Convert circle to radian

circle

Definition: Circle is a measure of plane angle or rotation. For conversion purposes, its value is 360 times the reference unit, degree.

radian

Definition: A radian (symbol: rad) is the standard unit of angular measure. It is a derived unit (meaning that it is a unit that is derived from one of the seven SI base units) in the International System of Units. An angle's measurement in radians is numerically equal to the length of a corresponding arc of a unit circle. One radian is equal to 180/π (~57.296) degrees.

History/origin: Measuring angles in terms of arc length has been used by mathematicians since as early as the year 1400. The concept of the radian specifically however, is credited to Roger Cotes who described the measure in 1714. Although he described the unit, Cotes did not name the radian, and it was not until 1873 that the term "radian" first appeared in print.

Current use: The radian is widely used throughout mathematics as well as in many branches of physics that involve angular measurements. Although the symbol "rad" is the accepted SI symbol, in practice, radians are often written without the symbol since a radian is a ratio of two lengths and is therefore, a dimensionless quantity. As such, when angle measures are written, the lack of a symbol implies that the measurement is in radians, while a ° symbol would be added if the measurement were in degrees.