What is "milliradian"

n. (context mathematics English) A unit of plane angular measure equal to one thousandth of a radian

n. a unit of angular distance equal to one thousandth of a radian

A milliradian, simply called a mil or mrad (often capitalized MRAD), is an angular measurement which is defined as a thousandth of the radian ( = 0.001 radian), meaning that a full circle of 360° ( degrees) corresponds to approximately 6283 milliradians. The exact definition of a mil (circle/6283.185…) is used in optical reticles and on scope adjustment knobs ("turrets").

There are also derived (approximated) definitions used for land mapping and artillery, where for instance a compass with 6400 NATO mils is used instead of a 360° compass, achieving higher precision. The term angular mil is used in artillery. Other derived definitions of 6000 or 6300 mils per circle also exist depending on country, see below.

Mil adjustment is commonly used in the mechanic adjustment of iron and scope sights in shooting sports, where sight adjustment using mils is particularly useful together with metric units when shooting at regular distances such as 100 m or 300 m, because for instance one click of a sight adjustment of 0.1 MRAD will move the point of impact exactly 1 cm at 100 m and 3 cm at 300 m respectively. This is not the case when using MOA with imperial units, where one often simplifies 1 MOA being equal to 1 inch at 100 yards while in reality 1 MOA at that distance equals 1.047 inches, producing a small error that will increase the more the sight is adjusted or the longer the shooting distance. Therefore in particular a spotter in long range shooting (i.e 1000 m and above) theoretically can provide more precise shot corrections using a mil reticle.

Mil reticles can easily be used in optics for range estimation because of the precise mathematical simplification that can be made with such small angular measurements, exploiting the attribute of radians that a small angle is a good approximation to its sine. For range estimation, using metric units will again be particularly useful because it requires less mental calculation in the field. Using the range estimation formula with the units meters for range and millimeters for target size it is just a matter of moving decimals and do the division, without the need of multiplication with additional constants, thus producing fewer rounding errors.