Tap Drill Size Chart — Metric & UNC
Find the correct tap drill size for any thread. Drill sizes use the d − P convention (nominal diameter minus pitch), the industry-standard recommendation for approximately 75% thread engagement.
ISO Metric Coarse — Tap Drills
| Thread | Calculated (mm) | Standard Drill (mm) | Inch Drill |
|---|---|---|---|
| M1×0.25 | 0.73 | 0.8 | 1/32" |
| M1.2×0.25 | 0.93 | 1.0 | #61 |
| M1.4×0.3 | 1.08 | 1.1 | #57 |
| M1.6×0.35 | 1.22 | 1.3 | #55 |
| M1.8×0.35 | 1.42 | 1.5 | #53 |
| M2×0.4 | 1.57 | 1.6 | 1/16" |
| M2.5×0.45 | 2.01 | 2.1 | #45 |
| M3×0.5 | 2.46 | 2.5 | #40 |
| M3.5×0.6 | 2.85 | 2.9 | #33 |
| M4×0.7 | 3.24 | 3.3 | #30 |
| M5×0.8 | 4.13 | 4.2 | #19 |
| M6×1.0 | 4.92 | 5.0 | #9 |
| M7×1.0 | 5.92 | 6.0 | B |
| M8×1.25 | 6.65 | 6.8 | H |
| M9×1.25 | 7.65 | 7.8 | N |
| M10×1.5 | 8.38 | 8.5 | Q |
| M12×1.75 | 10.11 | 10.5 | Z |
| M14×2.0 | 11.84 | 12.2 | 31/64" |
| 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 | 21.0 | 53/64" |
| M27×3.0 | 23.75 | 24.0 | 15/16" |
| M30×3.5 | 26.21 | 26.5 | 1-3/64" |
| M33×3.5 | 29.21 | 29.5 | 1-5/32" |
| M36×4.0 | 31.67 | 32.0 | 1-17/64" |
| M39×4.0 | 34.67 | 35.0 | 1-3/8" |
| M42×4.5 | 37.13 | 37.5 | 1-15/32" |
| M45×4.5 | 40.13 | 40.5 | 1-19/32" |
| M48×5.0 | 42.59 | 43.0 | 1-11/16" |
| M52×5.0 | 46.59 | 47.0 | 1-27/32" |
| M56×5.5 | 50.05 | 51.0 | 2-1/64" |
| M60×5.5 | 54.05 | 55.0 | 2-11/64" |
| M64×6.0 | 57.51 | 58.0 | 2-9/32" |
| M68×6.0 | 61.51 | 62.0 | 2-7/16" |
UNC — Tap Drills
| Thread | Calculated (in) | Standard Drill | Drill (mm) |
|---|---|---|---|
| #1-64 UNC | 0.0561" | #53 | 1.51 |
| #2-56 UNC | 0.0667" | #50 | 1.78 |
| #3-48 UNC | 0.0764" | #47 | 1.99 |
| #4-40 UNC | 0.0849" | #43 | 2.26 |
| #5-40 UNC | 0.0979" | #38 | 2.58 |
| #6-32 UNC | 0.1042" | #36 | 2.71 |
| #8-32 UNC | 0.1302" | #29 | 3.45 |
| #10-24 UNC | 0.1449" | #25 | 3.80 |
| #12-24 UNC | 0.1709" | 11/64" | 4.37 |
| 1/4-20 UNC | 0.1959" | #7 | 5.11 |
| 5/16-18 UNC | 0.2524" | F | 6.53 |
| 3/8-16 UNC | 0.3073" | 5/16" | 7.94 |
| 7/16-14 UNC | 0.3602" | U | 9.35 |
| 1/2-13 UNC | 0.4167" | 27/64" | 10.72 |
| 9/16-12 UNC | 0.4723" | 31/64" | 12.30 |
| 5/8-11 UNC | 0.5266" | 17/32" | 13.49 |
| 3/4-10 UNC | 0.6418" | 21/32" | 16.67 |
| 7/8-9 UNC | 0.7547" | 49/64" | 19.45 |
| 1-8 UNC | 0.8647" | 7/8" | 22.22 |
| 1-1/8-7 UNC | 0.9704" | 63/64" | 25.00 |
| 1-1/4-7 UNC | 1.0954" | 1-7/64" | 28.18 |
| 1-3/8-6 UNC | 1.1946" | 1-7/32" | 30.96 |
| 1-1/2-6 UNC | 1.3196" | 1-11/32" | 34.13 |
| 1-3/4-5 UNC | 1.5335" | 1-9/16" | 39.69 |
| 2-4.5 UNC | 1.7594" | 1-25/32" | 45.24 |
| 2-1/4-4.5 UNC | 2.0094" | 2-1/32" | 51.59 |
| 2-1/2-4 UNC | 2.2294" | 2-1/4" | 57.15 |
| 2-3/4-4 UNC | 2.4794" | 2-1/2" | 63.50 |
| 3-4 UNC | 2.7294" | 2-3/4" | 69.85 |
| 3-1/4-4 UNC | 2.9794" | 3" | 76.20 |
| 3-1/2-4 UNC | 3.2294" | 3-1/4" | 82.55 |
| 3-3/4-4 UNC | 3.4794" | 3-1/2" | 88.90 |
| 4-4 UNC | 3.7294" | 3-3/4" | 95.25 |
How Tap Drill Sizes Are Determined
The recommended tap drill is d − P (nominal diameter minus one pitch). This gives approximately 75% thread engagement — the industry-standard balance of joint strength against tapping torque and risk of tap breakage.
For metric threads: tap drill = d − P. For unified inch threads, refer to the published ASME B1.1 tap drill table.
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. Use d − P = 10 − 1.5 = 8.5 mm; round up to the next standard size ≥ 8.5 mm = 8.5 mm.
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 − pitch (d − P). For inch threads, use the published ASME B1.1 tap drill table. Always round UP to the next standard size.