Thou (length)

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.

As the metric system became more common in the States, use of the term mil began to decline among technical users due to the possible confusion with millimeters. However, the mil is still in common use in the United States for the thickness of plastic sheeting or bags.

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,, unlike the pronoun thou, , 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 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)