Use decimal equivalents to compare drill sizes across naming systems. This reference includes fractions in 1/64-inch steps through 1/2 inch, A–Z letter drills, selected common number sizes and a full #1–#80 table below.
| Fraction (unreduced) | Decimal in | mm |
|---|---|---|
| 1/64 | 0.015625 | 0.39687 |
| 2/64 | 0.03125 | 0.79375 |
| 3/64 | 0.046875 | 1.19062 |
| 4/64 | 0.0625 | 1.5875 |
| 5/64 | 0.078125 | 1.98438 |
| 6/64 | 0.09375 | 2.38125 |
| 7/64 | 0.109375 | 2.77812 |
| 8/64 | 0.125 | 3.175 |
| 9/64 | 0.140625 | 3.57187 |
| 10/64 | 0.15625 | 3.96875 |
| 11/64 | 0.171875 | 4.36562 |
| 12/64 | 0.1875 | 4.7625 |
| 13/64 | 0.203125 | 5.15937 |
| 14/64 | 0.21875 | 5.55625 |
| 15/64 | 0.234375 | 5.95312 |
| 16/64 | 0.25 | 6.35 |
| 17/64 | 0.265625 | 6.74687 |
| 18/64 | 0.28125 | 7.14375 |
| 19/64 | 0.296875 | 7.54062 |
| 20/64 | 0.3125 | 7.9375 |
| 21/64 | 0.328125 | 8.33437 |
| 22/64 | 0.34375 | 8.73125 |
| 23/64 | 0.359375 | 9.12812 |
| 24/64 | 0.375 | 9.525 |
| 25/64 | 0.390625 | 9.92188 |
| 26/64 | 0.40625 | 10.31875 |
| 27/64 | 0.421875 | 10.71562 |
| 28/64 | 0.4375 | 11.1125 |
| 29/64 | 0.453125 | 11.50937 |
| 30/64 | 0.46875 | 11.90625 |
| 31/64 | 0.484375 | 12.30312 |
| 32/64 | 0.5 | 12.7 |
| Letter | Decimal in | mm |
|---|---|---|
| A | 0.234 | 5.9436 |
| B | 0.238 | 6.0452 |
| C | 0.242 | 6.1468 |
| D | 0.246 | 6.2484 |
| E | 0.25 | 6.35 |
| F | 0.257 | 6.5278 |
| G | 0.261 | 6.6294 |
| H | 0.266 | 6.7564 |
| I | 0.272 | 6.9088 |
| J | 0.277 | 7.0358 |
| K | 0.281 | 7.1374 |
| L | 0.29 | 7.366 |
| M | 0.295 | 7.493 |
| N | 0.302 | 7.6708 |
| O | 0.316 | 8.0264 |
| P | 0.323 | 8.2042 |
| Q | 0.332 | 8.4328 |
| R | 0.339 | 8.6106 |
| S | 0.348 | 8.8392 |
| T | 0.358 | 9.0932 |
| U | 0.368 | 9.3472 |
| V | 0.377 | 9.5758 |
| W | 0.386 | 9.8044 |
| X | 0.397 | 10.0838 |
| Y | 0.404 | 10.2616 |
| Z | 0.413 | 10.4902 |
| Number drill | Decimal in | mm |
|---|---|---|
| #80 | 0.0135 | 0.3429 |
| #70 | 0.028 | 0.7112 |
| #60 | 0.04 | 1.016 |
| #55 | 0.052 | 1.3208 |
| #50 | 0.07 | 1.778 |
| #45 | 0.082 | 2.0828 |
| #43 | 0.089 | 2.2606 |
| #40 | 0.098 | 2.4892 |
| #36 | 0.1065 | 2.7051 |
| #35 | 0.11 | 2.794 |
| #30 | 0.1285 | 3.2639 |
| #29 | 0.136 | 3.4544 |
| #25 | 0.1495 | 3.7973 |
| #21 | 0.159 | 4.0386 |
| #20 | 0.161 | 4.0894 |
| #16 | 0.177 | 4.4958 |
| #10 | 0.1935 | 4.9149 |
| #7 | 0.201 | 5.1054 |
| #3 | 0.213 | 5.4102 |
| #1 | 0.228 | 5.7912 |
Nominal equivalents are not interchangeable tolerances
Multiply decimal inches by 25.4 to obtain millimeters. For example, a 1/4-inch drill is exactly 6.35 mm nominal, while a 6.5 mm drill is larger by 0.15 mm. Whether that difference is acceptable depends on the drawing and process; rounding a conversion does not authorize substitution.
Fractions are shown in sixty-fourths to make the progression visible: 16/64 is 1/4 and 32/64 is 1/2. Letter and number series do not follow the same increment. A larger number-drill designation generally means a smaller drill, whereas the letter series grows from A to Z.
Select a drill for the job, not just the nearest number
A tapping pre-hole, a bolt clearance hole and a reaming pre-hole have different requirements. For a #7 drill, the nominal size is 0.201 in or 5.1054 mm; its appearance in a tap chart does not make it a general-purpose clearance for every fastener near that diameter.
Check material, drill construction, coating, coolant delivery and available flute length. Hole size can differ from nominal drill diameter because of runout, point asymmetry, workpiece movement and wear. Reaming or boring may be needed for tighter finished-hole requirements.
