Tap Drill Chart

The 60-second rule

UNC tap drill (inches) = major diameter − 1 ÷ TPI. For 1/4-20: 0.250 − 1/20 = 0.250 − 0.050 = 0.200 in, and the nearest standard bit is #7 (0.201 in). That hole leaves about 75% thread engagement — the sweet spot where the thread is nearly full-strength but the tap still turns without heroic force. Metric coarse is even simpler: tap drill (mm) = diameter − pitch — M8 × 1.25 wants a 6.8 mm bit (6.75 theoretical, 6.8 is the standard size).

The worked example, line by line

Setup: you need to tap a 1/4-20 UNC hole in a steel bracket.

Major diameter. 1/4 inch = 0.250 in — the outside of the thread.

One thread pitch. 20 threads per inch means each thread is 1/20 = 0.050 in tall.

The 75% hole. 0.250 − 0.050 = 0.200 in — drill this and the tap cuts threads 75% as deep as a theoretically perfect (sharp-V) thread.

The standard bit. Nobody stocks a 0.2000-inch bit; the nearest standard is #7 at 0.201 in — one thousandth over, which costs you essentially nothing in thread strength.

Branch — #10-24: 0.190 − 1/24 = 0.190 − 0.0417 = 0.1483 in → use a #25 (0.1495 in) bit.

Branch — M6 × 1.0: 6 − 1 = 5.0 mm → use a 5.0 mm bit. Metric needs no chart at all — just subtract.

Branch — the 0.25-inch bit you already own: the closest 75%-thread hole is 5/16-18 (0.2569 in), only 0.0069 in off — a workable fit when the exact bit isn’t in your index.

The printable reference tables

Every common UNC machine-screw size with its 75%-thread tap drill decimal and the standard bit to use, plus the metric coarse table. The decimal column is the theoretical hole; the “use this drill” column is the nearest real bit. Use your browser’s print function for a shop-wall copy.

Print the chart

Output · formula · meaning

Who uses this

• The DIYer tapping a bracket: needs a 1/4-20 hole in steel, looks up #7 (0.201 in), and stops guessing between the 13/64 and the 7/32.
• The machinist in a hurry: the print calls out M12 × 1.75 — subtracts to 10.25, grabs the 10.2 mm bit, and never opens a handbook.
• The restorer chasing old threads: has a box of numbered bits but no chart — enters 0.25 in and learns it’s a workable 5/16-18 tap drill.
• The shop student: learns why the formula is D − 1/TPI — the hole is one full thread-pitch smaller than the outside of the thread — and why 75% is the standard, not 100%.

Accuracy & limitations

The decimals are theoretical 75%-thread holes for a sharp-V thread form; real Unified threads are slightly truncated, so actual engagement lands around 72–77% — exactly where you want it. The “use this drill” column rounds to the nearest standard bit, which can shift engagement a few percent either way — that is normal and safe. For hard or gummy materials (stainless steel, titanium), drop to 50–60% thread — use a bigger drill than the chart says — because full-depth threads in tough metal snap taps. This chart covers UNC (coarse) only; UNF fine threads, NPT pipe threads, and metric fine pitches need their own tables. Through-holes vs blind holes don’t change the drill size, but blind holes need a bottoming tap (or a plug tap run deeper) to thread to the bottom.