A radian is the angle at the centre where the arc is as long as the radius. So the angle in radians is just arc ÷ radius. Drag to open it, and watch the radius roll around the circle.
Arc equals radius
Angle—
In degrees—
—
1 radian ≈ 57.3° · π radians = 180° · 2π radians = a full turn (360°)