If a radian is defined as the angle formed by an arc length of a circle subtended at the center that is equal to its radius, then there are 2π radians, or 2π radii, that fit around the circumference of the circle. In other words, 360° equals 2π radians, or one radian is 180°/π, which is about 57.3°.
by MathPlus January 22, 2021