Calculate drainage fall from pipe run and gradient, such as 1 in 40 or 1 in 80, or find the gradient from run and fall, in metric or imperial units.

Use the level distance, not the sloping pipe length.

For 1 in 40, enter 40. Check the gradient your local building code requires for the pipe size.

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Drainage Fall Formula

The following formula is used to calculate the drainage fall:

F = (G / 100) × D

Variables:

  • F is the fall (meters)
  • G is the gradient (%)
  • D is the horizontal distance (meters), not the sloping pipe length

To calculate the drainage fall, multiply the gradient percentage by the distance and then divide by 100 to convert the percentage to a decimal.

What is Drainage Fall?

Drainage fall is the measure of vertical drop within a drainage system over a certain distance. It is an important aspect of civil engineering and building design, ensuring that water flows away from structures effectively. The gradient is typically expressed as a percentage, representing the ratio of the fall to the horizontal distance.

How to Calculate Drainage Fall?

The following steps outline how to calculate the Drainage Fall.

  1. First, determine the horizontal distance (D), in meters. Use the level run rather than the sloping pipe length.
  2. Next, determine the gradient (G) of the drainage system as a percentage.
  3. Use the formula F = (G / 100) * D to calculate the fall (F).
  4. Finally, calculate the Drainage Fall (F) in meters.
  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.

Distance (D) = 50 meters

Gradient (G) = 2%

Using the formula, the fall (F) would be calculated as follows:

F = (2 / 100) * 50 = 1 meter