Residential Cistern Size

Residential Cistern Sizing Calculator

Sizing a cistern for a home comes down to how much water each person uses. A household of 4 burns through something like 600–800 L/day (figure 150–200 L per person), so for 3 days of autonomy you work out V = P × C × D — that's 4 × 200 × 3 = 2,400 L. When you're harvesting rainwater instead, ABNT NBR 15527 says to size somewhere between 50% and 200% of the monthly demand, depending on how much rain falls locally and how big your roof is: V = A × P × C (area × precipitation × runoff coefficient).

For residential use, the buried polyethylene tanks you'll find (Fortlev, Tigre, Acqualimp) usually run 5,000 to 15,000 L. Add a first-flush diverter to throw away the first 1–2 mm of rain. That initial flush carries off the debris, bird droppings, and airborne grime that built up on the roof, and skipping it makes a big difference to the water you keep. Round out the basic treatment with a leaf screen at the gutter inlet and a calmed inlet inside the tank.

Applications

A backup when the utility supply cuts out, plus irrigation, flushing toilets, washing the car, and topping up the pool. Plenty of Brazilian municipalities (São Paulo, Curitiba, Salvador) require cisterns for new buildings past a certain footprint, so look up the local Lei das Cisternas before you start building.

FAQ

Buried or above-ground? Burying the tank saves space and keeps the water cool, which holds algae back, but you'll need a pump to lift the water out. Above-ground tanks let gravity do the work, though they want shade or insulation so UV doesn't degrade them.

Is cistern water potable? Not unless you disinfect it. Untreated rainwater is fine for non-potable uses only. If you want to drink it, you'll need filtration, UV sterilization, or chlorination as laid out in Portaria GM/MS 888/2021.

How much roof area do I need? Every 1 mm of rain falling on 1 m² of roof gives you roughly 0.8 L once you account for runoff losses. Scale that up and a 100 m² roof in São Paulo, where it rains about 1,400 mm/year, brings in around ≈ 112,000 L/year.