Inches to MM Converter

How to convert inches to millimeters

1 inch = 25.4 millimeters. To convert inches to millimeters, multiply by 25.4. The international inch is exactly 2.54 cm (agreed in 1959). A millimeter is one thousandth of a meter.

Worked examples

• 0.5 inches = 12.7 mm (0.5 × 25.4)
• 2 inches = 50.8 mm (2 × 25.4)
• 12 inches = 304.8 mm (12 × 25.4)

Where this conversion comes up

Inches are used for screens, tools, lumber and US body measurements; millimeters for screw and drill sizes, rainfall, paper and phone dimensions. Useful for matching imperial hardware to metric parts: 1/4 in is 6.35 mm, 1/2 in is 12.7 mm and 3/4 in is 19.05 mm, so none of them line up with a whole-millimeter size.

Quick estimate

Multiply by 25 and add 1.6%: 4 in ≈ 100 mm, exact 101.6 mm.

Common mistakes

• Dividing instead of multiplying. A millimeter is smaller than a inch, so the number of millimeters must come out larger: 0.5 inches = 12.7 mm.
• Rounding too early. Keep the full factor until the last step; rounding 25.4 to 25 first puts large values noticeably off.

Going the other way? Use the mm to inches converter. Related length converters: inches to feet, feet to inches, meters to feet, feet to meters, feet to cm, cm to mm.