Basics

Arc length formula: s = rθ

The arc cut off by a central angle θ on a circle of radius r has length s = rθ. θ must be in radians - the formula is only this clean because a radian is defined as the angle whose arc equals the radius. In degrees you would need s = (πrθ)/180 instead.

Related Basics identities

Practise it

Reading a formula is not the same as recalling it under time pressure. The workspace drills Basics questions in the quiz, and the triangle solver shows a worked solution for any triangle you type in.

Or browse all 45 identities in the full reference, or read the trigonometry guides for the method behind them.