Every standard tap drill: metric M2–M30 by the one rule that always works, inch UNC/UNF with number, letter and fractional drills at 75% and 50% thread engagement. Brand-neutral, with the arithmetic shown.
75% engagement: aluminum, brass, plastics. 50%: steel, stainless, cast iron — tapping torque drops sharply, joint strength barely changes.
| Thread | Tap drill (75%) | Closest US drill |
|---|---|---|
| M2 × 0.4 | 1.6 mm | 52 |
| M2.5 × 0.45 | 2.05 mm | 46 |
| M3 × 0.5 | 2.5 mm | 39 |
| M4 × 0.7 | 3.3 mm | 30 |
| M5 × 0.8 | 4.2 mm | 19 |
| M6 × 1.0 | 5.0 mm | 8 |
| M8 × 1.25 | 6.8 mm | H |
| M10 × 1.5 | 8.5 mm | R |
| M12 × 1.75 | 10.3 mm | 13/32 |
| M14 × 2.0 | 12.0 mm | 15/32 |
| M16 × 2.0 | 14.0 mm | 35/64 |
| M18 × 2.5 | 15.5 mm | 39/64 |
| M20 × 2.5 | 17.5 mm | 11/16 |
| M22 × 2.5 | 19.5 mm | 49/64 |
| M24 × 3.0 | 21.0 mm | 53/64 |
| M27 × 3.0 | 24.0 mm | 15/16 |
| M30 × 3.5 | 26.5 mm | 1-3/64 |
| Thread | Tap drill (75%) | Closest US drill |
|---|---|---|
| M8 × 1.0 | 7.0 mm | J |
| M10 × 1.25 | 8.8 mm | 11/32 |
| M10 × 1.0 | 9.0 mm | T |
| M12 × 1.5 | 10.5 mm | Z |
| M12 × 1.25 | 10.8 mm | 27/64 |
| M14 × 1.5 | 12.5 mm | 1/2 |
| M16 × 1.5 | 14.5 mm | 37/64 |
| M18 × 1.5 | 16.5 mm | 21/32 |
| M20 × 1.5 | 18.5 mm | 47/64 |
| M24 × 2.0 | 22.0 mm | 55/64 |
One rule covers all of it: tap drill = nominal diameter − pitch. M10 × 1.5 → 8.5 mm. No table needed once you know the pitch.
| Thread | Major ∅ | 75% — alu, brass | 50% — steel, iron |
|---|---|---|---|
| #4-40 UNC | .1120" | #43 (.0890") | #41 (.0960") |
| #5-40 UNC | .1250" | #38 (.1015") | 7/64 (.1094") |
| #6-32 UNC | .1380" | #36 (.1065") | #32 (.1160") |
| #8-32 UNC | .1640" | #29 (.1360") | #27 (.1440") |
| #10-24 UNC | .1900" | #25 (.1495") | #20 (.1610") |
| #10-32 UNF | .1900" | #21 (.1590") | #18 (.1695") |
| #12-24 UNC | .2160" | #16 (.1770") | #12 (.1890") |
| 1/4-20 UNC | .2500" | #7 (.2010") | 7/32 (.2188") |
| 1/4-28 UNF | .2500" | #3 (.2130") | #1 (.2280") |
| 5/16-18 UNC | .3125" | F (.2570") | J (.2770") |
| 5/16-24 UNF | .3125" | I (.2720") | 9/32 (.2812") |
| 3/8-16 UNC | .3750" | 5/16 (.3125") | Q (.3320") |
| 3/8-24 UNF | .3750" | Q (.3320") | S (.3480") |
| 7/16-14 UNC | .4375" | U (.3680") | 25/64 (.3906") |
| 7/16-20 UNF | .4375" | 25/64 (.3906") | 13/32 (.4062") |
| 1/2-13 UNC | .5000" | 27/64 (.4219") | 29/64 (.4531") |
| 1/2-20 UNF | .5000" | 29/64 (.4531") | 15/32 (.4688") |
| 9/16-12 UNC | .5625" | 31/64 (.4844") | 33/64 (.5156") |
| 5/8-11 UNC | .6250" | 17/32 (.5312") | 9/16 (.5625") |
| 5/8-18 UNF | .6250" | 37/64 (.5781") | 19/32 (.5938") |
| 3/4-10 UNC | .7500" | 21/32 (.6562") | 11/16 (.6875") |
| 3/4-16 UNF | .7500" | 11/16 (.6875") | 45/64 (.7031") |
Values match the standard published tap-drill charts; decimal equivalents in inches. The 75% formula: drill ≈ major − 0.974 / TPI.
Thread engagement above ~60% adds tapping torque much faster than it adds joint strength. In most steels the bolt breaks before a 50–65% nut thread strips, which is why the harder the material, the lower the sensible engagement — and why broken taps cluster in holes drilled for 75% in stainless. If the joint is critical, calculate stripping strength properly; if the tap is small and the material work-hardens, drop the engagement first.
Printable: laminate-ready tap drill chart ($5) · speeds & feeds wall chart ($5) · US ANSI/ISO reference pack ($9)
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#7 (0.201 inch) for 75% thread engagement, the standard choice in aluminum, brass and plastics. In steel, stainless and iron, 50% engagement with a 7/32 (0.2188 inch) drill taps far easier and loses little practical strength.
Tap drill = nominal diameter minus pitch, for roughly 75% thread. M10 x 1.5 gives 10 - 1.5 = 8.5 mm. This one rule covers every standard metric thread.
75% is the standard for softer materials. In harder materials 50-65% is often the better call: tapping torque drops sharply while assembled joint strength barely changes, because the bolt usually breaks before the nut threads strip. Deeper engagement mainly increases tap breakage risk.
Yes. Number drills #80-#1 and letter drills A-Z are the US wire-gauge system, used because standard fractional steps are too coarse for tap drills. Each has an exact decimal-inch equivalent, shown in the chart.