mil

Mil is a term used for two distinct measurement concepts in technical contexts.

As a unit of length, mil refers to one thousandth of an inch. Abbreviated as mil or thou, 1 mil equals 0.001 inch (0.0254 millimeters). This non-SI unit is commonly used in manufacturing and engineering to describe thicknesses and tolerances for materials such as plastic film, coatings, wire insulation, and printed circuit boards. Because it is not an official SI unit, mil measurements are often coordinated with decimals or converted to metric values for international production.

As an angular measure, mil denotes a unit of angle used in ballistics, surveying, and targeting. In this context, a circle is divided into a number of mils, allowing users to express small rotations conveniently. In the widely used NATO convention, a circle comprises 6,400 mils, making one mil about 0.05625 degrees or roughly 0.00098 radians. This enables practical calculations such as estimating range or drift: small angular changes measured in mils translate into linear displacements at distance. It is worth noting that several mil systems exist historically and regionally, so exact numbers can vary outside the NATO standard.

In summary, mil can denote a thickness unit (one thousandth of an inch) or an angular unit (a milliradian-like division of a circle), with the most common modern standard being 6,400 mils per circle in NATO practice.