Reference chart
Drill size chart
Decimal-inch and millimetre equivalents for fractional, number and letter drill sizes. For the hole to drill before tapping a thread, use the tap drill size calculator. Each row also shows the Unified threads that drill actually taps, and how much thread it leaves - so you can read the table in either direction.
Fractional drill sizes
| Fraction | Decimal (in) | mm | Tap drill for |
|---|---|---|---|
| 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 | #3-48 UNC (77%) |
| 3/32" | 0.0938 | 2.38 | #4-48 UNF (67%) |
| 7/64" | 0.1094 | 2.78 | #6-32 UNC (71%) |
| 1/8" | 0.1250 | 3.17 | — |
| 9/64" | 0.1406 | 3.57 | #8-36 UNF (65%) |
| 5/32" | 0.1563 | 3.97 | #10-32 UNF (83%), #10-24 UNC (62%) |
| 11/64" | 0.1719 | 4.37 | #12-24 UNC (82%) |
| 3/16" | 0.1875 | 4.76 | #12-28 UNF (61%) |
| 13/64" | 0.2031 | 5.16 | 1/4-20 UNC (72%) |
| 7/32" | 0.2188 | 5.56 | 1/4-28 UNF (67%) |
| 15/64" | 0.2344 | 5.95 | — |
| 1/4" | 0.2500 | 6.35 | — |
| 17/64" | 0.2656 | 6.75 | 5/16-18 UNC (65%) |
| 9/32" | 0.2813 | 7.14 | — |
| 19/64" | 0.2969 | 7.54 | — |
| 5/16" | 0.3125 | 7.94 | 3/8-16 UNC (77%) |
| 21/64" | 0.3281 | 8.33 | — |
| 11/32" | 0.3438 | 8.73 | — |
| 23/64" | 0.3594 | 9.13 | 7/16-14 UNC (84%) |
| 3/8" | 0.3750 | 9.52 | 7/16-14 UNC (67%) |
| 25/64" | 0.3906 | 9.92 | 7/16-20 UNF (72%) |
| 13/32" | 0.4063 | 10.32 | — |
| 27/64" | 0.4219 | 10.72 | 1/2-13 UNC (78%) |
| 7/16" | 0.4375 | 11.11 | 1/2-13 UNC (63%) |
| 29/64" | 0.4531 | 11.51 | 1/2-20 UNF (72%) |
| 15/32" | 0.4688 | 11.91 | — |
| 31/64" | 0.4844 | 12.30 | 9/16-12 UNC (72%) |
| 1/2" | 0.5000 | 12.70 | — |
Number drill sizes (#1 to #40)
| Size | Decimal (in) | mm | Tap drill for |
|---|---|---|---|
| #1 | 0.2280 | 5.79 | — |
| #2 | 0.2210 | 5.61 | 1/4-28 UNF (63%) |
| #3 | 0.2130 | 5.41 | 1/4-28 UNF (80%) |
| #4 | 0.2090 | 5.31 | 1/4-20 UNC (63%) |
| #5 | 0.2055 | 5.22 | 1/4-20 UNC (69%) |
| #6 | 0.2040 | 5.18 | 1/4-20 UNC (71%) |
| #7 | 0.2010 | 5.11 | 1/4-20 UNC (75%) |
| #8 | 0.1990 | 5.05 | 1/4-20 UNC (79%) |
| #9 | 0.1960 | 4.98 | 1/4-20 UNC (83%) |
| #10 | 0.1935 | 4.91 | — |
| #11 | 0.1910 | 4.85 | — |
| #12 | 0.1890 | 4.80 | — |
| #13 | 0.1850 | 4.70 | #12-28 UNF (67%) |
| #14 | 0.1820 | 4.62 | #12-28 UNF (73%), #12-24 UNC (63%) |
| #15 | 0.1800 | 4.57 | #12-28 UNF (78%), #12-24 UNC (67%) |
| #16 | 0.1770 | 4.50 | #12-24 UNC (72%), #12-28 UNF (84%) |
| #17 | 0.1730 | 4.39 | #12-24 UNC (79%) |
| #18 | 0.1695 | 4.31 | — |
| #19 | 0.1660 | 4.22 | — |
| #20 | 0.1610 | 4.09 | #10-32 UNF (71%) |
| #21 | 0.1590 | 4.04 | #10-32 UNF (76%) |
