### Millimeters

**Abbreviation/Symbol:**

- mm
- mil (informal)

**Unit of:**

- Length

**Worldwide use:**

- The millimetre, as part of the metric system, is used as a measure of length across the globe. The most notable exception is the United States, where the imperial system is still used for most purposes.

**Definition:**

The millimetre is a unit of length in the metric system, equivalent to one thousandth of a metre (the SI base unit of length).

One meter was defined in 1983 by the 17th conference of weights and measures as “the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second” and the millimetre by definition is derived as being 1/1000th of that value.

**Origin:**

John Wilkins first proposed the length of a “seconds pendulum” as a universal measurement – ie a measurement that could be carried out, to avoid a definition with respect to a stored reference unit. He thought that a pendulum that made one half oscillation per second would have a fixed length. He was almost correct, but it was later found there is a slight difference in this length around the world. Because of this variability, the Academy of Sciences devised a new measure, equal to 1/10000000 of the distance from the Equator to the North Pole, measured along the meridian through Paris. This measurement was used to create the international prototype meter – a metal bar stored at the BIPM’s headquarters. In time this standard was dropped in favour of more accurate measures – firstly in 1960 the 11th conference of weights and measures defined the meter as “1,650763.73 wavelengths of the orange-red emission line of a krypton-86 atom (in a vacuum)”. Finally in 1983 the 17th conference defined the meter as “the distance travelled by light in a vacuum in 1/299792458 of a second” which therefore leads to a definition of a millimetre as the distance travelled by light in a vacuum in 1/299792458000 of a second.

**Common references:**

- There are 25.4 millimetres in one inch.
- The head of a pin is approximately 2mm diameter.
- A CD is approximately 1.2mm thick.
- 00 gauge model railways measure 16.5mm between rails.
- Grade 1 hair clippers will cut hair to approximately 3 mm in length (grade 2 cuts to 6 mm, grade 3 to 9 mm etc)
- 1 metre, 1000 mm
- A piece of normal legal paper is approximately .1mm thick
- A fine human hair is approximately .04mm wide
- An average matchstick is approximately 2mm wide
- An AA battery can be between 49.2 and 50.5mm from its flat end to the end of the battery button

**Usage context:**

Millimetres are used as a standard measure of length in all manner of engineering and commercial applications where accuracy greater than the closest centimetre is required.

Where even greater accuracy must be measured or expressed, fractions of a millimetre are used to three decimal places.

Millimetres are commonly used to describe the calibre of small munitions and the weaponry used to fire them, for example the Uzi 9 mm assault rifle.

**Component units:**

- 1/1,000 mm = one micrometre
- 1/1,000,000 mm = one nanometre
- Further, progressively small units include the picometre, femtometre, attometre, zeptometre and yoctometre.

**Multiples:**

- There are numerous units for expressing multiples of the millimetre, but these are defined by their relationship to the metre (the SI base unit of length), rather than the millimetre.
- 10 mm = 1 centimetre (cm)
- 1000mm = 1 metre (m)