How it's calculated
fraction = round(decimal × D) ÷ D
decimal = inches, D = denominator (2, 4, 8, 16, 32, 64). Result reduced by greatest common divisor. mm = inches × 25.4.
Worked examples
| Decimal in | Precision | Fraction | Note |
|---|---|---|---|
| 0.5 | 1/16 | 1/2 | Exact |
| 0.375 | 1/16 | 3/8 | Exact |
| 0.3 | 1/16 | 5/16 | Rounds up 0.0125 in |
| 0.7 | 1/64 | 45/64 | Rounds down 0.003 in |
Common questions
Why use binary fractions?
Tape measures and rulers are marked in halves, quarters, eighths, sixteenths, and so on, so the nearest binary fraction is what you actually read.
What does the rounding error tell me?
It shows how far the chosen fraction sits from the true decimal, so you know whether a finer precision is worth it.