Thousandth of an inch
|25.4×10−6 m||25.40 μm|
|US customary / Imperial units|
|83.3×10−6 ft||1.00×10−3 in|
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"). Both words come from roots meaning "1000": "thou" from the English "thousand", which is from the Germanic root for 1000, and "mil" from the Romance root for 1000.
Contexts of use 
- The thickness of items such as paper, film, foil, wires, paint coatings, latex gloves, plastic sheeting, and fibers
- For example, most plastic ID cards are about 30 thou (30 mils in US) in thickness.
- Manufacturing dimensions and tolerances, such as:
- 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.
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". To aid in gaining a concept of relative size, it is useful to know that a tenth is about 2.5 microns; thus, a micron is about 40% of a tenth. 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 than "tenths", such as in plug gauge and gauge block calibration, is typically expressed in other units, such as millionths of an inch.
Usage notes on mil versus thou 
The unit is generally referred to as a thou outside of the United States; within the U.S., mil was once the more common term, although as use of the metric system has became common within U.S. industry in recent decades, thou has begun to replace mil among some technical users to avoid confusion with millimeters, and today both terms are used, but in specific contexts one is traditionally preferred over the other. Thus "mil" tends to be used more than "thou" for the thickness of plastic sheet, while "thou" or "thousandths" tends to be used when discussing machined dimensions.
"Thou" began as a colloquial abbreviation of "thousandth", and it is not much encountered in older published books, where it was copy-editorially avoided, for the same reason that other colloquialisms were usually purged—that is, from a feeling that their use was somehow indecorous in published writing. (For example, although physicians have commonly said "exam" and "lab" for many decades, traditional copy-editing rules specifying that they always be spelled out as "examination" and "laboratory" have been enforced even up to the present day.) Most engineers and machinists consider such register-proscribing strictures to be unnecessary, which explains why they are not as invariably enforced since the advent of internet publishing.
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)
For machinists who need to maintain a continual "horse sense" of relative size, a good rule of thumb for quick and sufficiently accurate mental guesstimation is that each millimeter is 40 thou (.040"); a tolerance of plus-or-minus a quarter millimeter is equivalent to plus-or-minus 10 thou; and a micron is almost half a tenth.
History of usage 
- ...instead of our engineers and machinists thinking in eighths, sixteenths and thirty-seconds of an inch, it is desirable that they should think and speak in tenths, hundredths, and thousandths...
Up until this era, workers such as millwrights, boilermakers, and machinists measured only in traditional fractions of an inch, divided as far as 64ths. Each 64th is about 15 thou. Communication about sizes smaller than a 64th of an inch was subjective and hampered by a degree of ineffability—while phrases such as "scant 64th" were used, their communicative ability was limited by subjectivity. Dimensions and geometry could be controlled to high accuracy, but this was done by comparative methods: comparison against templates or other gages, feeling the degree of drag of calipers, or simply repeatably cutting, relying on the positioning consistency of jigs, fixtures, and machine slides. Such work could only be done in craft fashion: on-site, by feel, rather than at a distance working from drawings and written notes. This in turn limited the kinds of process designs that could work, because they limited the degree of separation of concerns that could occur.
The introduction of thou as a base unit for machining work required the dissemination of vernier calipers and screw micrometers throughout the trade, as the unit is too small to be measured with practical repeatability using rules alone. During the following half century, such measuring instruments went from expensive rarities to widespread, everyday use among machinists. Bringing more metrology into machining increased the separation of concerns to make possible, for example, designing an assembly to the point of an engineering drawing, then having the mating parts made at different firms who did not have any contact with (or even awareness of) each other—yet still knowing with certainty that their products would have the desired fit.
See also 
- "Corrosion Rate Conversion", Corrosionist.com
- "...the smallest move of one-tenth (not 0.1 but 0.0001 of an inch)...", Dan Nelson, The CNC Toolbox, p89
- Mil at How Many? A Dictionary of Units of Measurement by Russ Rowlett
- University of Queensland: PCB design FAQ
- Thou at How Many? A Dictionary of Units of Measurement by Russ Rowlett
- Edkins, Jo. "Small units". Imperial Measures of Length. Jo Edkins. Retrieved 2009-09-23.
- "A Paper on Standard Decimal Measures of Length", Manchester, 1857