Be aware that the term grade should not be confused with an angle. The two are very different. For example, a flat surface can be referred to as a 0 percent grade or a 180-degree angle: there is no change in height as the road continues. (In mathematical terms, there is no rise over the run.) A 100 percent grade would be equivalent to a 45-degree angle or, in other words, the rise would be equal to the run. It is also possible to have an infinite percent grade as that is defined as any amount of rise with no run at all. This would be equivalent to the road going straight up at a 90-degree angle. Thus, not a realistic measurement for a roadway.
This script is made up of two calculators. The first is designed to give the percent of grade change on highways located in mountain terrains. The calculation is good for inclines (uphill), sometimes called upgrades or declines (downhill), often called downgrades. It is based on how much vertical change in feet takes place, up or down, over a given horizontal distance. The horizontal distance is often difficult to determine as it is NOT based on driving distance (including curves in the road). It is based on straight line distance. The lower calculator is designed to give the actual distance change if you know the percentage and horizontal distance. Our example (in both calculators) is for a 1% downgrade (or incline). Over a 1 mile horizontal distance, if the elevation change is 52.8 feet, that is a 1% grade change. You may wish to use our metric version instead.
Special thanks to John Bevan, Ph.D., for his help on this calculator.
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last updated 5/20/11