Drill size chart
Decimal-inch and millimeter equivalents for fractional, number, and letter drill bits. Use it to identify a marked bit or find the closest drill to a target diameter.
Fractional drills
| Fraction | Decimal (in) | mm |
|---|---|---|
| 1/64 | 0.0156 | 0.40 |
| 1/32 | 0.0313 | 0.79 |
| 3/64 | 0.0469 | 1.19 |
| 1/16 | 0.0625 | 1.59 |
| 5/64 | 0.0781 | 1.98 |
| 3/32 | 0.0938 | 2.38 |
| 7/64 | 0.1094 | 2.78 |
| 1/8 | 0.1250 | 3.17 |
| 9/64 | 0.1406 | 3.57 |
| 5/32 | 0.1563 | 3.97 |
| 11/64 | 0.1719 | 4.37 |
| 3/16 | 0.1875 | 4.76 |
| 13/64 | 0.2031 | 5.16 |
| 7/32 | 0.2188 | 5.56 |
| 15/64 | 0.2344 | 5.95 |
| 1/4 | 0.2500 | 6.35 |
| 17/64 | 0.2656 | 6.75 |
| 9/32 | 0.2813 | 7.14 |
| 19/64 | 0.2969 | 7.54 |
| 5/16 | 0.3125 | 7.94 |
| 21/64 | 0.3281 | 8.33 |
| 11/32 | 0.3438 | 8.73 |
| 23/64 | 0.3594 | 9.13 |
| 3/8 | 0.3750 | 9.52 |
| 25/64 | 0.3906 | 9.92 |
| 13/32 | 0.4063 | 10.32 |
| 27/64 | 0.4219 | 10.72 |
| 7/16 | 0.4375 | 11.11 |
| 29/64 | 0.4531 | 11.51 |
| 15/32 | 0.4688 | 11.91 |
| 31/64 | 0.4844 | 12.30 |
| 1/2 | 0.5000 | 12.70 |
Number drills
| Number | Decimal (in) | mm |
|---|---|---|
| #1 | 0.2280 | 5.79 |
| #2 | 0.2210 | 5.61 |
| #3 | 0.2130 | 5.41 |
| #4 | 0.2090 | 5.31 |
| #5 | 0.2055 | 5.22 |
| #6 | 0.2040 | 5.18 |
| #7 | 0.2010 | 5.11 |
| #8 | 0.1990 | 5.05 |
| #9 | 0.1960 | 4.98 |
| #10 | 0.1935 | 4.91 |
| #11 | 0.1910 | 4.85 |
| #12 | 0.1890 | 4.80 |
| #13 | 0.1850 | 4.70 |
| #14 | 0.1820 | 4.62 |
| #15 | 0.1800 | 4.57 |
| #16 | 0.1770 | 4.50 |
| #17 | 0.1730 | 4.39 |
| #18 | 0.1695 | 4.31 |
| #19 | 0.1660 | 4.22 |
| #20 | 0.1610 | 4.09 |
| #21 | 0.1590 | 4.04 |
| #22 | 0.1570 | 3.99 |
| #23 | 0.1540 | 3.91 |
| #24 | 0.1520 | 3.86 |
| #25 | 0.1495 | 3.80 |
| #26 | 0.1470 | 3.73 |
| #27 | 0.1440 | 3.66 |
| #28 | 0.1405 | 3.57 |
| #29 | 0.1360 | 3.45 |
| #30 | 0.1285 | 3.26 |
| #31 | 0.1200 | 3.05 |
| #32 | 0.1160 | 2.95 |
| #33 | 0.1130 | 2.87 |
| #34 | 0.1110 | 2.82 |
| #35 | 0.1100 | 2.79 |
| #36 | 0.1065 | 2.71 |
| #37 | 0.1040 | 2.64 |
| #38 | 0.1015 | 2.58 |
| #39 | 0.0995 | 2.53 |
| #40 | 0.0980 | 2.49 |
Letter drills
| Letter | Decimal (in) | mm |
|---|---|---|
| A | 0.234 | 5.94 |
| B | 0.238 | 6.05 |
| C | 0.242 | 6.15 |
| D | 0.246 | 6.25 |
| E | 0.250 | 6.35 |
| F | 0.257 | 6.53 |
| G | 0.261 | 6.63 |
| H | 0.266 | 6.76 |
| I | 0.272 | 6.91 |
| J | 0.277 | 7.04 |
| K | 0.281 | 7.14 |
| L | 0.290 | 7.37 |
| M | 0.295 | 7.49 |
| N | 0.302 | 7.67 |
| O | 0.316 | 8.03 |
| P | 0.323 | 8.20 |
| Q | 0.332 | 8.43 |
| R | 0.339 | 8.61 |
| S | 0.348 | 8.84 |
| T | 0.358 | 9.09 |
| U | 0.368 | 9.35 |
| V | 0.377 | 9.58 |
| W | 0.386 | 9.80 |
| X | 0.397 | 10.08 |
| Y | 0.404 | 10.26 |
| Z | 0.413 | 10.49 |
Number and letter drills fall between fractional sizes and are handy for tapping and close-fit holes. Decimal values are nominal; real bit diameters vary slightly by manufacturer and wear.
Need speeds and feeds too?
Once you have the bit size, the Drill Speed Calculator gives the right RPM and feed for your material.
The drill size systems
Fractional drills step in 64ths of an inch. Number drills run from #1 (largest) to #80 (smallest). Letter drills A to Z fill the gap just above 1/4 inch. This chart converts them all to a common decimal so you can compare across systems.
Finding the closest drill
To hit a target diameter, scan the decimal column for the nearest value. Drilling slightly undersize and reaming, or stepping up through sizes, gives a cleaner, more accurate hole than forcing one large bit.
FAQ
What is the decimal size of a 1/4 inch drill?
Exactly 0.250 inch, or 6.35 mm. The nearest letter drills are E (0.250) and F (0.257).
Do number drills get smaller as the number rises?
Yes. #1 is 0.228 in and #80 is about 0.0135 in, so a higher number means a finer bit.
How do I convert a drill size to millimeters?
Multiply the decimal-inch size by 25.4. A 0.250 in bit is 0.250 x 25.4 = 6.35 mm.
Worked example: A #7 drill is 0.2010 inch, an F letter drill is 0.2570 inch, and a 5 mm drill is 5 ÷ 25.4 ≈ 0.1969 inch. A 6.8 mm drill is about 0.2677 inch, which is close to 17/64 inch (0.2656).
Common mistakes: Do not assume number, letter and fractional sizes line up; look up the decimal. Metric and inch drills can be near-matches but not identical, which matters for tapped holes and press fits.
Related: the Fractional Drill Chart, the Tap Drill Chart and the Inches to Millimeters Converter.