Angles in a triangle add to 180°. Enter two angles and the working finds the third. In an isosceles triangle the base angles are equal: enter the angle between the equal sides, or one base angle. For parallel lines, use angles in parallel lines.
The red label is the angle this page will find. The picture is not drawn to the size of your angles.
Each given angle must be greater than 0° and less than 180°. The angles you enter must leave a positive angle in the triangle.