Thou (length)

1 thou =

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 (pronounced /ˈθaʊ/) also known as a mil or point, is the verbalized abbreviation for "thousandth(s) of an inch." It is a unit of length equal to 0.001 inch.

The plural of thou is also thou; e.g., a measurement of one hundredth of an inch is written as 10 thou, while the plural of mil is mils. The th in thou is pronounced as in thousand, /ˈθaʊ/, unlike the pronoun thou, /ðaʊ/, where the th is pronounced as in that.

The introduction of the thou in 1844 is generally attributed to Joseph Whitworth.^[1]

In the United States, the usual term has been mil, whereas thou has been more common elsewhere.^[2][3]

Usage

The thou (or mil in US) is sometimes used in engineering, machining, 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 automobile engines [A typical example is the thickness of the head gasket, or the amount of material to be removed from the head to adjust the compression ratio of the cylinders.]
• in the servicing of automobile engines [Typical examples include a spark-plug gap or ignition points gap]
• the manufacture of printed circuit boards (PCBs)
• tolerance specifications on hydraulic cylinders.

There are also compound units such as "mils per year" used to express corrosion rates.^[4]

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

In machining

In machining, it is common to work to positions with accuracies of one thou, several thou, or even a fraction of a thou. One tenth of a thou is one ten-thousandth of an inch (0.0001"). Because machinists often think in terms of the thou as the "base unit" in their context, it follows that they call 0.0001" "one tenth", meaning "one tenth of a thou". Machining "to within a few tenths" is usually considered very accurate, with few and special exceptions. This level of accuracy is more common in Tool and die work than in production or MRO work.

The metric analog of thou and tenths [referring back to the inch] is hundredths and microns [referring back to the millimeter]. (The name "micron" is now deprecated by SI in favor of "micrometre", but "micron" is still commonly heard in industry.) This metric "scale of thinking" represents slightly smaller distances than does the corresponding inch-based scale of thinking (thou and tenths). Therefore, machining in metric units is occasionally preferred to working in thou for extremely precise work. But there are other factors involved in deciding which one is used in any particular case, and this difference rarely matters. On CNC machine tools, all linear motion is based on the minimum increment of the control system, which is either one "micron" (one micrometre, 0.001 mm) or one "tenth" (0.0001"). The metric standard widely prevails today, both because it is physically smaller and because SI prevails in measurement worldwide, and globalized manufacturing uses ISO standards as its benchmarks. But the control allows the user to work in either system, because the computer does the unit conversions automatically (rounded to the nearest minimum increment).

mil versus thou

In the United States, the usual term has been mil, but as use of the metric system became more common thou has begun to replace mil among some technical users to avoid confusion with millimeters.^[2][5][3]

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 millimeter is about equal to 39.37 thou)

The most common thickness of most plastic ID cards, 30 thou (30 mils in US), is equal to 0.03 inches (0.76 mm).

See also

• Angular mil

Notes

1. ^ Edkins, Jo. "Small units". Imperial Measures of Length. Jo Edkins. http://gwydir.demon.co.uk/jo/units/length.htm#small. Retrieved 2009-09-23. 
2. ^ ^{a b} Mil at How Many? A Dictionary of Units of Measurement by Russ Rowlett
3. ^ ^{a b} University of Queensland: PCB design FAQ
4. ^ "Corrosion Rate Conversion", Corrosionist.com
5. ^ Thou at How Many? A Dictionary of Units of Measurement by Russ Rowlett