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.
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.
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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.