Rainwater Calculator

How the calculator works

The calculator gives a preliminary size for a residential rainwater cistern using the two sizing methods in Annex A of the Brazilian standard ABNT NBR 15527: the German Practical Method and the Rippl method. It multiplies the roof area by the city's rainfall and a runoff coefficient for the roof material, compares that supply with the household's demand, and rounds the result up to the next standard polyethylene tank size.

Inputs and assumptions

• Daily demand per person: 53 L for toilet flushing, 10 L for garden irrigation, 63 L for both.
• Rainfall: each city's average monthly rainfall in millimetres (the annual total for the German method, month by month for Rippl).
• Tank size: rounded up to the next standard size, from 500 to 30.000 litres.

German Practical Method (NBR 15527, Annex A.5)

Works with yearly totals. The tank is the smaller of 6% of the volume the roof captures in a year and half of the household's annual demand:

V = min(0.06 × P × A × C × (1 − f), D ÷ 2)

P = annual rainfall (m) · A = catchment area (m²) · C = runoff coefficient · f = first-flush loss (10%) · D = annual demand (m³).

Rippl method (NBR 15527, Annex A.4)

Works month by month. Each month the roof captures Q = P × A × C and the house uses the same demand D. The running deficit (D − Q, added up month after month) grows through the dry season and resets to zero whenever the tank overflows; the tank has to hold the largest deficit. The year is walked twice, so a dry spell that runs from December into January is counted once. If every month captures more than the demand, the tank only needs to buffer one month of demand.

Worked example

A 100 m² ceramic-tile roof in Maringá, PR, for 4 residents using rainwater to flush toilets:

• NBR 15527 German: 10.000 L tank
• NBR 15527 Rippl: 500 L tank

The methods can disagree by a lot: the German method sizes from yearly totals, while Rippl only stores what the driest stretch of the year needs — small in a city with rain all year round, like Maringá.

Where it comes from

The calculator grew out of Ricardo Gomes's published civil engineering capstone: Gomes & Azenha (2018), “Sistema gravitacional para captação e uso de água da chuva em residências unifamiliares na cidade de Maringá/PR”, UniCesumar institutional repository (RDU). Maringá's rainfall is the 2007–2017 INMET station average compiled there.

It's a preliminary estimate, not a design. Check the final system against the full standard, with a qualified professional.