Thou (unit of length)

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A thou, also known as a mil, is a unit of length equal to 0.001 inches (a "milli-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.

The older name mil originated in the United States, with the alternate thou being used in Britain. 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.^[1]^[2] However, the mil is still in common use in the United States for the thickness of plastic sheeting or bags.

The term "mil" is still commonly used in the United States in the field of property taxation. A "mil" refers to the tax rate on $1000 of assessed property value. For example, a property tax rate of 0.001 would commonly be referred to as a 1 mil tax rate.

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.

The plural of "thou" is also "thou"; for example, a measurement of one hundredth of an inch is described as "10 thou". The "th" in "thou" is pronounced as in "thousand", IPA: /θaʊ/, and not as in "that" or the pronoun "thou", /ðaʊ/.

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.