CNC TRUECUT / MACHINING REFERENCE
Fractional drill sizes: inches to millimetres
Convert common fractional drill diameters to decimal inches and millimetres, and understand where number and letter drills fit.
Reading a fractional drill diameter
A fractional drill designation describes a nominal diameter in inches. Reduce the fraction when comparing sizes: 8/64 and 1/8 are the same diameter. Conversion is exact arithmetic: divide the numerator by the denominator for decimal inches, then multiply by 25.4 for millimetres. A nominal size is not a guarantee of the drilled hole’s final size.
Common fractional sizes
The table below is calculated from fractions rather than copied from a manufacturer catalogue. Use a measured tool diameter and an appropriate hole-making process when a controlled tolerance is required.
| Fraction | Decimal inch | Millimetres |
|---|---|---|
| 1/64 | 0.015625 | 0.39687 |
| 1/32 | 0.031250 | 0.79375 |
| 3/64 | 0.046875 | 1.19062 |
| 1/16 | 0.062500 | 1.58750 |
| 5/64 | 0.078125 | 1.98438 |
| 3/32 | 0.093750 | 2.38125 |
| 7/64 | 0.109375 | 2.77812 |
| 1/8 | 0.125000 | 3.17500 |
| 9/64 | 0.140625 | 3.57187 |
| 5/32 | 0.156250 | 3.96875 |
| 11/64 | 0.171875 | 4.36562 |
| 3/16 | 0.187500 | 4.76250 |
| 13/64 | 0.203125 | 5.15937 |
| 7/32 | 0.218750 | 5.55625 |
| 15/64 | 0.234375 | 5.95313 |
| 1/4 | 0.250000 | 6.35000 |
| 17/64 | 0.265625 | 6.74687 |
| 9/32 | 0.281250 | 7.14375 |
| 19/64 | 0.296875 | 7.54062 |
| 5/16 | 0.312500 | 7.93750 |
| 21/64 | 0.328125 | 8.33437 |
| 11/32 | 0.343750 | 8.73125 |
| 23/64 | 0.359375 | 9.12812 |
| 3/8 | 0.375000 | 9.52500 |
| 25/64 | 0.390625 | 9.92188 |
| 13/32 | 0.406250 | 10.31875 |
| 27/64 | 0.421875 | 10.71562 |
| 7/16 | 0.437500 | 11.11250 |
| 29/64 | 0.453125 | 11.50937 |
| 15/32 | 0.468750 | 11.90625 |
| 31/64 | 0.484375 | 12.30312 |
| 1/2 | 0.500000 | 12.70000 |
| 33/64 | 0.515625 | 13.09687 |
| 17/32 | 0.531250 | 13.49375 |
| 35/64 | 0.546875 | 13.89063 |
| 9/16 | 0.562500 | 14.28750 |
| 37/64 | 0.578125 | 14.68437 |
| 19/32 | 0.593750 | 15.08125 |
| 39/64 | 0.609375 | 15.47812 |
| 5/8 | 0.625000 | 15.87500 |
| 41/64 | 0.640625 | 16.27187 |
| 21/32 | 0.656250 | 16.66875 |
| 43/64 | 0.671875 | 17.06563 |
| 11/16 | 0.687500 | 17.46250 |
| 45/64 | 0.703125 | 17.85938 |
| 23/32 | 0.718750 | 18.25625 |
| 47/64 | 0.734375 | 18.65312 |
| 3/4 | 0.750000 | 19.05000 |
| 49/64 | 0.765625 | 19.44687 |
| 25/32 | 0.781250 | 19.84375 |
| 51/64 | 0.796875 | 20.24062 |
| 13/16 | 0.812500 | 20.63750 |
| 53/64 | 0.828125 | 21.03437 |
| 27/32 | 0.843750 | 21.43125 |
| 55/64 | 0.859375 | 21.82813 |
| 7/8 | 0.875000 | 22.22500 |
| 57/64 | 0.890625 | 22.62187 |
| 29/32 | 0.906250 | 23.01875 |
| 59/64 | 0.921875 | 23.41562 |
| 15/16 | 0.937500 | 23.81250 |
| 61/64 | 0.953125 | 24.20937 |
| 31/32 | 0.968750 | 24.60625 |
| 63/64 | 0.984375 | 25.00312 |
| 1/1 | 1.000000 | 25.40000 |
Number, letter and metric drills
Number and letter drill designations use their own size series; they are not fractions and cannot be decoded arithmetically. Confirm those sizes in your drill supplier’s chart. Metric drill sizes state the diameter directly in millimetres. Do not choose the nearest converted drill solely because it looks close: the allowed hole tolerance and tapping recommendation still decide whether a substitute is acceptable.
Drill speed and feed
Once you know the actual drill diameter, use the drilling calculator with cutting speed and feed per revolution for the drill, material and hole depth. Point height is separate from full-diameter depth; the drill point calculator explains that geometry.