The radian, denoted by the symbol ,[1] is the SI unit for measuring angles, and is the standard unit of angular measure used in many areas of mathematics. The unit was formerly an SI supplementary unit (before that category was abolished in 1995) and the radian is now an SI derived unit.[2] The radian is defined in the SI as being a dimensionless value, and its symbol is accordingly often omitted, especially in mathematical writing.

Unit systemSI derived unit
Unit ofAngle
In unitsDimensionless with an arc length equal to the radius, i.e. 1 m/m
1 rad in ...... is equal to ...
   milliradians   1000 mrad
   turns   1/2π turn
   degrees   180/π ≈ 57.296°
   gradians   200/π ≈ 63.662g
An arc of a circle with the same length as the radius of that circle subtends an angle of 1 radian. The circumference subtends an angle of 2π radians.