Thou (length)

Thousandth of an inch
Unit conversions
1 thousandth of an inch in ...	... is equal to ...
imperial & US customary systems	0.001 in
SI units	25.40 μm

A thousandth of an inch is a derived unit of length in an inch-based system of units. Equal to 0.001 inches, it is normally referred to as a thou /ˈθaʊ/, a thousandth, or (particularly in the United States) a mil.

The plural of thou is also thou (thus one hundredth of an inch is "10 thou"), while the plural of mil is mils (thus "10 mils"). The words are shortened forms of the English and Latin words for "thousand" (mille). The US Customary mil can be confused with the millimetre, which is the standard meaning for "mil" or "mils" (plural) in British English and European engineering circles. This can cause problems with spoken dimensions or with those who are not familiar with alternative uses of the term. One US mil is approximately 1/40th of a millimetre at 0.0254 mm, or 25.40 μm.

The thou, or mil, is most commonly used in engineering and manufacturing. For example in specifying:

There are also compound units such as "mils per year" used to express corrosion rates.

A related measurement for area known as the circular mil, is based on a circle having a diameter of one mil.

In machining, where the thou is often treated as a basic unit, 0.0001 inches can be referred to as "one tenth", meaning "one tenth of a thou". (The metric comparison is discussed below.) Machining "to within a few tenths" is often considered very accurate, and at or near the extreme limit of tolerance capability in most contexts. Greater accuracy (tolerance ranges inside one tenth) apply in only a few contexts: in plug gauge and gauge block manufacturing or calibration, they are typically expressed in millionths of an inch or, alternatively, in micrometres; in nanotechnology, nanometres or picometres are used.

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