.0068 × %thread × pitch, about 65 % thread, and the formed minor diameter should be gauged and the hole adjusted by trial for the material.
What percent of thread should I use?
Going from 75% to 100% thread engagement adds only a few percent of thread strength but sharply increases tapping torque. Most shops tap at 70–75%, inside the class’s minor diameter. Whitworth and BSP count 100% as their basic depth, so their drills read higher, about 75–85%.