Millimeters Conversion

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  • 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.


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

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)

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.


  • 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)