Convert grad to radian

The gon, also called grad or grade, divides the full circle into 400 parts, so a right angle is 100 gon and 1 gon = 0.9°. Its attraction is decimal subdivision: no minutes and seconds. It is the working unit of surveyors in much of continental Europe, and theodolites and total stations sold there read in gon; the 'grad' button on calculators refers to it.

The radian is the natural unit of angle: the angle whose arc is as long as the radius. A full circle is 2π rad, so 1 rad ≈ 57.3°, and the formulas of physics and calculus (arc length = r·θ, sin x ≈ x for small x) only hold in radians. Calculators and programming languages expect radians unless told otherwise, which is the source of many wrong answers.