LatheCalculator

Tap drill finder

Choose a thread, then set how much thread engagement you want. You get the ideal drill and the nearest stock metric and imperial drills, with the % of thread each one actually gives.

75% suits most steel and brass work. Use 60–65% in stainless, duplex and other tough materials, or with small taps by hand. 100% needs about 5% more torque for only a few percent more strength.

How it works

For standard sizes the answer is the size on the usual tapping chart (for example M12 = 10.2 mm, ¼" UNC = #7). Switch to Detailed to see the formula value for any % of thread and the nearest imperial drills. See the glossary for the terms.

Percentage of thread follows the usual workshop convention: 100% means the drill equals the basic minor diameter of a full-height thread. For 60° threads, drill = D − 1.299 × P × %/100. For Whitworth forms the thread height is 0.6403P, and for BA it is 0.6P.

Worked example, M6 × 1: at 77% the drill is 6 − 1.299 × 1 × 0.77 = 5.0 mm. This matches the standard “drill = D − P” rule for metric coarse threads. For 1/4" UNC at 75%: 0.25 − 0.01299 × 75 / 20 = 0.2013", so a #7 drill (0.201").

Number and letter drill sizes were compiled from memory and are being checked against BS 328 / ANSI B94.11M. They are marked est. until confirmed. Acme, Trapezoidal and BSP threads use a fixed drill-to-minor-diameter recommendation, so the % control does not apply to them.

Safety: a broken tap in a part is hard to remove. Use plenty of cutting fluid, back off to break the chip, and use a tapping guide so the tap starts square.

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