Determine the horizontal distance between you (or any starting point) and the top of the mountain or hill.
Using a topographic map, or other information, determine the difference in elevation between the start and end points being measured. The elevation and distance measurements need to be in the same units.
Divide the height (elevation) of the incline by the horizontal distance between the start and end points. Enter these two amounts into your calculator, elevation first, divided by the distance.
Press the inverse tangent key on your calculator. On most calculators, you will need to press a "shift" key followed by the "tan" key. Press the equals key to get the angle of elevation. For example, if a mountain was 100 feet high, and looking on a map, you were 200 feet away from the top, 100/200 = 0.5, shift tan = 26.57. In this case the angle of elevation would be 26.57 degrees.