A thou, also known as a mil, is a unit of length equal to one thousandth of an international inch. It is sometimes used in engineering and in the specification of:

• the thickness of items such as film, foil, wires, and fibers; and
• manufacturing dimensions and tolerances.

Use of the thou is now generally deprecated in favour of the use of SI units of length such as the micrometre.

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 .

Equivalence to other units of length

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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 equal to about 39.37 thou)

Units of length | Imperial units