The percent slope is a ratio between the change in the elevation versus the distance covered, also known as the rise over the run. The slope is important when building roads because some vehicles cannot withstand slopes that are too steep, either going up or going down. Similarly, if you were making a wheelchair ramp, you would want to make sure the slope of the ramp is manageable. To calculate the percent slope, you need to know the change in elevation and the distance.
Determine the change in elevation from the start to the finish and call this E. If you are measuring a ramp, you can use a measuring tape. If you are measuring the percent slope of a road, you can use a topographic map.
Determine the distance of the change in elevation takes place over and call this D. For small ramps, you could use a measuring tape. For longer distances, you can use the scale on a map to figure the distance.
Calculate the slope by plugging D and E into the following formula: E/D. For example, if there were a 100 foot change in elevation over 500 feet of road, you would get 0.2.
Multiply the result from step 3 by 100 to calculate the percent slope. Finishing the example, you would multiply 0.2 times 100 to find that the percent slope equals 20 percent.