Shop reference tools
Tap Drill Calculator
Estimate tap drill size from major diameter and pitch, then compare to standard drill sizes.
Tap Drill Calculator lookup
Start with the table, then use the related pages for tapping, clearance holes, pilot holes, and printable shop sheets.
| Thread | Tap drill | Decimal inch | Millimeters | Working note |
|---|---|---|---|---|
| #4-40 UNC | #43 | 0.0890 | 2.26 | Common machine-screw tap size. |
| #6-32 UNC | #36 | 0.1065 | 2.71 | Useful for light machine screws. |
| #8-32 UNC | #29 | 0.1360 | 3.45 | Check thin material and thread depth. |
| #10-24 UNC | #25 | 0.1495 | 3.80 | Coarse #10 machine screw thread. |
| #10-32 UNF | #21 | 0.1590 | 4.04 | Fine #10 machine screw thread. |
| 1/4-20 UNC | #7 | 0.2010 | 5.11 | Common quarter-inch coarse tap. |
| 1/4-28 UNF | #3 | 0.2130 | 5.41 | Fine quarter-inch tap. |
| 5/16-18 UNC | F | 0.2570 | 6.53 | Coarse 5/16 inch tap. |
| 5/16-24 UNF | I | 0.2720 | 6.91 | Fine 5/16 inch tap. |
| 3/8-16 UNC | 5/16 | 0.3125 | 7.94 | Common coarse 3/8 inch tap. |
| 3/8-24 UNF | Q | 0.3320 | 8.43 | Fine 3/8 inch tap. |
| 1/2-13 UNC | 27/64 | 0.4219 | 10.72 | Coarse half-inch tap. |
| 1/2-20 UNF | 29/64 | 0.4531 | 11.51 | Fine half-inch tap. |
| M3 x 0.5 | 2.5 mm | 0.0984 | 2.50 | Approximate metric tap drill: major diameter minus pitch. |
| M4 x 0.7 | 3.3 mm | 0.1299 | 3.30 | Use published chart for production work. |
| M5 x 0.8 | 4.2 mm | 0.1654 | 4.20 | Common metric coarse thread. |
| M6 x 1 | 5.0 mm | 0.1969 | 5.00 | Common metric coarse thread. |
| M8 x 1.25 | 6.8 mm | 0.2677 | 6.80 | Common metric coarse thread. |
| M10 x 1.5 | 8.5 mm | 0.3346 | 8.50 | Common metric coarse thread. |
| M12 x 1.75 | 10.2 mm | 0.4016 | 10.20 | Common metric coarse thread. |
Tap drill calculator
For metric threads, a common starting estimate is major diameter minus pitch. For inch threads, this calculator estimates pitch from threads per inch. Use the result as a starting point and compare it with a published chart.