How to Convert Millimeters to Inches
Converting millimeters (mm) to inches is straightforward: divide the millimeter value by 25.4. This exact factor comes from the 1959 international agreement that defined 1 inch = 25.4 mm exactly. So for any measurement in mm, inches = mm ÷ 25.4.
MM to Inches Conversion Table
Common millimeter values and their inch equivalents:
| Millimeters (mm) | Inches (in) | Fraction |
|---|---|---|
| 1 mm | 0.03937 in | ~1/25 |
| 2 mm | 0.07874 in | ~5/64 |
| 3 mm | 0.11811 in | ~3/25 |
| 5 mm | 0.19685 in | ~13/64 |
| 10 mm | 0.39370 in | ~25/64 |
| 12.7 mm | 0.50000 in | 1/2 |
| 15 mm | 0.59055 in | ~19/32 |
| 20 mm | 0.78740 in | ~25/32 |
| 25 mm | 0.98425 in | ~63/64 |
| 25.4 mm | 1.00000 in | 1 |
| 30 mm | 1.18110 in | ~1 3/16 |
| 50 mm | 1.96850 in | ~1 31/32 |
| 100 mm | 3.93701 in | ~3 15/16 |
| 150 mm | 5.90551 in | ~5 29/32 |
| 200 mm | 7.87402 in | ~7 7/8 |
| 300 mm | 11.81102 in | ~11 13/16 |
| 500 mm | 19.68504 in | ~19 11/16 |
A Brief History of the Millimeter and Inch
The millimeter was born from the metric system introduced in France in 1795 during the French Revolution — part of a sweeping effort to standardize measurement across Europe and, eventually, the world. One meter was defined as one ten-millionth of the distance from the equator to the North Pole. A millimeter is one thousandth of that meter.
The inch has a murkier origin. It was historically defined as the width of a man's thumb, and later (in 14th-century England) as three barleycorns laid end to end. The modern inch was standardized internationally in 1959, when the US, UK, Canada, Australia, New Zealand, and South Africa agreed that exactly 1 inch = 25.4 mm — the definition used today.
Where Are MM and Inches Used?
Millimeters dominate in engineering, manufacturing, medicine, and science worldwide. Inches remain standard in the United States for everyday measurement, construction lumber, screen sizes, and firearms. Many industries — notably aerospace and automotive — maintain dual specifications in both systems. Understanding the mm-to-inch conversion is essential for anyone importing products, reading international technical drawings, or working with tools from different markets.