Letter drill chart
Letter drill sizes A through Z with decimal-inch and millimeter equivalents. The letter series covers the range just above 1/4 inch, filling the gaps between fractional sizes from 0.234 to 0.413 inch.
Letter drill sizes
| Letter | Decimal (in) | mm |
|---|---|---|
| A | 0.2340 | 5.944 |
| B | 0.2380 | 6.045 |
| C | 0.2420 | 6.147 |
| D | 0.2460 | 6.248 |
| E | 0.2500 | 6.350 |
| F | 0.2570 | 6.528 |
| G | 0.2610 | 6.629 |
| H | 0.2660 | 6.756 |
| I | 0.2720 | 6.909 |
| J | 0.2770 | 7.036 |
| K | 0.2810 | 7.137 |
| L | 0.2900 | 7.366 |
| M | 0.2950 | 7.493 |
| N | 0.3020 | 7.671 |
| O | 0.3160 | 8.026 |
| P | 0.3230 | 8.204 |
| Q | 0.3320 | 8.433 |
| R | 0.3390 | 8.611 |
| S | 0.3480 | 8.839 |
| T | 0.3580 | 9.093 |
| U | 0.3680 | 9.347 |
| V | 0.3770 | 9.576 |
| W | 0.3860 | 9.804 |
| X | 0.3970 | 10.084 |
| Y | 0.4040 | 10.262 |
| Z | 0.4130 | 10.490 |
Letter drills run from A, the smallest at 0.234 inch, up to Z at 0.413 inch, getting larger as the alphabet advances. They overlap the larger number drills and the fractional series, giving fine control over hole size in the quarter-inch-and-up range often needed for tap drills.
Need the number drills or a tap drill?
See the full Drill Size Chart for number and fractional sizes, or the Tap Drill Calculator for threads.
Where letter drills are used
Letter drills shine when a hole needs to land between fractional sizes in the range above a quarter inch, which happens often for tap drills on larger threads and for clearance holes that must be just so. An F drill (0.257 in), for example, is the standard tap drill for a 5/16-18 thread, sitting neatly between 1/4 and 5/16 inch.
Letter, number, and fractional together
The three series interleave to give a near-continuous range of sizes. Below about 0.234 inch the number drills take over; above 0.413 the fractional series continues alone. In the overlap, you simply pick whichever series has the exact size you need from the chart.
FAQ
What size is a letter F drill?
0.257 inch (6.528 mm). It is the common tap drill for a 5/16-18 thread.
Are letter drills bigger than number drills?
Yes. Letter drills start at 0.234 inch, just above the largest number drill (#1 at 0.228 inch), and run up to 0.413 inch.
Which letter drill is the largest?
Z, at 0.413 inch (10.490 mm). The series runs A through Z from smallest to largest.