# Inches to Millimeters (in to mm)

Source: https://www.unitconvertercalculator.com/length/inches-to-millimeters/

**1 in = 25.4 mm**

One inch equals exactly 25.4 millimeters. This precise, exact conversion is one of the most important in engineering and manufacturing, forming the bridge between imperial and metric precision measurement. Every workshop, factory, and design office that handles both systems relies on this relationship daily.

## Formula

Convert Inches to Millimeters

## Conversion Table

| Inches (in) | Millimeters (mm) |
|---|---|
| 0.25 in | 6.35 mm |
| 0.5 in | 12.7 mm |
| 1 in | 25.4 mm |
| 2 in | 50.8 mm |
| 3 in | 76.2 mm |
| 4 in | 101.6 mm |
| 5 in | 127 mm |
| 6 in | 152.4 mm |
| 8 in | 203.2 mm |
| 10 in | 254 mm |
| 12 in | 304.8 mm |
| 18 in | 457.2 mm |
| 24 in | 609.6 mm |
| 36 in | 914.4 mm |
| 48 in | 1219.2 mm |
| 72 in | 1828.8 mm |
| 100 in | 2540 mm |

## Units

### Inch (in)

An imperial unit of length equal to exactly 2.54 centimeters. Commonly used in the US and UK for screen sizes, body measurements, and construction.

### Millimeter (mm)

A metric unit of length equal to one thousandth of a meter. Used in engineering, manufacturing, and precise measurements.

## Background

A 1/4-inch drill bit is exactly 6.35 mm. Pipe fittings labeled in inches need conversion to millimeters for metric plumbing systems. CNC machining programs may specify dimensions in either unit. The 25.4 mm/inch relationship is so fundamental that it was the basis for the 1959 International Yard and Pound Agreement that standardized all imperial length units.

## Good to Know

The relationship 1 inch = 25.4 mm is the single most important bridge between imperial and metric systems. Every other imperial-to-metric length conversion derives from this one definition.

## FAQ

### How many millimeters are in 1 inch?

One inch equals exactly 25.4 millimeters. This is an exact definition, not an approximation. It was established by international agreement in 1959.

### How do I convert inches to millimeters?

Multiply the number of inches by 25.4. For example, 3 inches × 25.4 = 76.2 mm. For fractions, convert to decimals first: 3/8 inch = 0.375 × 25.4 = 9.525 mm.

### What is 1/2 inch in millimeters?

1/2 inch equals exactly 12.7 millimeters. Common fractional conversions: 1/4" = 6.35 mm, 3/8" = 9.525 mm, 1/2" = 12.7 mm, 3/4" = 19.05 mm, 1" = 25.4 mm.

## Non-Frequently Asked Questions

### Why is 25.4 such an ugly number for 1 inch in mm?

Because inches evolved from barleycorns and thumbs, while millimeters evolved from the meter (which was based on the Earth's circumference). When two measurement systems derived from completely different starting points finally meet, the conversion factor is whatever number the math produces. 25.4 is not ugly - it is honest.

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## See Also

- [Millimeters to Inches](https://www.unitconvertercalculator.com/length/millimeters-to-inches/)