Slope in landscaping is also known as grade. It shows how much change in elevation there is over a given distance. It is important to keep both of these measurements in the same unit; for example, feet. If you imagine the lawn as a right triangle, the slope is the hypotenuse. The change in elevation then is the base of the triangle and the distance is the third side. As such, the distance must be flat and does not take into account any slope.
Measure the elevation at one point in the lawn. Measure the elevation at a second point in the lawn. Subtract the results to find the change in elevation. For example, assume at point one you are at an elevation of 1000 feet and at point two you are at an elevation of 1010 feet. So, 1100 feet minus 1000 feet equals 10 feet.
Measure the distance between the two points. In the example, assume the distance is 50 feet.
Divide the change in elevation by the distance to find slope. In the example, 10 feet divided by 50 feet equals 0.2 or 20 percent slope.