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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.
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 — and not as in "that" or the pronoun "thou" — IPA .
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.

Contents

Equivalence to other units of length

Notes

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)

Notes

1. Mil at How Many? A Dictionary of Units of Measurement by Russ Rowlett
2. Thou at How Many? A Dictionary of Units of Measurement by Russ Rowlett