Inches to mm Converter

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Enter a decimal or common fraction in inches to get its millimeter measurement, with the unrounded answer shown first.

Advanced options
Millimeters from entered inchesThis is the unrounded conversion using exactly 25.4 mm per inch.
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How to use our Inches to mm Converter

  1. Enter one measurement in inches, such as 2.5, 3/8, or 1 1/2.
  2. Use inches only. If you have feet and inches, first change the full measurement into inches.
  3. Select Calculate to see the unrounded millimeter conversion.
  4. If a drawing, worksheet, or record requires a set number of decimal places, open Advanced options and choose that format.
  5. Check that a positive or negative mm answer has the same sign as your inch entry, and that any rounded value is close to the unrounded value.
Example inputs for Inches to mm Converter
Example inputs for Inches to mm Converter

Definitions

Inch (in): A unit of length. One international inch equals exactly 25.4 millimeters.

Millimeter (mm): A metric unit of length equal to one thousandth of a meter.

Decimal inch: An inch measurement written with a decimal point, such as 2.5 in.

Simple fraction: Part of an inch written as one number over another, such as 3/8 in.

Mixed fraction: A whole number plus a fraction, such as 1 1/2 in, which equals 1.5 in.

Rounded value: A display shortened to a chosen number of decimal places. It can differ slightly from the unrounded value.

Common mistakes and quick fixes

Mistake: Entering a feet-and-inches measurement such as 5 ft 8 in.
Fix: Convert the full measurement to inches first, then enter one inch value.

Mistake: Using a comma in a measurement, such as 1,2.
Fix: Use a decimal point, such as 1.2, or a fraction, such as 1 1/2.

Mistake: Entering a fraction with a zero denominator, such as 1/0.
Fix: The bottom number of a fraction cannot be zero. Use a valid fraction such as 1/2 or 3/8.

Mistake: Copying the rounded mm value into later calculations.
Fix: Keep the unrounded mm value for calculations. Use the rounded value only when a required decimal-place format is stated.

Limitations & Key Assumptions / Boundary Conditions

  • The entry must be one measurement written entirely in inches; combined feet-and-inches entries are not accepted.
  • The 25.4 mm per inch conversion is exact, but a measurement copied from a tape measure or drawing may already be approximate.
  • Choosing decimal places changes only the displayed rounded value, not the underlying conversion.
  • Very small nonzero values may appear in scientific notation so they are not mistaken for zero.
  • Fractions need a nonzero denominator, and comma-based text is rejected because its meaning can be unclear.

Methodology

Conversion calculation

The converter reads a decimal, simple fraction, or mixed fraction as one inch measurement. It then multiplies that value by 25.4 because the international inch is defined as exactly 25.4 millimeters.

millimeters = inches * 25.4

Worked example

For 3/8 in, the fraction equals 0.375 in. Multiplying 0.375 by 25.4 gives 9.525 mm.

0.375 * 25.4 = 9.525 mm

Rounding

A decimal-place choice formats the same millimeter value for copying into a drawing, worksheet, or record. For example, 12.7 mm displayed to three decimal places is 12.700 mm; the unrounded conversion stays 12.7 mm.

rounded display = formatted unrounded millimeters

Zero inches converts to exactly zero millimeters, and a negative inch entry keeps its negative sign. The calculator rejects a zero fraction denominator, feet-and-inches wording, commas, and other text that does not state one unambiguous inch measurement.