What is a Millimeter?

The millimeter equals one thousandth of a meter (0.001 m) and is the precision unit of choice in engineering, manufacturing, electronics, and meteorology. Its small size makes it ideal for tolerances in mechanical parts, paper thickness, and rainfall measurements. A standard credit card is 0.76 mm thick, and a sheet of office paper is about 0.1 mm. The millimeter is the universal unit for tire-tread depth, weather-station rainfall reports, and 3D printer resolution. It relates to the centimeter (10 mm = 1 cm), the inch (25.4 mm = 1 in exactly), and the micrometer (1 mm = 1,000 µm). Engineering drawings worldwide default to millimeters for dimensions, except in the United States where inches remain dominant in mechanical engineering.

Visual reference for how the millimeter relates to other length units. Each row links to the full converter for that pair.

See all 15 length units in matrix view →

• Rainfall measurements in weather reports
• Precision engineering and manufacturing tolerances
• Medical imaging — tumor and wound size

• ✓1 mm is the smallest graduation on a standard ruler. 10 mm = 1 cm exactly.
• ✓Rainfall in mm is depth — 25 mm of rain over 1 m² equals 25 litres of water.
• ✓For very small measurements switch to micrometres (µm) — 1 mm = 1000 µm.

• ✗Reading an engineering drawing dimensioned in mm as if it were cm — a factor-of-10 error.
• ✗Using 25 instead of 25.4 when converting mm to inches — the error matters in CNC machining.
• ✗Confusing millimetre (length) with millilitre (volume) — both abbreviated with "ml" but on different scales.