Thou (unit)

Definition and conversions

One thou = 0.001 inch. In metric terms this is 0.0254 millimetres (25.4 micrometres). Simple conversions often used in workshops and engineering notes include:

• 5 thou = 0.005 in = 0.127 mm
• 10 thou = 0.010 in = 0.254 mm
• 100 thou = 0.100 in = 2.54 mm

History and terminology

The word "thou" arose as an informal short form of "thousandth" and was historically used in British engineering. In the United States the alternative name "mil" became widespread. The abbreviation "th" is sometimes used, but context matters because it can be mistaken for other notations. The term "thou" is distinct from the archaic English pronoun of the same spelling; engineers and technicians typically avoid confusion by writing "thou (th)" or "mil."

Common uses and importance

Thousandths of an inch are widely used where fine control is required: machining and metalworking tolerances, gasket and film thicknesses, wire and sheet-metal specifications, and printed circuit board trace widths. In electronics, PCB designers often specify trace widths and spacing in mils. The unit provides a practical, familiar scale for many tradespeople and engineers operating in imperial units.

Practical notes and distinctions

Although convenient, the thou is non-SI; many international standards encourage using millimetres. The word "mil" can be ambiguous in global contexts: outside North America it may refer to other measures (for example, the Scandinavian "mil" is a different length). When precision and international exchange matter, professionals usually give both the thousandth-of-an-inch value and the millimetre equivalent for clarity. For more technical context see the Imperial system and the United States customary units.