Thou (unit of length)
Encyclopedia
A thou also known as a mil or point, is the verbalized abbreviation for "thousandth(s) of an inch." It is a unit
Units of measurement
A unit of measurement is a definite magnitude of a physical quantity, defined and adopted by convention and/or by law, that is used as a standard for measurement of the same physical quantity. Any other value of the physical quantity can be expressed as a simple multiple of the unit of...

 of length
Length
In geometric measurements, length most commonly refers to the longest dimension of an object.In certain contexts, the term "length" is reserved for a certain dimension of an object along which the length is measured. For example it is possible to cut a length of a wire which is shorter than wire...

 equal to 0.001 inch
Inch
An inch is the name of a unit of length in a number of different systems, including Imperial units, and United States customary units. There are 36 inches in a yard and 12 inches in a foot...

.

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
Thou
The word thou is a second person singular pronoun in English. It is now largely archaic, having been replaced in almost all contexts by you. It is used in parts of Northern England and by Scots. Thou is the nominative form; the oblique/objective form is thee , and the possessive is thy or thine...

, /ðaʊ/, where the th is pronounced as in that.

The introduction of the thou in 1844 is generally attributed to Joseph Whitworth
Joseph Whitworth
Sir Joseph Whitworth, 1st Baronet was an English engineer, entrepreneur, inventor and philanthropist. In 1841, he devised the British Standard Whitworth system, which created an accepted standard for screw threads...

.

In the United States, the usual term has been mil, whereas thou has been more common elsewhere.

Usage

The thou (or mil in US) is sometimes used in engineering
Engineering
Engineering is the discipline, art, skill and profession of acquiring and applying scientific, mathematical, economic, social, and practical knowledge, in order to design and build structures, machines, devices, systems, materials and processes that safely realize improvements to the lives of...

, 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
    Tolerance (engineering)
    Engineering tolerance is the permissible limit or limits of variation in# a physical dimension,# a measured value or physical property of a material, manufactured object, system, or service,# other measured values ....

    .
  • 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
    Printed circuit board
    A printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components using conductive pathways, tracks or signal traces etched from copper sheets laminated onto a non-conductive substrate. It is also referred to as printed wiring board or etched wiring...

     (PCBs)
  • tolerance specifications on hydraulic cylinder
    Hydraulic cylinder
    A Hydraulic cylinder is a mechanical actuator that is used to give a unidirectional force through a unidirectional stroke. It has many applications, notably in engineering vehicles.- Operation :...

    s.


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

There is a related measurement for area
Area
Area is a quantity that expresses the extent of a two-dimensional surface or shape in the plane. Area can be understood as the amount of material with a given thickness that would be necessary to fashion a model of the shape, or the amount of paint necessary to cover the surface with a single coat...

 known as the circular mil
Circular mil
A circular mil is a unit of area, equal to the area of a circle with a diameter of one mil . It is a convenient unit for referring to the area of a wire with a circular cross section, because the area in circular mils can be calculated without reference to pi .The area in circular mils, A, of a...

, based on a circle
Circle
A circle is a simple shape of Euclidean geometry consisting of those points in a plane that are a given distance from a given point, the centre. The distance between any of the points and the centre is called the radius....

 having a diameter
Diameter
In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints are on the circle. The diameters are the longest chords of the circle...

 of one mil.

In machining

In machining
Machining
Conventional machining is a form of subtractive manufacturing, in which a collection of material-working processes utilizing power-driven machine tools, such as saws, lathes, milling machines, and drill presses, are used with a sharp cutting tool to physical remove material to achieve a desired...

, 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
Tool and die maker
Tool and die makers are workers in the manufacturing industry who make jigs, fixtures, dies, molds, machine tools, cutting tools , gauges, and other tools used in manufacturing processes...

 than in production or MRO
Maintenance, repair, and operations
Maintenance, repair, and operations or maintenance, repair, and overhaul involves fixing any sort of mechanical or electrical device should it become out of order or broken...

 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
International System of Units
The International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...

 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.

Equivalence to other units of length

1 thou is exactly equal to:
  • 0.001 international inch
    Inch
    An inch is the name of a unit of length in a number of different systems, including Imperial units, and United States customary units. There are 36 inches in a yard and 12 inches in a foot...

    es (1 international inch is equal to 1,000 thou)
  • 0.0254 mm, or 25.4 μm
    Micrometre
    A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...

     (1 millimeter is about equal to 39.37 thou)


The most common thickness of most plastic ID card
Identity document
An identity document is any document which may be used to verify aspects of a person's personal identity. If issued in the form of a small, mostly standard-sized card, it is usually called an identity card...

s, 30 thou (30 mils in US), is equal to 0.03 inch (0.762 mm).
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