HOW TO USE IT
How to use the Drainage Swale Calculator
- 1
Measure the cross-section: Record the top width, bottom width and depth. The average of the widths times the depth is the section area.
- 2
Compute the section area: A trapezoid uses the average width times the depth. (6 + 2) ÷ 2 × 1.5 = 6 sq ft.
- 3
Multiply by the length: The volume is the section area times the swale length. 6 sq ft × 50 ft = 300 cu ft.
- 4
Check grade and fall: The calculator converts longitudinal grade into total fall. It estimates excavation geometry only; hydraulic capacity, erosion control and outlet design still require professional verification.
THE MATH
Drainage swale calculation method
Average the top and bottom width, multiply by depth and length for excavation volume. A 50 ft swale with 6 ft and 2 ft widths at 1.5 ft deep is 300 cu ft; a 1% grade adds 0.5 ft of fall.A trapezoid uses the average width times the depth. (6 + 2) ÷ 2 × 1.5 = 6 sq ft.
PRACTICAL EXAMPLE
Worked example for Drainage swale
Average the top and bottom width, multiply by depth and length for excavation volume. A 50 ft swale with 6 ft and 2 ft widths at 1.5 ft deep is 300 cu ft; a 1% grade adds 0.5 ft of fall.
FAQ
Frequently asked questions
How do you calculate a drainage swale?
Average the top and bottom width, multiply by depth and length for excavation volume. A 50 ft swale with 6 ft and 2 ft widths at 1.5 ft deep is 300 cu ft; a 1% grade adds 0.5 ft of fall.
What should I verify before using the result?
The volume is for excavation planning. Verify the hydraulic design before you dig.
Is this a final quote or design?
No. Use it as a planning estimate, then confirm measurements, local rates, product requirements, permits and jobsite conditions before bidding or building.