Channel volume
The calculator estimates a trapezoid cross-section from bottom width, depth, and side slope, then multiplies by swale length.
RainYard Tools
Shallow channel
Swale Capacity Calculator
Estimate how much water a simple trapezoid swale can temporarily hold.
Swale storage524 galTemporary volume at selected flow depth.
Cross section1.75 sq ftApproximate trapezoid area.
Volume70.0 cu ftSame capacity in cubic feet.
Example
A 40 ft swale with 2 ft bottom width, 0.5 ft flow depth, and 3:1 side slopes holds about 524 gallons before overflow.
Swales slow water down
A planted swale is often better for shallow sheet flow than a buried pipe, especially where water needs to spread and soak in.
Capacity is temporary
This is a temporary volume check, not a guarantee that water will infiltrate fast enough.
Questions
Common checks.
Can a swale stay wet?
It should drain in a reasonable time. Persistent standing water may mean soil or outlet issues.
Should a swale be planted?
Vegetation helps slow flow and stabilize soil.