Drill sizes and reaming allowance
A #7 drill is 0.2010 in (5.105 mm); the nearest metric drills are 5.1 and 5.2 mm. Type any size, as a number drill, a letter, a fraction or a decimal in mm or inch, to get the drills either side of it in both systems, drawn to scale.
To ream, enter the finished size and the material: Sandvik Coromant, M.A. Ford and Tungaloy each publish the stock to leave, and the pre-drill follows. Where they differ, the maker of the reamer you are using wins.
- Drill #7
- Inch, under: #8
- Inch, over: 13/64 in
- Metric, under: 5.1 mm
- Metric, over: 5.2 mm
How it’s calculated
The size is read as typed (a number drill, a letter, a fraction of an inch or a decimal in mm or inch) and set against every drill in the chart: the drills either side are the largest under it and the smallest over it, in each family.
| Maker | Pre-drill | Stock on Ø (mm) | Drill between (mm) | Read from |
|---|---|---|---|---|
| Sandvik Coromant | 9.8 mm | 0.200 | 9.800 | 5.1–10 mm, P1–P5 |
| M.A. Ford | 9.7 mm | 0.310 – 0.330 | 9.670 – 9.690 | at Ø 10 mm, steel, under 0.35 % C; steel, over 0.35 % C; tool steel |
| Tungaloy | Tungaloy's table runs 11.5–32 mm. | |||
| Drill | in | mm | Nearest metric drill |
|---|---|---|---|
| #80 | 0.0135 | 0.343 | — |
| #79 | 0.0145 | 0.368 | — |
| #78 | 0.0160 | 0.406 | — |
| #77 | 0.0180 | 0.457 | — |
| #76 | 0.0200 | 0.508 | — |
| #75 | 0.0210 | 0.533 | — |
| #74 | 0.0225 | 0.571 | — |
| #73 | 0.0240 | 0.610 | — |
| #72 | 0.0250 | 0.635 | — |
| #71 | 0.0260 | 0.660 | — |
| #70 | 0.0280 | 0.711 | — |
| #69 | 0.0292 | 0.742 | — |
| #68 | 0.0310 | 0.787 | — |
| #67 | 0.0320 | 0.813 | — |
| #66 | 0.0330 | 0.838 | — |
| #65 | 0.0350 | 0.889 | — |
| #64 | 0.0360 | 0.914 | 1.0 mm (+0.086 mm) |
| #63 | 0.0370 | 0.940 | 1.0 mm (+0.060 mm) |
| #62 | 0.0380 | 0.965 | 1.0 mm (+0.035 mm) |
| #61 | 0.0390 | 0.991 | 1.0 mm (+0.009 mm) |
| #60 | 0.0400 | 1.016 | 1.0 mm (−0.016 mm) |
| #59 | 0.0410 | 1.041 | 1.0 mm (−0.041 mm) |
| #58 | 0.0420 | 1.067 | 1.1 mm (+0.033 mm) |
| #57 | 0.0430 | 1.092 | 1.1 mm (+0.008 mm) |
| #56 | 0.0465 | 1.181 | 1.2 mm (+0.019 mm) |
| #55 | 0.0520 | 1.321 | 1.3 mm (−0.021 mm) |
| #54 | 0.0550 | 1.397 | 1.4 mm (+0.003 mm) |
| #53 | 0.0595 | 1.511 | 1.5 mm (−0.011 mm) |
| #52 | 0.0635 | 1.613 | 1.6 mm (−0.013 mm) |
| #51 | 0.0670 | 1.702 | 1.7 mm (−0.002 mm) |
| #50 | 0.0700 | 1.778 | 1.8 mm (+0.022 mm) |
| #49 | 0.0730 | 1.854 | 1.9 mm (+0.046 mm) |
| #48 | 0.0760 | 1.930 | 1.9 mm (−0.030 mm) |
| #47 | 0.0785 | 1.994 | 2.0 mm (+0.006 mm) |
| #46 | 0.0810 | 2.057 | 2.1 mm (+0.043 mm) |
| #45 | 0.0820 | 2.083 | 2.1 mm (+0.017 mm) |
