Use a protractor to measure the angle directly
Identify the two rays or lines that form the angle
Place the protractor’s center on the vertex
Align one ray with the 0° line
Read the degree mark where the other ray crosses
Use the formula for complementary angles: angle = 90° – given angle
Use the formula for supplementary angles: angle = 180° – given angle
Use the formula for a triangle’s angles: angle = 180° – sum of other two angles
Use the formula for a quadrilateral’s angles: angle = 360° – sum of other three angles
Use trigonometry if side lengths are known
Use sine: sin(angle) = opposite/hypotenuse
Use cosine: cos(angle) = adjacent/hypotenuse
Use tangent: tan(angle) = opposite/adjacent
Solve for the angle with inverse trig functions
Use a calculator in degree mode if needed