Calculate fall percentage from vertical drop and horizontal run, or work out the fall over a distance for a 1 in 60, 1:80 or percentage gradient.

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Fall Percentage Formula

The following formula is used to calculate the fall percentage:

FP = (EC / HD) × 100

Variables:

  • FP is the fall percentage (%)
  • EC is the elevation change (feet)
  • HD is the horizontal distance (feet)

To calculate fall percentage, divide the nonnegative vertical drop by positive horizontal run in matching units, then multiply by 100. Use horizontal distance, not sloped length. A 100% fall is 45 degrees; percentages above 100 are valid steeper gradients. Mixed input units are converted by the calculator.

What is Fall Percentage?

Fall percentage is a way of expressing the steepness or slope of a terrain. It is calculated by dividing the change in elevation by the horizontal distance over which the change occurs, and then converting this ratio into a percentage. This measurement is used in civil engineering, construction and road design. The geometry alone does not establish safety, drainage performance or compliance with an applicable design standard.

How to Calculate Fall Percentage?

The following steps outline how to calculate the Fall Percentage.

  1. First, determine the elevation change (EC) in feet.
  2. Next, determine the horizontal distance (HD) in feet over which the elevation change occurs.
  3. Use the formula from above = FP = (EC / HD) * 100.
  4. Finally, calculate the Fall Percentage (FP).
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem:

Use the following variables as an example problem to test your knowledge.

elevation change (EC) = 15 feet

horizontal distance (HD) = 200 feet