Thou (thou)
The thou (also called the mil) is a unit of length equal to one-thousandth of an inch or 25.4 micrometers (0.0254 mm). It is used in engineering and manufacturing for specifying tight tolerances, wire gauges, paint film thickness, and plastic film specifications. The name "thou" is short for "thousandth" (of an inch).
Definition
One thou (mil) equals exactly 0.001 inches, 25.4 micrometers, 0.0254 millimeters, or approximately 0.0000254 meters. There are 1,000 thou in one inch. The term "mil" in this context specifically means one-thousandth of an inch; it is unrelated to the millimeter (mm) or the military angular mil.
History
The thou emerged in English engineering during the Industrial Revolution as machine tools became precise enough to work to one-thousandth of an inch. Joseph Whitworth, the pioneer of precision engineering in 19th-century England, promoted measurement to the thou as a manufacturing standard. In the United States, the unit is more commonly called a "mil" (not to be confused with the millimeter or the angular mil). As metric measurement gained ground, micrometers increasingly replaced thou in specifications, but the unit persists in American manufacturing, electronics, and certain specialised industries.
Common Uses
In the US electronics industry, PCB (printed circuit board) trace widths, copper foil thickness, and solder mask specifications use mils (thou). Plastic wrap and trash bags are specified in mils - typical kitchen cling wrap is about 0.5 mil thick. Wire gauges and coating thicknesses in automotive, aerospace, and paint industries use thou. Paper and card stock thickness is sometimes given in mils. In the UK, thou appears in traditional engineering workshops alongside or in place of metric equivalents.
Did You Know? Facts About Thou
- Standard kitchen cling wrap is about 0.5 thou (12.7 micrometers) thick.
- Human hair is typically 2-3 thou (50-75 micrometers) in diameter.
- A standard business card is about 12 thou (0.3 mm) thick.
- In American PCB manufacturing, trace widths are specified in mils - a common minimum is 6 mil (0.15 mm).
- The thou and the mil are the same unit - "thou" is British usage and "mil" is American, but both equal one-thousandth of an inch.