| #22 | 0.1570 | 3.99 | #10-32 UNF (81%), #10-24 UNC (61%) |
| #23 | 0.1540 | 3.91 | #10-24 UNC (67%) |
| #24 | 0.1520 | 3.86 | #10-24 UNC (70%) |
| #25 | 0.1495 | 3.80 | #10-24 UNC (75%) |
| #26 | 0.1470 | 3.73 | #10-24 UNC (79%) |
| #27 | 0.1440 | 3.66 | #10-24 UNC (85%) |
| #28 | 0.1405 | 3.57 | #8-36 UNF (65%) |
| #29 | 0.1360 | 3.45 | #8-36 UNF (78%), #8-32 UNC (69%) |
| #30 | 0.1285 | 3.26 | — |
| #31 | 0.1200 | 3.05 | — |
| #32 | 0.1160 | 2.95 | #6-40 UNF (68%) |
| #33 | 0.1130 | 2.87 | #6-40 UNF (77%), #6-32 UNC (62%) |
| #34 | 0.1110 | 2.82 | #6-40 UNF (83%), #6-32 UNC (67%) |
| #35 | 0.1100 | 2.79 | #6-32 UNC (69%) |
| #36 | 0.1065 | 2.71 | #6-32 UNC (78%), #5-44 UNF (63%) |
| #37 | 0.1040 | 2.64 | #5-44 UNF (71%), #6-32 UNC (84%) |
| #38 | 0.1015 | 2.58 | #5-40 UNC (72%), #5-44 UNF (80%) |
| #39 | 0.0995 | 2.53 | #5-40 UNC (79%) |
| #40 | 0.0980 | 2.49 | #5-40 UNC (83%) |
Letter drill sizes (A to Z)
| Size | Decimal (in) | mm | Tap drill for |
|---|---|---|---|
| A | 0.2340 | 5.94 | — |
| B | 0.2380 | 6.05 | — |
| C | 0.2420 | 6.15 | — |
| D | 0.2460 | 6.25 | — |
| E | 0.2500 | 6.35 | — |
| F | 0.2570 | 6.53 | 5/16-18 UNC (77%) |
| G | 0.2610 | 6.63 | 5/16-18 UNC (71%) |
| H | 0.2660 | 6.76 | 5/16-18 UNC (64%) |
| I | 0.2720 | 6.91 | 5/16-24 UNF (75%) |
| J | 0.2770 | 7.04 | 5/16-24 UNF (66%) |
| K | 0.2810 | 7.14 | — |
| L | 0.2900 | 7.37 | — |
| M | 0.2950 | 7.49 | — |
| N | 0.3020 | 7.67 | — |
| O | 0.3160 | 8.03 | 3/8-16 UNC (73%) |
| P | 0.3230 | 8.20 | 3/8-16 UNC (64%) |
| Q | 0.3320 | 8.43 | 3/8-24 UNF (79%) |
| R | 0.3390 | 8.61 | 3/8-24 UNF (67%) |
| S | 0.3480 | 8.84 | — |
| T | 0.3580 | 9.09 | — |
| U | 0.3680 | 9.35 | 7/16-14 UNC (75%) |
| V | 0.3770 | 9.58 | 7/16-14 UNC (65%) |
| W | 0.3860 | 9.80 | 7/16-20 UNF (79%) |
| X | 0.3970 | 10.08 | 7/16-20 UNF (62%) |
| Y | 0.4040 | 10.26 | — |
| Z | 0.4130 | 10.49 | — |
Number and letter sizes are the standard published decimal equivalents; fractional sizes are exact (the fraction times one inch). Millimetre values are the decimal inch times 25.4.
The tap drill column is computed, not looked up: for every drill we take each UNC and UNF thread in the common range and work out the engagement that drill would leave, keeping any between 60% and 85% and showing the two closest to the usual 75%. Of the 98 sizes here, 63 are the tap drill for at least one standard thread. Below about 60% the thread is weak; above about 85% the tap is fighting the hole and likely to break. A dash means no common Unified thread lands in that band for that drill.