mm to Inches Converter

How to use the millimeters to inches converter

1. Enter a value in the Millimeters field or the Inches field.
2. The other field updates automatically with the converted length.
3. Check Show nearest fraction if you need the inch value as a common fraction like 3/16 or 1 1/2.
4. Copy the result directly from the output area.

How the mm to inches conversion works

The relationship between these units is fixed by international agreement: one inch equals exactly 25.4 millimeters. To convert mm to inches, divide the millimeter value by 25.4. To go the other way, multiply inches by 25.4.

For example, a standard credit card is 85.60 mm wide. Dividing by 25.4 gives 85.60 / 25.4 = 3.370079 inches. Because 25.4 is an exact definition rather than an approximation, the only limit on precision is how many decimal places you choose to display.

Common mm to inches reference table

This table covers frequently used measurements in manufacturing, hardware and everyday objects. Values are rounded to four decimal places where needed.

Reading fractional inches

In woodworking, machining and construction in the United States, inches are often expressed as fractions with denominators of 2, 4, 8, 16 or 32. When you enable the fraction option, the converter rounds the decimal result to the nearest sixteenth of an inch.

The algorithm multiplies the decimal portion by 16, rounds to the nearest whole number, then simplifies the fraction by dividing numerator and denominator by their greatest common divisor. For instance, 1.9685 inches has a fractional part of 0.9685. Multiplying by 16 gives 15.496, which rounds to 15. The result is displayed as 1 15/16 inches.

Keep in mind that rounding to 1/16 introduces a small error of up to 0.03125 inches (about 0.8 mm). For precision engineering work, use the full decimal output instead.

Where this conversion matters

Metric fasteners such as M6 or M8 bolts are specified in millimeters, but the holes they fit into may be drilled using imperial bit sizes. Converting mm to inches ensures the right fit without guesswork.

3D printing slicers typically accept models in millimeters, yet many US-published plans list dimensions in inches. A quick conversion avoids scaling errors that would ruin a print.

If you also need to move between centimeters and millimeters, the cm to mm converter handles that step before you convert the final figure to inches.

Precision and floating point limits

JavaScript uses IEEE 754 double-precision floats, which provide about 15 to 17 significant decimal digits. For any practical length measurement, this is far more precision than physical tools can achieve. The converter cleans up floating-point noise by rounding intermediate results to 12 significant digits before formatting.

Very large inputs (above roughly 10¹⁵ mm) will start losing integer precision due to the float format, but such lengths exceed real-world engineering needs by many orders of magnitude.

Worked examples

Each result below was produced by the same code the tool runs, so you can check your own numbers against them.