Millimeter (mm)
A millimeter is one thousandth of a meter: 1 mm = 0.001 m. Ten millimeters make one centimeter.
- Symbol
- mm
- System
- Metric
- Meter value
- 0.001 m
- Singular
- millimeter
- Plural
- millimeters
- Aliases
- millimetre, millimetres
What a millimeter measures
The millimeter sits between the micrometer and centimeter on the metric scale. It is small enough for thicknesses and tolerances but still easy to mark on a handheld ruler.
- The smallest numbered interval on many metric rulers is 1 millimeter.
- A stack of 10 millimeters spans exactly 1 centimeter.
- A 1-meter length contains exactly 1,000 millimeters.
- One inch contains exactly 25.4 millimeters.
Where millimeters are used
Definition and standardization
The prefix milli means one thousandth, so the unit follows directly from the SI meter and its decimal prefix system.
The symbol is mm, written in lowercase and without a period in ordinary SI notation.
Because an inch is defined as exactly 25.4 millimeters, millimeter-to-inch calculations have an exact underlying relationship.
Tools used to measure millimeters
- Metric ruler for everyday dimensions
- Vernier caliper for outside, inside, and depth measurements
- Micrometer for fine thickness and diameter work
Common mistakes
- Reading centimeter numbers but forgetting the ten smaller millimeter divisions
- Writing MM instead of the correct lowercase symbol mm
- Reporting more decimal places than the measuring tool can support
Millimeter questions
How many millimeters are in a centimeter?
There are exactly 10 millimeters in 1 centimeter.
Is a millimeter smaller than an inch?
Yes. One inch is exactly 25.4 millimeters.
When should I use millimeters?
Use millimeters for dimensions too small to describe comfortably in centimeters, especially thicknesses, hardware, and technical tolerances.