Millimetres and Inches in Precision Work
One inch equals exactly 25.4 millimetres — a fixed conversion since the 1959 international agreement. To convert millimetres to inches, divide by 25.4. To convert inches to millimetres, multiply by 25.4. Millimetres are the standard unit in engineering, manufacturing, and science for dimensions under a metre. Inches persist in the US and UK for construction lumber, screen sizes, pipe diameters, and fastener thread sizing. Metric fasteners (M4, M6, M8) use millimetre diameters; imperial fasteners use inch fractions (1/4", 3/8", 1/2").
Rainfall is measured in millimetres globally (except the US, which uses inches). A typical heavy rainfall event is 25 mm (1 inch) in one hour. Tyre tread depth in Europe is measured in millimetres (legal minimum 1.6 mm); in the US it's 2/32 inch (1.59 mm).
Millimetres to Inches Reference
| Millimetres | Inches | Common Item |
|---|---|---|
| 1 mm | 0.0394" | Credit card thickness |
| 6 mm | 0.236" | Pencil diameter (standard) |
| 10 mm | 0.394" | Metric bolt (M10 diameter) |
| 25.4 mm | 1.000" | Exact 1 inch |
| 50 mm | 1.969" | ~2 inches |
| 100 mm | 3.937" | ~4 inches |
| 254 mm | 10.000" | 10-inch blade |
