Convert millimeters, centimeters, or meters to the nearest inch fraction, or enter an inch fraction to get millimeters and centimeters.
- All Unit Converters
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- Fraction to mm Calculator
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- Fractional Ruler Layout Calculator
Watch the full explainer with transcript and worked examples.
mm to Fraction Formula
Convert millimeters to a decimal inch, then snap to the nearest fraction with your chosen denominator. Here x is the decimal inch length, n is the rounded numerator, and f is the reduced fraction. For nonnegative lengths, round exactly halfway values upward.
- mm — input length in millimeters (cm × 10, m × 1000)
- 25.4 — exact mm per inch
- D — fraction denominator (8, 16, 32, 64, or 128)
- numerator / D — reduced to lowest terms with whole-inch part separated
The reverse mode uses mm = inches × 25.4 and accepts mixed input like 1 3/8, 3/8, or 0.375. Rounding error in mm is reported as (numerator/D − inches_decimal) × 25.4.
Reference Tables
Common millimeter values with their nearest 1/64 inch fraction. Error is how far the rounded fraction sits from the exact value.
| mm | Decimal in | Nearest 1/64 | Error mm |
|---|---|---|---|
| 1 | 0.0394 | 3/64 | +0.1906 |
| 3 | 0.1181 | 1/8 | +0.1750 |
| 5 | 0.1969 | 13/64 | +0.1594 |
| 6 | 0.2362 | 15/64 | −0.0469 |
| 10 | 0.3937 | 25/64 | −0.0781 |
| 12 | 0.4724 | 15/32 | −0.0938 |
| 15 | 0.5906 | 19/32 | +0.0813 |
| 20 | 0.7874 | 25/32 | −0.1563 |
| 25 | 0.9843 | 63/64 | +0.0031 |
| 25.4 | 1.0000 | 1 | 0 |
Pick precision based on the work, not habit. Coarser denominators are easier to read; finer denominators reduce error.
| Precision | Max error | Typical use |
|---|---|---|
| 1/8 | ±1.59 mm | Rough framing, layout |
| 1/16 | ±0.79 mm | General woodworking, trim |
| 1/32 | ±0.40 mm | Cabinetry, joinery |
| 1/64 | ±0.20 mm | Machining, drill sizes |
| 1/128 | ±0.10 mm | Precision fits, gauging |
Worked Example
Convert 6 mm to the nearest 1/64 inch.
- Decimal inches: 6 ÷ 25.4 = 0.23622
- Multiply by 64: 0.23622 × 64 = 15.118
- Round: 15
- Fraction: 15/64 in (already reduced)
- Check: 15/64 × 25.4 = 5.9531 mm, error −0.0469 mm
FAQ
Is 1 inch exactly 25.4 mm? Yes. The international inch has been defined as exactly 25.4 mm since 1959.
Why does my fraction not match the input exactly? Most metric values do not land on a clean inch fraction. The calculator picks the closest fraction at your chosen denominator and reports the leftover difference in mm.
Which precision should I use for drill or wrench sizing? Use the required tool size and the job’s stated tolerance. The fraction precision controls numerical rounding; it does not establish a manufacturing tolerance or prove that a metric and inch tool are interchangeable.
Can I enter mixed numbers in the reverse mode? Yes. Formats like 1 3/8, 3/8, 0.375, and 1.375" all parse correctly.