| #44 | 0.0860 | 2.184 | 2.2 mm (+0.016 mm) |
| #43 | 0.0890 | 2.261 | 2.3 mm (+0.039 mm) |
| #42 | 0.0935 | 2.375 | 2.4 mm (+0.025 mm) |
| #41 | 0.0960 | 2.438 | 2.4 mm (−0.038 mm) |
| #40 | 0.0980 | 2.489 | 2.5 mm (+0.011 mm) |
| #39 | 0.0995 | 2.527 | 2.5 mm (−0.027 mm) |
| #38 | 0.1015 | 2.578 | 2.6 mm (+0.022 mm) |
| #37 | 0.1040 | 2.642 | 2.6 mm (−0.042 mm) |
| #36 | 0.1065 | 2.705 | 2.7 mm (−0.005 mm) |
| #35 | 0.1100 | 2.794 | 2.8 mm (+0.006 mm) |
| #34 | 0.1110 | 2.819 | 2.8 mm (−0.019 mm) |
| #33 | 0.1130 | 2.870 | 2.9 mm (+0.030 mm) |
| #32 | 0.1160 | 2.946 | 2.9 mm (−0.046 mm) |
| #31 | 0.1200 | 3.048 | 3.0 mm (−0.048 mm) |
| #30 | 0.1285 | 3.264 | 3.3 mm (+0.036 mm) |
| #29 | 0.1360 | 3.454 | 3.5 mm (+0.046 mm) |
| #28 | 0.1405 | 3.569 | 3.6 mm (+0.031 mm) |
| #27 | 0.1440 | 3.658 | 3.7 mm (+0.042 mm) |
| #26 | 0.1470 | 3.734 | 3.7 mm (−0.034 mm) |
| #25 | 0.1495 | 3.797 | 3.8 mm (+0.003 mm) |
| #24 | 0.1520 | 3.861 | 3.9 mm (+0.039 mm) |
| #23 | 0.1540 | 3.912 | 3.9 mm (−0.012 mm) |
| #22 | 0.1570 | 3.988 | 4.0 mm (+0.012 mm) |
| #21 | 0.1590 | 4.039 | 4.0 mm (−0.039 mm) |
| #20 | 0.1610 | 4.089 | 4.1 mm (+0.011 mm) |
| #19 | 0.1660 | 4.216 | 4.2 mm (−0.016 mm) |
| #18 | 0.1695 | 4.305 | 4.3 mm (−0.005 mm) |
| #17 | 0.1730 | 4.394 | 4.4 mm (+0.006 mm) |
| #16 | 0.1770 | 4.496 | 4.5 mm (+0.004 mm) |
| #15 | 0.1800 | 4.572 | 4.6 mm (+0.028 mm) |
| #14 | 0.1820 | 4.623 | 4.6 mm (−0.023 mm) |
| #13 | 0.1850 | 4.699 | 4.7 mm (+0.001 mm) |
| #12 | 0.1890 | 4.801 | 4.8 mm (−0.001 mm) |
| #11 | 0.1910 | 4.851 | 4.9 mm (+0.049 mm) |
| #10 | 0.1935 | 4.915 | 4.9 mm (−0.015 mm) |
| #9 | 0.1960 | 4.978 | 5.0 mm (+0.022 mm) |
| #8 | 0.1990 | 5.055 | 5.1 mm (+0.045 mm) |
| #7 | 0.2010 | 5.105 | 5.1 mm (−0.005 mm) |
| #6 | 0.2040 | 5.182 | 5.2 mm (+0.018 mm) |
| #5 | 0.2055 | 5.220 | 5.2 mm (−0.020 mm) |
| #4 | 0.2090 | 5.309 | 5.3 mm (−0.009 mm) |
| #3 | 0.2130 | 5.410 | 5.4 mm (−0.010 mm) |
| #2 | 0.2210 | 5.613 | 5.6 mm (−0.013 mm) |
| #1 | 0.2280 | 5.791 | 5.8 mm (+0.009 mm) |
Where do number and letter drill sizes come from?
Number drills run from #80, the smallest, to #1 at 0.2280 in, and letter drills carry on from A at 0.2340 in to Z at 0.4130 in. They grew out of wire gauges, so the steps are uneven, but together with the fractional sizes they cover the inch range closely enough that a tap drill or a clearance hole can be matched to within a few thousandths.
Metric drills step by 0.1 mm through the common sizes. A drilled hole usually runs a little over the drill, which is why a reamer is left some stock: too little and it rubs and glazes the hole, too much and it chatters and runs off size.