Tap Drill Size Chart
Find the correct tap drill size for any thread. Drill sizes are calculated from internal minor diameter (D₁) per ISO 724 and ASME B1.1, giving approximately 75% thread engagement.
ISO Metric Coarse — Tap Drills
| Thread | Calculated (mm) | Standard Drill (mm) | Inch Drill |
|---|---|---|---|
| M1×0.25 | 0.73 | 0.7 | #69 |
| M1.2×0.25 | 0.93 | 0.9 | #63 |
| M1.4×0.3 | 1.08 | 1.1 | #57 |
| M1.6×0.35 | 1.22 | 1.2 | 3/64" |
| M1.8×0.35 | 1.42 | 1.4 | #54 |
| M2×0.4 | 1.57 | 1.6 | 1/16" |
| M2.5×0.45 | 2.01 | 2.0 | #47 |
| M3×0.5 | 2.46 | 2.5 | #41 |
| M3.5×0.6 | 2.85 | 2.9 | #33 |
| M4×0.7 | 3.24 | 3.2 | #30 |
| M5×0.8 | 4.13 | 4.1 | #20 |
| M6×1.0 | 4.92 | 5.0 | #10 |
| M7×1.0 | 5.92 | 6.0 | A |
| M8×1.25 | 6.65 | 6.8 | G |
| M9×1.25 | 7.65 | 7.5 | N |
| M10×1.5 | 8.38 | 8.5 | 21/64" |
| M12×1.75 | 10.11 | 10.2 | X |
| M14×2.0 | 11.84 | 12.0 | 15/32" |
| M16×2.0 | 13.84 | 14.0 | 35/64" |
| M18×2.5 | 15.29 | 15.5 | 39/64" |
| M20×2.5 | 17.29 | 17.5 | 11/16" |
| M22×2.5 | 19.29 | 19.5 | 49/64" |
| M24×3.0 | 20.75 | 20.5 | 13/16" |
| M27×3.0 | 23.75 | 24.0 | 15/16" |
| M30×3.5 | 26.21 | 26.0 | 1-1/32" |
| M33×3.5 | 29.21 | 29.0 | 1-5/32" |
| M36×4.0 | 31.67 | 32.0 | 1-1/4" |
| M39×4.0 | 34.67 | 35.0 | 1-23/64" |
| M42×4.5 | 37.13 | 38.0 | 1-15/32" |
| M45×4.5 | 40.13 | 40.0 | 1-37/64" |
| M48×5.0 | 42.59 | 42.0 | 1-43/64" |
| M52×5.0 | 46.59 | 46.0 | 1-53/64" |
| M56×5.5 | 50.05 | 50.0 | 1-31/32" |
| M60×5.5 | 54.05 | 55.0 | 2-1/8" |
| M64×6.0 | 57.51 | 58.0 | 2-17/64" |
| M68×6.0 | 61.51 | 62.0 | 2-27/64" |
UNC — Tap Drills
| Thread | Calculated (in) | Standard Drill | Drill (mm) |
|---|---|---|---|
| #1-64 UNC | 0.0561" | #54 | 1.4 |
| #2-56 UNC | 0.0667" | #51 | 1.7 |
| #3-48 UNC | 0.0764" | #48 | 1.93 |
| #4-40 UNC | 0.0849" | #44 | 2.18 |
| #5-40 UNC | 0.0979" | #40 | 2.49 |
| #6-32 UNC | 0.1042" | #37 | 2.64 |
| #8-32 UNC | 0.1302" | #30 | 3.26 |
| #10-24 UNC | 0.1449" | #27 | 3.66 |
| #12-24 UNC | 0.1709" | 11/64" | 4.37 |
| 1/4-20 UNC | 0.1959" | #9 | 4.98 |
| 5/16-18 UNC | 0.2524" | E | 6.35 |
| 3/8-16 UNC | 0.3073" | 5/16" | 7.94 |
| 7/16-14 UNC | 0.3602" | 23/64" | 9.13 |
| 1/2-13 UNC | 0.4167" | Z | 10.49 |
| 9/16-12 UNC | 0.4723" | 15/32" | 11.91 |
| 5/8-11 UNC | 0.5266" | 17/32" | 13.49 |
| 3/4-10 UNC | 0.6418" | 41/64" | 16.27 |
| 7/8-9 UNC | 0.7547" | 3/4" | 19.05 |
| 1-8 UNC | 0.8647" | 55/64" | 21.83 |
| 1-1/8-7 UNC | 0.9704" | 31/32" | 24.61 |
| 1-1/4-7 UNC | 1.0954" | 1-3/32" | 27.78 |
| 1-3/8-6 UNC | 1.1946" | 1-3/16" | 30.16 |
| 1-1/2-6 UNC | 1.3196" | 1-5/16" | 33.34 |
| 1-3/4-5 UNC | 1.5335" | 1-17/32" | 38.89 |
| 2-4.5 UNC | 1.7594" | 1-49/64" | 44.85 |
| 2-1/4-4.5 UNC | 2.0094" | 2-1/64" | 51.2 |
| 2-1/2-4 UNC | 2.2294" | 2-15/64" | 56.75 |
| 2-3/4-4 UNC | 2.4794" | 2-31/64" | 63.1 |
| 3-4 UNC | 2.7294" | 2-47/64" | 69.45 |
| 3-1/4-4 UNC | 2.9794" | 2-63/64" | 75.8 |
| 3-1/2-4 UNC | 3.2294" | 3-15/64" | 82.15 |
| 3-3/4-4 UNC | 3.4794" | 3-31/64" | 88.5 |
| 4-4 UNC | 3.7294" | 3-47/64" | 94.85 |
How Tap Drill Sizes Are Determined
The tap drill diameter equals the internal minor diameter (D₁) of the thread, which produces approximately 75% thread engagement — the industry-standard recommendation. Higher thread engagement (smaller drill) increases tapping torque and risk of tap breakage without significantly increasing joint strength.
For metric threads: D₁ = d − 1.0825 × P. For unified inch threads: D₁ = d − 1.0825 × (1/TPI).
Frequently Asked Questions
What size drill bit do I need for an M10 bolt?
For an M10×1.5 coarse thread, use an 8.5mm tap drill. This is based on the internal minor diameter D1 = 10 - 1.0825 × 1.5 = 8.38mm, rounded to the nearest standard drill size.
What is 75% thread engagement?
75% thread engagement means the thread depth is 75% of the full theoretical depth. This is the industry standard because going higher (e.g., 100%) doubles tapping torque and risk of tap breakage while only increasing strength by about 5%.
How do I calculate a tap drill size?
For metric threads: tap drill = nominal diameter - (1.0825 × pitch). For inch threads: tap drill = nominal diameter - (1.0825 / TPI). Then round to the nearest standard drill size.