Measure the horizontal span of the angle, the run, and call this number "x." For example, if you want to find the angle of a symmetrically sloped roof whose total width is 20 feet, then the run of the angle is half this distance. This gives you x = 10.
Measure the vertical span of the angle, the rise, and call this number "y." For instance, if the roof is 7.5 feet tall, then y = 7.5.
Divide y by x and call this number "z." This number is the tangent of the angle. For example, since 7.5/10 = 0.75, z = 0.75.
Compute the inverse tangent of z. On your calculator, this is the button that says either "arctan," "atan" or the word "tan" with a "-1" superscript. For example, if z = 0.75, then arctan(z) = 36.87. This number is the degree measure of the angle. In this example, the roof's pitch is 36.87 degrees.