Find the contour interval amount on the map. This is the amount each contour line is a change in elevation. For example, assume each contour line represents 20 feet.
Find the elevation points of the two points. Use the contour interval if the contour line does not have a label on it. In the example, assume one elevation point is two lines above 4,000 feet. So the elevation point is 4,040 feet. Then assume the other elevation point is 3,800 feet.
Find the difference between the two elevation points to calculate the rise. In the example, 4,040 feet minus 3,800 feet equals 240 feet.
Measure the distance between the two points and convert it to actual feet using the map's scale. For example, assume the measurement is 1.5 inches with a map scale of 1 inch for 3,000 feet. Then, 3,000 feet times 1.5 equals 4,500 feet. This is the run.
Divide rise by run to calculate your slope. In the example, 240 feet divided by 4,500 feet equals 0.0533. Multiply this by 100 to get your percent slope -- 5.33 percent.