Calculate ditch grade, angle and 1-in-N ratio from an explicitly selected input, or estimate trapezoidal-channel Manning flow for steady, uniform conditions. Positive rise is upward and negative rise is downward.

Signed rise is positive upward and negative downward.

Manning estimates assume steady, uniform flow. Roughness presets are examples; verify site conditions. Results do not establish safe excavation, erosion control or drainage design.

Ditch Slope Formula

The following formula is used to calculate the slope of a ditch:

S = (VD / HD) × 100

Variables:

  • S is the slope (%)
  • VD is the signed vertical rise (use the same length unit as HD)
  • HD is the positive horizontal distance (same unit as VD)

To calculate the slope of a ditch, divide the vertical drop by the horizontal distance and then multiply by 100 to convert it to a percentage.

What is Ditch Slope?

Ditch slope is a measure of the steepness or incline of a ditch or channel. It is typically expressed as a percentage, representing the vertical drop per unit of horizontal distance. Slope influences drainage, but this calculation alone does not establish adequate capacity, erosion control or a safe ditch design.

How to Calculate Ditch Slope?

The following steps outline how to calculate the Ditch Slope.

  1. First, measure the horizontal distance (HD) of the ditch in feet.
  2. Next, measure the vertical drop (VD) of the ditch in feet.
  3. Use the formula S = (VD / HD) * 100 to calculate the slope (S).
  4. Finally, calculate the Ditch Slope (S) as a percentage.
  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.

Horizontal distance (HD) = 100 feet

Signed vertical rise (VD) = −10 feet for a downward slope. With 100 feet of horizontal run, grade = −10%, angle ≈ −5.7106°, and N = −10.