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- See also: Mil
| 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)