Cistern Size Calculator
Estimate cistern storage from daily demand and dry-period duration, then compare roof catchment from a design rainfall event and estimate tank cost.
Default project preview
2,415 gal
Base storage demand
2,100 gal
Event roof harvest
954 gal
Cistern fill equivalent
39.5%
Harvested days of supply
6.4 days
Calculator
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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 area
Enter this project assumption in ft². The current imperial default is 1800 ft².
Design rainfall event
Enter this project assumption in in. The current imperial default is 1 in.
Runoff coefficient
Enter this project assumption in a unitless value. The current imperial default is 0.85. Minimum: 0.1. Maximum: 1.
Daily cistern demand
Enter this project assumption in gal/day. The current imperial default is 150 gal/day.
Design dry period
Enter this project assumption in days. The current imperial default is 14 days.
Storage reserve
Enter this project assumption in %. The current imperial default is 15 %. Minimum: 0. Maximum: 50.
Tank price
Enter this project assumption in per 1,000 gal. The current imperial default is 950 per 1,000 gal. Minimum: 0.
How to read the outputs
Base storage demand
before reserve. Default preview: 2,100 gal.
Event roof harvest
1 in rainfall. Default preview: 954 gal.
Cistern fill equivalent
one design event. Default preview: 39.5%.
Harvested days of supply
at entered demand. Default preview: 6.4 days.
Runoff loss
entered coefficient. Default preview: 15%.
Tank cost allowance
storage vessel only. Default preview: $2,294.25.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Storage demand = daily demand × dry days × reserve factor. Roof harvest equals catchment area × rainfall depth × runoff coefficient, converted to gallons or litres.
Related tools
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FAQ
Cistern Size Calculator questions
Why can one storm harvest less than the required storage?+
Storage sizing is based on demand through a dry period, while one rainfall event may refill only part of the tank.
What does the runoff coefficient represent?+
It accounts for splash, evaporation, first flush and other losses between rainfall and collected water.
Can collected rainwater be used for drinking?+
Only with an approved treatment, monitoring and plumbing system that meets applicable requirements.