Tap Drill Calculator & Chart
A tap drill estimate depends on thread pitch and the selected engagement. Choose an available drill using the tap and material manufacturer recommendations.
Maintained by mmtoinches.im · Updated
NIST describes the standard inch as exactly equivalent to 25.4 mm
. NIST length definition. ASME — unified inch screw threads. Method and limitations.
Need mm to inch drill bit conversions? Visit Drill Bit Size Conversion.
Tap Drill Calculator
Compute recommended tap drill using an engagement slider. Formula: drill = major − (1.082 × pitch × engagement). For imperial, pitch = 1 ÷ TPI.
Nearest Standard Bit Mapping
For critical fits, consult manufacturer charts. Engagement slider uses 60° V-thread geometry with factor 1.082; actual fit depends on tap class and hole quality.
Tap Drill Chart (UNC/UNF & Metric)
Common thread combinations and recommended drills. Adjust per manufacturer guidance and material.
Open thread conversion| Thread | Recommended Drill | Inches | mm |
|---|---|---|---|
| #6-32 UNC | #36 | 0.1065" | 2.705 mm |
| #8-32 UNC | #29 | 0.1360" | 3.454 mm |
| #10-24 UNC | #25 | 0.1495" | 3.797 mm |
| #10-32 UNF | #21 | 0.1590" | 4.039 mm |
| 1/4-20 UNC | #7 | 0.2010" | 5.105 mm |
| 5/16-18 UNC | F | 0.2570" | 6.528 mm |
| 3/8-16 UNC | 5/16" | 0.3125" | 7.938 mm |
| M3×0.5 | 2.5 mm | 0.0984" | 2.499 mm |
| M4×0.7 | 3.3 mm | 0.1299" | 3.299 mm |
| M5×0.8 | 4.2 mm | 0.1654" | 4.201 mm |
| M6×1.0 | 5.0 mm | 0.1969" | 5.001 mm |
| M8×1.25 | 6.8 mm | 0.2677" | 6.8 mm |
| #4-40 UNC | #43 | 0.0890" | 2.261 mm |
| #6-40 UNF | #33 | 0.1130" | 2.87 mm |
| #8-36 UNF | #28 | 0.1405" | 3.569 mm |
| 1/4-28 UNF | #3 | 0.2130" | 5.41 mm |
| 5/16-24 UNF | I | 0.2720" | 6.909 mm |
| 3/8-24 UNF | Q | 0.3320" | 8.433 mm |
| 7/16-14 UNC | 23/64 | 0.3594" | 9.129 mm |
| 7/16-20 UNF | U | 0.3680" | 9.347 mm |
| 1/2-13 UNC | 27/64 | 0.4219" | 10.716 mm |
| 1/2-20 UNF | 29/64 | 0.4531" | 11.509 mm |
| M2×0.4 | 1.6 mm | 0.0630" | 1.6 mm |
| M10×1.5 | 8.5 mm | 0.3346" | 8.499 mm |
| M12×1.75 | 10.2 mm | 0.4016" | 10.201 mm |
| M3×0.35 (fine) | 2.65 mm | 0.1043" | 2.649 mm |
| M4×0.5 (fine) | 3.5 mm | 0.1378" | 3.5 mm |
| M5×0.5 (fine) | 4.5 mm | 0.1772" | 4.501 mm |
| M6×0.75 (fine) | 5.2 mm | 0.2047" | 5.199 mm |
| M8×1.0 (fine) | 7.0 mm | 0.2756" | 7 mm |
| M10×1.25 (fine) | 8.8 mm | 0.3465" | 8.801 mm |
| M12×1.5 (fine) | 10.5 mm | 0.4134" | 10.5 mm |
| M14×1.0 (fine) | 13.0 mm | 0.5118" | 13 mm |
| M16×1.5 (fine) | 14.5 mm | 0.5709" | 14.501 mm |
| M20×1.5 (fine) | 18.5 mm | 0.7283" | 18.499 mm |
| M18×1.0 (fine) | 17.0 mm | 0.6693" | 17 mm |
| M22×1.5 (fine) | 20.5 mm | 0.8068" | 20.493 mm |
Note: Manufacturer and material recommendations may vary. For high precision, consult official charts or test on scrap.
ISO tolerance note: Metric threads commonly use classes like 6H (internal) and 6g (external). Tap drill sizes are approximate; final fit depends on ISO 261/965 tolerances, tap class, and hole quality.
Sources and calculation notes
A tap drill estimate depends on thread pitch and the selected engagement. Choose an available drill using the tap and material manufacturer recommendations.
Method: Divide millimeters by 25.4 to obtain inches. The factor is exact; displayed decimal values are rounded. Fractional inches are rounded to the selected denominator.
Scope: Calculated engagement and reference charts are planning aids, not a guarantee of thread strength. Material, tap type and hole tolerances affect the result.
Content maintained by mmtoinches.im. Updated and reference notes reviewed . How we calculate and present measurements. Sources support the stated definitions; examples and calculators are not certification of a product or compliance with a standard.