▶ Radian

A radian is a unit used to measure angles.

One radian is the angle formed when the length of an arc of a circle is equal to the radius of that circle.

• A full circle measures 2π radians.
• Half a circle measures π radians.
• 180° = π radians.

Radians are often used in higher-level mathematics, especially in trigonometry and calculus, because they make formulas simpler and more natural to work with.

Key Idea: A radian measures angles using the radius of a circle instead of degrees.