How to Find Reference Angle in Radians?

Identify the angle in standard position

Determine the quadrant of the terminal side

Use the appropriate reference-angle formula:

Quadrant I: reference angle = θ

Quadrant II: reference angle = π – θ

Quadrant III: reference angle = θ – π

Quadrant IV: reference angle = 2π – θ

If the angle is negative, first find a positive coterminal angle or use the quadrant based on rotation direction

If the angle is greater than 2π, subtract multiples of 2π until it lies between 0 and 2π

Express the reference angle in radians