Tapping Torque Calculator

0.201 in — the #7 drill — at 75% thread. The approximation tap drill = D − 1/pitch gives 0.25 − 0.05 = 0.200 in. For M8×1.25 the same rule gives 8 − 1.25 = 6.75 mm.

Thread strength curves flatten past ~75% engagement: 75→85% buys a few percent of strength but sharply raises torque — the classic broken-tap setup. 65–70% is common in tough alloys where the fastener, not the thread, is the weak link.

There is no closed-form equation shops trust. Torque depends on tap geometry, coating, lubrication, and hole condition — not just size and material. That is why this page gives empirical reference ranges across tool-maker data instead of a formula pretending to be exact.

Roll (form) taps need a larger hole — roughly the pitch diameter, several percent bigger than a cut-tap drill — because they displace metal instead of cutting it. A cut-tap drill with a roll tap seizes. Percent thread also reads differently on a form tap.

In order: drill size (too small is the #1 cause), tap sharpness (a dull tap multiplies torque), chip packing (switch to spiral-flute or peck), lubrication (tapping fluid, not flood coolant alone), alignment. Don't just slow down — a stalled tap work-hardens the thread at any speed.

From surface speed: RPM = SFM × 12 / (π × D). HSS taps: ~30 SFM stainless, ~60 mild steel, ~90 aluminum. 1/4-20 in mild steel: 60 × 12 / (π × 0.25) = 917 rpm. Rigid tapping holds these; hand tapping ignores them.

Same rule in mm: tap drill = major diameter − pitch. M6×1 → 5.0 mm, M8×1.25 → 6.75 mm, M10×1.5 → 8.5 mm. Inch sizes convert at 25.4 mm/in; round to the nearest standard metric drill.

Compare the reference band for your thread and material against spindle torque at the tapping RPM — not peak power. #10 aluminum needs 1–2 lb·ft; a 1/2-13 in stainless demands 35–60 lb·ft plus ~50% margin.