Millimetre

The millimetre (International spelling as used by the International Bureau of Weights and Measures) or millimeter (American spelling) (SI unit symbol mm) is a unit of length in the metric system, equal to one thousandth of a metre, which is the SI base unit of length.

It is equal to 1,000 micrometres and 1,000,000 nanometres. There are 25.4 mm in one inch by definition, so a millimetre is exactly equal to ⁵⁄₁₂₇ inch.

Definition

Since 1983, the metre has been defined as "the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second." A millimetre, 1/1000 of a metre, is therefore the distance travelled by light in 1/299,792,458,000 of a second.

Unicode symbols

For the purposes of compatibility with Chinese, Japanese and Korean (CJK) characters, Unicode has symbols for:

Measurement

On a metric ruler, the smallest measurements are normally millimetres. High-quality engineering rules may be graduated in increments of 0.5 mm. Digital Vernier callipers are commonly capable of reading increments as small as 0.01 mm.

1 decimetre

To help compare different orders of magnitude this page lists lengths between 10 centimetres and 100 centimetres (10⁻¹ metre and 1 metre).

Distances shorter than 10 centimetres

Conversions

10 centimetres (abbreviated to 10 cm) is equal to:

Wavelengths

Human-defined scales and structures

1 centimetre

To help compare different orders of magnitude this page lists lengths between 10⁻²m and 10⁻¹ m (1 cm and 10 cm).

See also

References