Engineering calculators · Water reuse
Rainwater harvesting tank size calculator
Size a rainwater harvesting tank for a building or site using the simplified approach in BS 8515: the tank holds 5% of whichever is smaller, the rainwater you can collect in a year or the non-potable water you use in a year. The calculator also shows how much of the demand rain can meet and the mains water it could replace.
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The tank holds about 14.4 days of average demand.
How the calculation works
- Annual yield (litres) = collection area (m²) × annual rainfall (mm) × drainage coefficient × filter efficiency [DB]. The drainage coefficient allows for rain lost on the roof: 0.9 for a pitched roof, 0.8 for a tiled pitched roof or a flat roof with a gravel layer. Filter efficiency is commonly 90% [DB].
- Annual demand (litres) = people × use per person × 365 + 60 L per m² of garden + other demand. The published domestic form is 16,000 L per person plus 60 L per m² of garden a year [NEST].
- Tank size = 5% of the lesser of annual yield and annual demand. This is the BS 8515 simplified approach; the source notes that larger tanks are not allowed because of the risk of bacteria breeding in stored water [NEST]. Five per cent of a year is about 18 days of storage.
- Mains water replaced is the lesser of yield and demand. This is an upper bound: in a dry spell the tank runs empty and the system tops up from the mains.
What this cannot tell you. The simplified approach suits steady demand and fairly even rainfall. It does not model month-by-month rainfall, long dry spells, or large or irregular demands. For those, BS 8515 has a detailed approach based on daily rainfall and demand records. BS 8515:2009 has since been replaced by BS EN 16941-1, so check which standard your project must meet. Tank, filter, overflow and backflow-protection details also need to follow the standard and the supplier's instructions.