Since 1983, the metre has been defined as "the length of the path travelled by light in vacuum during a time interval of ⁠1/299792458⁠ of a second".

A common shortening of millimetre in spoken English is "mil".

This can cause confusion in the United States, where "mil" traditionally means a thousandth of an inch.

[3] High-quality engineering rulers may be graduated in increments of 0.5 mm.

Digital callipers are commonly capable of reading increments as small as 0.01 mm.