Thou_(unit_of

Thou (unit of length)

See also: Mil

**1 thou** =
SI units
25.4×10⁻⁶ m	25.40 μm
US customary / Imperial units
83.3×10⁻⁶ ft	1.00×10⁻³ in

A thou, also known as a mil, is a unit of length equal to 0.001 inches (a "milli-inch" or a "thousandth of one inch"). It is sometimes used in engineering and in the specification of:

the thickness of items such as paper, film, foil, wires, paint coatings, latex gloves, plastic sheeting, and fibers; and
manufacturing dimensions and tolerances.
in the manufacture of certain racing engines [Normally the space between the valves and pistons, or camshaft and valves]

Internationally, use of the thou is now generally deprecated in favour of the use of SI units of length such as the micrometre. In the United States, the mil/thou is still in use extensively in certain industries such as in the manufacture of printed circuit boards (PCBs).

The plural of thou is also thou, e.g. a measurement of one hundredth of an inch is described as "10 thou". The th in thou is pronounced as in thousand, IPA: /θaʊ/, unlike the pronoun thou, /ðaʊ/, where the th is pronounced as in that.

The plural of mil is mils.

There is a related measurement for area known as the circular mil, based on a circle having a diameter of one mil. Because of this relationship, a mil and a circular mil might be easily confused.

Equivalence to other units of length

1 thou is exactly equal to:

0.001 international inches (1 international inch is equal to 1,000 thou)
0.0254 mm, or 25.4 μm (1 millimetre is about equal to 39.37 thou)

Equivalence to other units of length

Notes