Formula
For a 60° cutting tap the shop drill is the major diameter minus 1.299 times the pitch times the percent divided by 100. At 75 percent that is major minus 0.97425 times the pitch. The 1.299 is twice the sharp-V depth per unit of pitch, (3√3)/4. Actual percent from a real drill is (major minus drill) divided by 1.299 times the pitch, times 100. The same constant is used for inch and metric so the two systems are not mixed.
drill = major − 1.299 × P × (% / 100)
basic pitch Ø = major − 0.64952 × P
basic minor Ø = major − 1.08253 × P
ASME B1.1 and ISO 68-1 truncate the sharp V. The basic height used there is 5/8 of the sharp triangle, so the basic minor sits about 1.0825 pitch under the major, roughly 83 percent on this page’s sharp-V scale. A 75 percent shop drill is larger than that basic minor on purpose. Class of fit, 1B, 2B, or 3B, is a gauging limit on the drawing. This page does not compute it. Forming taps use a larger hole; they are omitted because this page is cutting taps only. NPT is omitted.
Worked examples
1/4-20 at 75%. Pitch 0.050. Drill 0.2013 in, 5.113 mm. Nearest #7 at 0.2010 in, about 75.4 percent.
#10-24 at 75%. Drill 0.1494 in. Nearest #25 at 0.1495 in, about 74.8 percent. 1/2-13 at 75%. Drill 0.4251 in. Nearest 27/64 at 0.4219 in.
M6×1 at 75%. 5.026 mm, nearest 5.0 mm, about 77 percent. M10×1.5 at 75%. 8.539 mm, nearest 8.5 mm. 0.375-16 at 60%. 0.3263 in.
Reference
Number, letter, and 1/64-inch drills, plus metric drills in 0.1 mm steps from 1.0 to 20.0. The nearest size is the smallest gap. A tie keeps the smaller drill.
Questions
How do you calculate tap drill size?
Subtract 1.299 times the pitch times the percent of thread from the major diameter. At 75 percent that factor is about 0.974 times the pitch. The nearest number, letter, fraction, or 0.1 mm metric drill is what you put in the chuck.
What size drill for 1/4-20?
At 75 percent the theoretical hole is 0.2013 inch. The nearest drill on this chart is a #7, 0.2010 inch, about 75.4 percent.
What is 75 percent thread engagement?
It is a shop convenience: most of the strength of a full thread, with less tapping torque than a deeper hole. Above about 80 percent the torque climbs quickly. A drawing’s 2B callout is a class of fit, not this percent.
What is the difference between UNC and UNF?
UNC is the coarse series, fewer threads per inch. UNF is fine, more threads on the same major diameter, so the tap drill is larger. 1/4-20 and 1/4-28 share a 0.250 inch major.
How do I find a metric tap drill?
The same 1.299 factor, with pitch in millimetres. M6×1 at 75 percent is 5.026 mm, and the nearest 0.1 mm drill here is 5.0 mm, about 77 percent.
What is thread pitch?
The distance from one crest to the next. Inch pitch is 1 divided by the threads per inch. Metric pitch is the millimetre figure in the callout.
Can I use this for pipe threads?
No. NPT and BSPT are tapered pipe threads. Use the tap maker’s chart for those. This page is straight Unified and ISO metric only.
Sources
- Shop percent-of-thread drill: major − 1.299 × pitch × percent/100. The 1.299 factor is the sharp-V constant used in Machinery’s Handbook tap-drill practice and on published cutting-tap charts. At 75 percent, 1/4-20 is 0.2013 in, and the common chart drill is a #7 at 0.2010 in.
- ASME B1.1 and ISO 68-1 basic 60° profile. Basic pitch diameter is major minus about 0.64952 × pitch. Basic minor is major minus about 1.08253 × pitch. Those are profile references, not a class-of-fit limit. ISO 261 is the metric coarse and fine series the presets follow (M6×1, M8×1.25 and M8×1, M10×1.5 and M10×1.25).
- Inch drill diameters are the published number, letter, and fractional series (#7 = 0.2010, #25 = 0.1495, 27/64 = 0.421875). Metric drills are 0.1 mm steps. 1 in = 25.4 mm.