Length converter
Inches to millimeters.
Convert a decimal, fraction, or mixed inch measurement into millimeters. No need to turn 2 3/8 into a decimal first.
The marker shows the fractional part of one inch. This diagram illustrates tick positions; it is not a physical-size ruler.
Conversion formula
millimeters = inches × 25.4
Enter the source length above. The result updates immediately. These calculations use exact length definitions: 1 m = 100 cm = 1000 mm, 1 in = 25.4 mm, and 1 ft = 12 in. Only the displayed answer and fractional ruler mark are rounded. The feet-and-inches output splits a length into whole feet and remaining inches; decimal feet do not encode an inch count.
Worked example
A 3/8-inch measurement equals 0.375 inches. Multiply 0.375 by 25.4 to get exactly 9.525 mm. If you report it as 9.5 mm, that is a rounded value—not a change in the underlying unit definition.
Conversion table: in → mm
Select a source value to load it into the calculator. Decimal results are displayed to at most six places; inch fractions in this table are rounded to 1/16 inch.
| Inches (in) | Millimeters (mm) | Nearest inch fraction |
|---|---|---|
| 1.5875 | ≈ 1/16 in | |
| 3.175 | ≈ 1/8 in | |
| 6.35 | ≈ 1/4 in | |
| 9.525 | ≈ 3/8 in | |
| 12.7 | ≈ 1/2 in | |
| 19.05 | ≈ 3/4 in | |
| 25.4 | ≈ 1 in | |
| 50.8 | ≈ 2 in | |
| 76.2 | ≈ 3 in | |
| 152.4 | ≈ 6 in | |
| 254 | ≈ 10 in | |
| 304.8 | ≈ 12 in |
When to use this conversion
Use this for hardware dimensions, tape-measure readings, sheet thickness, and drawings that must be communicated in metric units. Fractional nominal names for pipes or fasteners may not describe the measured outside diameter.
Type fractions with a slash, such as 3/8, or a whole number and a fraction separated by a space, such as 2 3/8. The denominator must be greater than zero.
Exact conversion versus a ruler reading
The unit relationship is exact, but a measured object may have uncertainty. At 1/16 inch, the maximum rounding difference is half a tick: 1/32 inch, or 0.79375 mm. The calculator shows the actual rounding difference for your value. Use a finer division only if the source measurement and physical instrument support that precision.
Next steps
Numerical conversion needs no display calibration. Comparing a physical object against your screen does. Use the ruler for small objects, a verified paper ruler away from your display, or an appropriate calibrated instrument for safety-critical measurements.
Related converters
Reference: NIST unit conversion factors. Calculations use the international foot, not historical survey-foot units. Fractions are explicitly approximate. NIST ↗