Stormwater Calculator
Estimate runoff volume and rain-barrel count from roof area, impervious area, rainfall, and runoff coefficient.
Interactive calculator · Method STORMWATER-2.1 · Last reviewed: 2026-11-03
Estimate event runoff and barrel count
Enter catchment area, event rainfall, an estimating runoff coefficient and usable barrel capacity.
Your inputs
Default worked example. Edit the inputs to calculate your scenario.
Default worked example
(1,000 + 500) sq ft × 1/12 ft rain × 0.9 runoff coefficient = 112.5 cubic feet. Barrel count uses 55 US gal each, before any volume rounding.
How to use the calculator
Enter non-overlapping horizontal roof and other contributing plan areas in square feet, event rainfall in inches, an entered runoff fraction from 0 to 1 and usable empty capacity per identical container in US gallons. Roof area or rainfall may be 0; no forecast is supplied.
What the result means
For 1000 + 500 sq ft, one inch of rain and fraction 0.9, modeled runoff is 112.5 cubic feet or 841.6 US gal as displayed; 55 US gal usable containers require a count of 16. The exact boundary 77 sq ft × 1 inch × fraction 1 gives 48 US gal and exactly 48 one-gallon containers. The contributing plan area card retains tenths; tiny positive volumes use a less-than display.
Formula and units
Event cubic feet = (roof plan area + other plan area) × rainfall inches/12 × entered runoff fraction. One square foot × one inch is exactly 144/231 = 48/77 US liquid gallons. Containers = ceiling(unrounded event gallons ÷ entered usable capacity); displayed volumes are not reused for this ceiling.
Worked examples
1-inch rain on 1,500 sq ft: Barrel capacity (US gal): 55 US gal; Roof area (sq ft): 1000 sq ft; Other impervious area: 500 sq ft; Rainfall (in): 1 in; Runoff coefficient: 0.9 coefficient.
Example result: Runoff volume: 841.6 US gal; Cubic feet: 112.5 cu ft; Rain barrel count: 16; Contributing plan area: 1,500 sq ft.
Barrel capacity (US gal): 82.5 US gal; Roof area (sq ft): 1500 sq ft: Barrel capacity (US gal): 82.5 US gal; Roof area (sq ft): 1500 sq ft; Other impervious area: 750 sq ft; Rainfall (in): 1 in; Runoff coefficient: 0.9 coefficient.
Example result: Runoff volume: 1,262.3 US gal; Cubic feet: 168.75 cu ft; Rain barrel count: 16; Contributing plan area: 2,250 sq ft.
Limitations and common mistakes
The entered fraction is an aggregate event assumption. Containers start empty, modeled runoff is fully routed to them and none is emptied during the event. Timing, routing losses, existing contents, water quality and overflow hydraulics are outside this static volume model. It does not predict actual capture, peak flow, drainage capacity or flood safety. Unsupported non-finite outputs or counts beyond 9,007,199,254,740,991 are rejected.
Use horizontal contributing area rather than pitched roof-material area, and avoid overlapping catchments. Enter usable empty capacity, not nominal exterior size. Keep US liquid gallons distinct from other gallons; do not round event volume before choosing a whole count.
Sources and assumptions
USGS supports runoff terminology; the coefficient is a user-entered modeling assumption and the result is not a drainage design.
Method STORMWATER-2.1; last formula review 2026-11-03. Sources and their supported claims. Calculation methodology.
NIST unit definitions: 144 square inches per square foot and 231 cubic inches per US liquid gallon. USGS runoff terminology does not supply a fitted event fraction or drainage design.