Inch fractions to millimetres
Imperial tapes, drill sets and timber plans work in fractions of an inch. Metric ones don't. This table converts between them, and the notes below explain how to read a fractional scale without counting every tick.
Conversion table
One inch is exactly 25.4 mm, so every value here is exact before rounding. Millimetres are rounded to two decimal places; the rows in bold are the sixteenths that most tapes and rules mark.
| Fraction | Decimal inch | Millimetres |
|---|---|---|
| 1/32" | 0.0313" | 0.79 mm |
| 1/16" | 0.0625" | 1.59 mm |
| 3/32" | 0.0938" | 2.38 mm |
| 1/8" | 0.1250" | 3.18 mm |
| 5/32" | 0.1563" | 3.97 mm |
| 3/16" | 0.1875" | 4.76 mm |
| 7/32" | 0.2188" | 5.56 mm |
| 1/4" | 0.2500" | 6.35 mm |
| 9/32" | 0.2813" | 7.14 mm |
| 5/16" | 0.3125" | 7.94 mm |
| 11/32" | 0.3438" | 8.73 mm |
| 3/8" | 0.3750" | 9.53 mm |
| 13/32" | 0.4063" | 10.32 mm |
| 7/16" | 0.4375" | 11.11 mm |
| 15/32" | 0.4688" | 11.91 mm |
| 1/2" | 0.5000" | 12.70 mm |
| 17/32" | 0.5313" | 13.49 mm |
| 9/16" | 0.5625" | 14.29 mm |
| 19/32" | 0.5938" | 15.08 mm |
| 5/8" | 0.6250" | 15.88 mm |
| 21/32" | 0.6563" | 16.67 mm |
| 11/16" | 0.6875" | 17.46 mm |
| 23/32" | 0.7188" | 18.26 mm |
| 3/4" | 0.7500" | 19.05 mm |
| 25/32" | 0.7813" | 19.84 mm |
| 13/16" | 0.8125" | 20.64 mm |
| 27/32" | 0.8438" | 21.43 mm |
| 7/8" | 0.8750" | 22.23 mm |
| 29/32" | 0.9063" | 23.02 mm |
| 15/16" | 0.9375" | 23.81 mm |
| 31/32" | 0.9688" | 24.61 mm |
| 1" | 1.0000" | 25.40 mm |
For lengths over an inch, add 25.4 mm for each whole inch. 2 3/8" is 2 × 25.4 + 9.53 = 60.33 mm.
Reading a fractional rule
The ticks on an imperial rule get shorter as the fraction gets finer. The longest tick between two inch marks is the half, the next longest are the quarters, then eighths, then sixteenths. To read a length, find the last inch mark, then name the shortest tick the edge lands on and count back from the nearest longer one: two sixteenths past 3/8" is 7/16".
Fractions are always written in lowest terms. 8/16" is 1/2" and 12/16" is 3/4". If you count an even number of sixteenths, halve both numbers until one of them is odd.
Fractions on Screen Ruler
In settings, Inch Measurements switches the readout between fractions rounded to the nearest 1/16" and decimals to two places. The fraction is a rounded view of the same measurement: a reading anywhere from about 0.97" to 1.03" displays as 1". When two parts need to match closely, switch to decimals or millimetres to see the difference a fraction hides. The accuracy guide explains why the readout can be finer than the measurement itself.
When rounding to 1/16" is enough
A sixteenth of an inch is about 1.6 mm. That is fine for marking out timber, choosing a drill bit for a pilot hole or checking that a shelf will fit. It is not fine for a joint that has to be snug, a hinge recess or a part that slides into another. For those, measure in millimetres or thousandths and use a physical tool. The screws and drill bits guide shows where fractional and metric sizes sit close together.