Use a drill-size chart to identify a bit by its designation, decimal inch size, and millimetre size. A number drill, letter drill, fractional drill, and metric drill can be close without being equal. The “nearest alternate” column below is a comparison aid, not permission to treat two different diameters as interchangeable in a precision hole.
Search the drill-size chart
| Designation | Type | Decimal inch | Millimetres | Nearest common alternate |
|---|---|---|---|---|
| #80 | Number | 0.0135 | 0.34 | 0.35 mm, difference 0.01 mm |
| #70 | Number | 0.0280 | 0.71 | 0.7 mm, difference 0.01 mm |
| #60 | Number | 0.0400 | 1.02 | 1.0 mm, difference 0.02 mm |
| 1/16 | Fractional | 0.0625 | 1.59 | 1.6 mm, difference 0.01 mm |
| #50 | Number | 0.0700 | 1.78 | 1.8 mm, difference 0.02 mm |
| 3/32 | Fractional | 0.0938 | 2.38 | 2.4 mm, difference 0.02 mm |
| #40 | Number | 0.0980 | 2.49 | 2.5 mm, difference 0.01 mm |
| 1/8 | Fractional | 0.1250 | 3.18 | 3.2 mm, difference 0.02 mm |
| #30 | Number | 0.1285 | 3.26 | 3.3 mm, difference 0.04 mm |
| 9/64 | Fractional | 0.1406 | 3.57 | 3.6 mm, difference 0.03 mm |
| 5/32 | Fractional | 0.1563 | 3.97 | 4.0 mm, difference 0.03 mm |
| #21 | Number | 0.1590 | 4.04 | 4.0 mm, difference 0.04 mm |
| #20 | Number | 0.1610 | 4.09 | 4.1 mm, difference 0.01 mm |
| 3/16 | Fractional | 0.1875 | 4.76 | 4.8 mm, difference 0.04 mm |
| #13 | Number | 0.1850 | 4.70 | 3/16 in, difference 0.06 mm |
| #10 | Number | 0.1935 | 4.92 | 5.0 mm, difference 0.08 mm |
| 13/64 | Fractional | 0.2031 | 5.16 | #6 at 5.18 mm, difference 0.02 mm |
| #7 | Number | 0.2010 | 5.11 | 5.1 mm, difference 0.01 mm |
| 1/4 | Fractional | 0.2500 | 6.35 | F at 6.53 mm, difference 0.18 mm |
| F | Letter | 0.2570 | 6.53 | 1/4 in, difference 0.18 mm |
| H | Letter | 0.2660 | 6.75 | 17/64 in, difference 0.09 mm |
| 9/32 | Fractional | 0.2813 | 7.14 | K at 7.14 mm, difference 0.00 mm rounded |
| K | Letter | 0.2810 | 7.14 | 9/32 in, difference 0.01 mm |
| 5/16 | Fractional | 0.3125 | 7.94 | O at 8.03 mm, difference 0.09 mm |
| O | Letter | 0.3160 | 8.03 | 5/16 in, difference 0.09 mm |
| Q | Letter | 0.3320 | 8.43 | 21/64 in, difference 0.11 mm |
| 3/8 | Fractional | 0.3750 | 9.53 | U at 9.35 mm, difference 0.18 mm |
| U | Letter | 0.3680 | 9.35 | 3/8 in, difference 0.18 mm |
| 7/16 | Fractional | 0.4375 | 11.11 | 27/64 in, difference 0.40 mm |
| 1/2 | Fractional | 0.5000 | 12.70 | 12.7 mm, exact decimal conversion |
Number drill sizes
Number drills use a numbered series. The number increases as the diameter becomes smaller, so #80 is much smaller than #1. Common shop sizes include #80 (0.0135 inch), #60 (0.0400 inch), #40 (0.0980 inch), #30 (0.1285 inch), #21 (0.1590 inch), #20 (0.1610 inch), #13 (0.1850 inch), #10 (0.1935 inch), #7 (0.2010 inch), #6 (0.2040 inch), #4 (0.2090 inch), and #1 (0.2280 inch).
The number designation is not a thread recommendation. For example, #7 is often used as a reference size in fastener work, but the correct hole depends on the screw, thread, material, engagement, and whether the hole is for tapping, clearance, or a pilot. Use a tap-drill chart for a thread recommendation instead of reusing a conversion row as if it were a tap rule.
Letter drill sizes
Letter drills run from A through Z and fill useful gaps between fractional and number sizes. Common values include F at 0.2570 inch, H at 0.2660, K at 0.2810, O at 0.3160, Q at 0.3320, U at 0.3680, and Z at 0.4130 inch. A letter is a diameter designation, not a material grade or a drill series with a universal metric name.
If a letter size is unavailable, compare the decimal difference before choosing a substitute. F is not the same as 1/4 inch: F is 0.007 inch larger. That difference can matter in a reamed hole, a dowel fit, a tapped hole, or a close-clearance application. A loose hand-held clearance hole may tolerate a different choice, but the drawing or fastener specification controls.
Fractional drill sizes
Fractional bits are usually sold in 1/64-inch steps, although sets may include other increments. Useful conversions include 1/16 inch = 0.0625 inch, 3/32 = 0.0938, 1/8 = 0.1250, 5/32 = 0.1563, 3/16 = 0.1875, 1/4 = 0.2500, 5/16 = 0.3125, 3/8 = 0.3750, 7/16 = 0.4375, and 1/2 = 0.5000 inch.
Convert a fraction by dividing the numerator by the denominator, then multiply the decimal inch result by 25.4 to obtain millimetres. Round the displayed metric value for practical selection, but retain the decimal inch value when the hole is fit-critical.
Metric equivalents and nearest sizes
Metric and inch sizes rarely match exactly except at familiar conversions such as 1/2 inch and 12.7 mm. A 5/16-inch bit is 7.9375 mm, which is close to 8.0 mm but not equal. A 1/4-inch bit is 6.3500 mm, which is smaller than a 6.5 mm bit. The chart shows the difference so you can judge whether the alternate is acceptable.
When the exact drill is unavailable, choose the substitute based on the hole’s purpose. A pilot hole, tap drill, dowel hole, bearing seat, and clearance hole have different tolerances. Do not enlarge a hole to the nearest larger metric size if the larger diameter would reduce thread engagement or weaken a part. Do not use a smaller drill where the fastener, tap, or shaft requires the specified diameter.
Accuracy and tolerance notes
Published drill charts show nominal sizes. Actual drill diameter can vary with manufacturing tolerance, wear, coating, sharpening, runout, and the way the hole is produced. A twist drill can cut a hole larger than its measured flute diameter because of runout, material movement, or an unsupported workpiece.
For precision work, use the drawing’s tolerance and verify the finished hole with a pin gauge, plug gauge, micrometer, or calibrated measuring method. A digital caliper can identify a rough drilled hole, but it is not a substitute for a proper gauge when a close fit is required.
For threading guidance, see the tap drill size chart. For related references, use the GarageSanctum size-chart hub, bolt chart with wrench sizes, and tap and die set guide.
Sources and method notes
The decimal values were checked against the current LittleMachineShop drill and tap reference and the ICS Cutting Tools reference library. Metric values are inch-to-millimetre conversions rounded to two decimal places for display. “Nearest common alternate” means closest among the listed reference choices; it does not mean equal, interchangeable, or suitable for tapping without checking the thread specification.
