Imperial sizes are built on 64ths of an inch, so almost every fractional tool size, drill and stock thickness resolves to one of the 64 rows below. Each is shown as a reduced fraction, its decimal inch value to four places, and its exact millimetre equivalent at 25.4 mm to the inch.
Reading the chart
Fractions are reduced to their simplest form, so 32/64 appears as 1/2 and 8/64 as 1/8. That matters when reading a tool: a spanner stamped 1/2 inch and a drawing calling for 32/64 are the same dimension. Decimal values are exact to four places, which is the resolution most workshop calipers read to.
Complete 64ths conversion table
| Fraction | Decimal inch | mm |
|---|---|---|
| 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 |
| 3/32 | 0.0938 | 2.38 |
| 7/64 | 0.1094 | 2.78 |
| 1/8 | 0.1250 | 3.17 |
| 9/64 | 0.1406 | 3.57 |
| 5/32 | 0.1563 | 3.97 |
| 11/64 | 0.1719 | 4.37 |
| 3/16 | 0.1875 | 4.76 |
| 13/64 | 0.2031 | 5.16 |
| 7/32 | 0.2188 | 5.56 |
| 15/64 | 0.2344 | 5.95 |
| 1/4 | 0.2500 | 6.35 |
| 17/64 | 0.2656 | 6.75 |
| 9/32 | 0.2813 | 7.14 |
| 19/64 | 0.2969 | 7.54 |
| 5/16 | 0.3125 | 7.94 |
| 21/64 | 0.3281 | 8.33 |
| 11/32 | 0.3438 | 8.73 |
| 23/64 | 0.3594 | 9.13 |
| 3/8 | 0.3750 | 9.52 |
| 25/64 | 0.3906 | 9.92 |
| 13/32 | 0.4063 | 10.32 |
| 27/64 | 0.4219 | 10.72 |
| 7/16 | 0.4375 | 11.11 |
| 29/64 | 0.4531 | 11.51 |
| 15/32 | 0.4688 | 11.91 |
| 31/64 | 0.4844 | 12.30 |
| 1/2 | 0.5000 | 12.70 |
| 33/64 | 0.5156 | 13.10 |
| 17/32 | 0.5313 | 13.49 |
| 35/64 | 0.5469 | 13.89 |
| 9/16 | 0.5625 | 14.29 |
| 37/64 | 0.5781 | 14.68 |
| 19/32 | 0.5938 | 15.08 |
| 39/64 | 0.6094 | 15.48 |
| 5/8 | 0.6250 | 15.88 |
| 41/64 | 0.6406 | 16.27 |
| 21/32 | 0.6563 | 16.67 |
| 43/64 | 0.6719 | 17.07 |
| 11/16 | 0.6875 | 17.46 |
| 45/64 | 0.7031 | 17.86 |
| 23/32 | 0.7188 | 18.26 |
| 47/64 | 0.7344 | 18.65 |
| 3/4 | 0.7500 | 19.05 |
| 49/64 | 0.7656 | 19.45 |
| 25/32 | 0.7813 | 19.84 |
| 51/64 | 0.7969 | 20.24 |
| 13/16 | 0.8125 | 20.64 |
| 53/64 | 0.8281 | 21.03 |
| 27/32 | 0.8438 | 21.43 |
| 55/64 | 0.8594 | 21.83 |
| 7/8 | 0.8750 | 22.22 |
| 57/64 | 0.8906 | 22.62 |
| 29/32 | 0.9063 | 23.02 |
| 59/64 | 0.9219 | 23.42 |
| 15/16 | 0.9375 | 23.81 |
| 61/64 | 0.9531 | 24.21 |
| 31/32 | 0.9688 | 24.61 |
| 63/64 | 0.9844 | 25.00 |
| 1/1 | 1.0000 | 25.40 |
Method and precision
The inch is defined as exactly 25.4 mm, so every value here is an exact conversion rather than an approximation. Millimetres are rounded to two decimal places for display only; the converter above uses the exact factor.
Why the millimetre column rarely lands round
The inch is defined as exactly 25.4 mm, so every imperial fraction converts to an awkward metric number and every round metric size converts to an awkward fraction. There is no rounding convention that makes them meet. This is the underlying reason mixed-standard fastener work needs both sets of tools rather than one set and a conversion in your head.
Measuring equipment is in measuring equipment.