There are exactly [math]2\pi[/math] radii (approximately 6.28) radii that wrap perfectly around a circle. [br]Thus, [math]2\pi[/math] radians = 1 revolution = 360 degrees.[br]Also, the circumference of a circle = (number of radii that wrap around circle)*(length of each radius)[br]So, [math]C=2\pi\cdot r[/math].