Tap drill calculator
Any 60-degree thread, any percentage, cut or forming tap. Pick a standard thread or type your own diameter and pitch.
| Nearest drills | Size | Thread % |
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How the calculator works
A tap cuts threads into the wall of a drilled hole, so the hole sets how deep those threads can be. The calculator finds the hole size for the thread percentage you choose, then lists the real drills nearest to it.
The formulas
Inch, cut tap: drill = major − (0.01299 × % ÷ TPI)
Metric, cut tap: drill = major − (% × pitch ÷ 76.98)
Forming tap: drill = major − (0.0068 × % ÷ TPI), or major − (% × pitch ÷ 147.06) in mm
The cut-tap constants are the ones printed by OSG and Tapmatic (OSG Tapping Guide, Vol. 3; Tapmatic tapping formulas). The forming-tap version comes from the same Tapmatic page and from Missouri Carbide's forming tap data. The 0.01299 constant is twice the old basic thread depth of 0.6495 times the pitch, divided by 100, so the percentage is measured against that depth.
Worked example: 1/4-20 at 75%
0.2500 − (0.01299 × 75 ÷ 20) = 0.2500 − 0.0487 = 0.2013 in
Nearest drills: #7 at 0.2010 in (75%) and 13/64 at 0.2031 in (72%)
Both work. The #7 is the textbook answer; 13/64 is what most home shops have.
What the numbers do not tell you
Drills cut slightly oversize. A Greenfield Industries tap specialist measured #7 drills averaging 0.0038 in over their nominal size, which turns a 75% drill into about a 70% hole (Cutting Tool Engineering). If the thread has to meet a class 2B or 6H gauge, the hole also has to land inside the minor-diameter limits for that class, which can rule out drills that look fine by percentage alone.
Frequently asked questions
Why is my answer different from the chart?
Charts list a stocked drill near 75% thread. The calculator shows the exact hole for the percentage you choose, then the nearest drills on either side with the thread each would leave.
What percentage should I pick?
75% is the usual target for cut taps in steel, aluminum and brass. Tap makers suggest less for deep holes and tough metals, and more for thin sheet; OSG's table ranges from 55% for hard materials in deep holes to 85% for thin sheet (OSG Tapping Guide, Vol. 3). See the thread percentage guide.
Does it work for thread-forming taps?
Yes. Choose "Forming tap" and it switches to the forming formula with the 0.0068 constant used by Tapmatic and Missouri Carbide. Forming taps need a much larger hole, and makers advise not going above about 75% thread. See the forming tap guide.
Can I use it for odd or special threads?
Yes. Pick "Custom" and type any major diameter and pitch. It works for UNS and other special 60-degree threads. It does not apply to pipe threads, which are tapered, or to Acme and other non-60-degree forms.