Use a protractor to measure the angle directly
If two angles are complementary, subtract the known angle from 90°
If two angles are supplementary, subtract the known angle from 180°
In a triangle, use 180° minus the sum of the other two angles
In a quadrilateral, use 360° minus the sum of the other three angles
Use angle relationships such as vertical angles being equal
Use parallel line rules such as corresponding angles being equal
Use alternate interior angles being equal when lines are parallel
Use the exterior angle theorem in triangles
Use trigonometric ratios if side lengths are known
Use the sine rule when one side and its opposite angle are known
Use the cosine rule when two sides and the included angle are known
Use algebra to solve for the unknown angle if an equation is given