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About these units
Inch (in)
An inch is defined as exactly 25.4 millimeters, a precise metric-based definition that preserves its usefulness within imperial systems. Historically, the inch was based on the width of three barleycorns placed end-to-end, a charming relic of medieval measurement practices. Today, the inch is vital in manufacturing, woodworking, consumer electronics (e.g., screen sizes), and tooling standards across the US and partially in the UK. Its size is small enough to offer usable precision yet large enough to avoid unwieldy fractions for many everyday objects. Even in predominantly metric industries, certain products—such as plumbing parts, bicycle rims, and camera mounts—retain inch-based standards for compatibility. This persistence shows how technological ecosystems can outlive their measurement origins.
Mil (mil)
A mil is an imperial-derived engineering unit equal to one-thousandth of an inch. It is used extensively in manufacturing, machining, and electrical engineering. Printed circuit board (PCB) trace widths, for instance, are often measured in mils because the unit provides a convenient scale for precision without resorting to decimals in inches. Because 1 mil equals 25.4 micrometers, it occupies a scale appropriate for tolerances in industrial production, coatings, film thickness, and gaskets. The mil is especially common in the United States, where certain engineering standards still rely on the imperial system. It provides an intuitive small-scale measurement for technicians accustomed to inches, helping avoid errors that might result from metric conversions.