Scope of this chart
The first number-drill table is a quick selection; the expanded #1–#80 table below provides the complete numbered series covered here. Confirm the tool’s marked size and the supplier’s dimensional listing before purchase or setup. Use a calibrated inspection method for the finished feature, not only the size printed on the drill case.
Complete number-drill reference: #1 through #80
This expands the selected number sizes above. Decimal-inch designations are checked against Champion’s numbered-drill product listing and its linked decimal-equivalency reference. Millimeter values are calculated as inches × 25.4. These are nominal sizes, not finished-hole tolerances.
| No. | in | mm | No. | in | mm |
|---|---|---|---|---|---|
| 1 | 0.2280 | 5.7912 | 41 | 0.0960 | 2.4384 |
| 2 | 0.2210 | 5.6134 | 42 | 0.0935 | 2.3749 |
| 3 | 0.2130 | 5.4102 | 43 | 0.0890 | 2.2606 |
| 4 | 0.2090 | 5.3086 | 44 | 0.0860 | 2.1844 |
| 5 | 0.2055 | 5.2197 | 45 | 0.0820 | 2.0828 |
| 6 | 0.2040 | 5.1816 | 46 | 0.0810 | 2.0574 |
| 7 | 0.2010 | 5.1054 | 47 | 0.0785 | 1.9939 |
| 8 | 0.1990 | 5.0546 | 48 | 0.0760 | 1.9304 |
| 9 | 0.1960 | 4.9784 | 49 | 0.0730 | 1.8542 |
| 10 | 0.1935 | 4.9149 | 50 | 0.0700 | 1.7780 |
| 11 | 0.1910 | 4.8514 | 51 | 0.0670 | 1.7018 |
| 12 | 0.1890 | 4.8006 | 52 | 0.0635 | 1.6129 |
| 13 | 0.1850 | 4.6990 | 53 | 0.0595 | 1.5113 |
| 14 | 0.1820 | 4.6228 | 54 | 0.0550 | 1.3970 |
| 15 | 0.1800 | 4.5720 | 55 | 0.0520 | 1.3208 |
| 16 | 0.1770 | 4.4958 | 56 | 0.0465 | 1.1811 |
| 17 | 0.1730 | 4.3942 | 57 | 0.0430 | 1.0922 |
| 18 | 0.1695 | 4.3053 | 58 | 0.0420 | 1.0668 |
| 19 | 0.1660 | 4.2164 | 59 | 0.0410 | 1.0414 |
| 20 | 0.1610 | 4.0894 | 60 | 0.0400 | 1.0160 |
| 21 | 0.1590 | 4.0386 | 61 | 0.0390 | 0.9906 |
| 22 | 0.1570 | 3.9878 | 62 | 0.0380 | 0.9652 |
| 23 | 0.1540 | 3.9116 | 63 | 0.0370 | 0.9398 |
| 24 | 0.1520 | 3.8608 | 64 | 0.0360 | 0.9144 |
| 25 | 0.1495 | 3.7973 | 65 | 0.0350 | 0.8890 |
| 26 | 0.1470 | 3.7338 | 66 | 0.0330 | 0.8382 |
| 27 | 0.1440 | 3.6576 | 67 | 0.0320 | 0.8128 |
| 28 | 0.1405 | 3.5687 | 68 | 0.0310 | 0.7874 |
| 29 | 0.1360 | 3.4544 | 69 | 0.0292 | 0.7417 |
| 30 | 0.1285 | 3.2639 | 70 | 0.0280 | 0.7112 |
| 31 | 0.1200 | 3.0480 | 71 | 0.0260 | 0.6604 |
| 32 | 0.1160 | 2.9464 | 72 | 0.0250 | 0.6350 |
| 33 | 0.1130 | 2.8702 | 73 | 0.0240 | 0.6096 |
| 34 | 0.1110 | 2.8194 | 74 | 0.0225 | 0.5715 |
| 35 | 0.1100 | 2.7940 | 75 | 0.0210 | 0.5334 |
| 36 | 0.1065 | 2.7051 | 76 | 0.0200 | 0.5080 |
| 37 | 0.1040 | 2.6416 | 77 | 0.0180 | 0.4572 |
| 38 | 0.1015 | 2.5781 | 78 | 0.0160 | 0.4064 |
| 39 | 0.0995 | 2.5273 | 79 | 0.0145 | 0.3683 |
| 40 | 0.0980 | 2.4892 | 80 | 0.0135 | 0.3429 |
Read the reduced fraction on the drill case
The fraction progression above uses sixty-fourths; the tool label usually uses the reduced fraction. For example, 8/64 = 1/8, 12/64 = 3/16, 16/64 = 1/4, 20/64 = 5/16, 24/64 = 3/8 and 32/64 = 1/2. Reducing the fraction changes the notation, not the diameter.
For a hole with a design range of 5.00–5.05 mm, a #8 drill is nominally 5.0546 mm and a #9 is 4.9784 mm. Neither nominal size falls inside that interval. Picking whichever looks nearest is not a tolerance check, and actual drilling variation still applies even if the nominal tool size is inside the interval. Select the process and verify the finished hole.
Content updated October 8, 2026. Sources and numerical examples are explained above; examples are not production test results. Editorial method · Report a correction.
References and verification
Original explanations and worked examples by MfgWorkbench. Sources below support the stated concepts or selected reference data; they do not certify every example or a production application.
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