Thou (unit)

small unit of length in the imperial and US customary systems

The thou (abbreviation th) or mil (mainly United States) is a unit of length in the Imperial System and the United States Customary Units. It is equal to ​11,000 of an inch. This is not to be confused with the old form of "you".



Thou comes from the Germanic languages, a shortening of thousandth.

Mil comes from the Romance languages, a shortening of mille.



The thou was introduced in 1844 by Joseph Whitworth as a substitute to traditional fractions such as ​16413211618 which cannot be defined simply in hundredths unlike ​14 (0.25) or ​12 (0.5). Traditional fractions continued to be used, as they were a substantial part of other units of length.


International Thou Inch Millimetre
Thou 1 11,000 0.0254

The thou is used in engineering and manufacturing mostly.