Rain Barrel Size Calculator
Estimate rainfall captured from a rectangular roof, target storage volume, full rain-barrel quantity, purchased capacity and barrel cost.
Default project preview
7 barrels
Event roof yield
673.2 gal
Target storage
336.6 gal
Purchased capacity
385 gal
Roof catchment area
1,200 ft²
Calculator
Build the material order
Your order will appear here
Enter the project details and calculate a rounded material estimate.
Input and output guide
What each value means
The labels and units below are part of this calculator’s visible methodology. Replace every default with measurements or product data from the actual project.
Roof catchment length
Enter this project assumption in ft. The current imperial default is 40 ft.
Roof catchment width
Enter this project assumption in ft. The current imperial default is 30 ft.
Design rainfall event
Enter this project assumption in in. The current imperial default is 1 in.
Roof runoff coefficient
Enter this project assumption in a unitless value. The current imperial default is 0.9. Minimum: 0.1. Maximum: 1.
Storage target from event
Enter this project assumption in %. The current imperial default is 50 %. Minimum: 1. Maximum: 100.
Volume per rain barrel
Enter this project assumption in gal. The current imperial default is 55 gal.
Price per barrel
Enter this project assumption in a unitless value. The current imperial default is 95. Minimum: 0.
How to read the outputs
Event roof yield
runoff coefficient applied. Default preview: 673.2 gal.
Target storage
selected event percentage. Default preview: 336.6 gal.
Purchased capacity
full barrels. Default preview: 385 gal.
Roof catchment area
horizontal catchment. Default preview: 1,200 ft².
Unused barrel capacity
above target volume. Default preview: 48.4 gal.
Barrel cost
storage barrels only. Default preview: $665.00.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Captured volume = roof area × rainfall depth × runoff coefficient. Target storage applies the selected percentage, then barrel quantity is rounded up.
FAQ
Rain Barrel Size Calculator questions
Why use a runoff coefficient below one?+
Some rainfall is lost to wetting, splash, debris, first flush and gutter inefficiency.
Can rain barrels prevent all stormwater runoff?+
Usually not. Once barrels fill, overflow must be routed safely away from foundations.
Is captured rainwater safe to drink?+
Not without an approved collection, treatment and monitoring system.