Rainwater Harvesting Calculator — Tank Size & Annual Yield

This calculator estimates the annual rainwater yield from your roof and recommends a tank size based on your household demand.

Annual yield formula

Annual Yield (litres) = Roof Area (m²) × Annual Rainfall (m) × Yield Coefficient × Filter Coefficient × 1,000

This follows BS EN 16941-1:2024 (the current UK code of practice, which superseded BS 8515:2009), sizing yield from the collecting area, rainfall depth, a surface yield coefficient and a hydraulic filter (treatment) efficiency coefficient. The preliminary treatment is required to be at least 90% efficient, so a filter coefficient of 0.9 is used.

Yield coefficients

• Pitched tile / slate: 0.80
• Pitched metal: 0.90
• Flat felt / membrane: 0.70
• Green roof: 0.50

Daily demand estimates

Indicative UK non-potable figures (not a single standard) — actual demand depends on appliances and habits:

• Garden only: ~30 litres per person per day (summer average)
• Garden + WC: ~50 litres per person per day
• Full non-potable: ~80 litres per person per day (includes washing machine)

Tank sizing

Tank size = the lesser of (5% of Annual Yield) and (5% of Annual Demand)
Per the BS EN 16941-1:2024 (formerly BS 8515:2009) simplified rule, 5% of annual demand equates to about 18 days. The result is rounded up to the nearest standard tank size (500-litre increments). Capping at 5% prevents both oversizing in high-rainfall areas and excessive stagnation when demand is the limiting factor.

For managing excess rainwater, see our Gutter Sizing Calculator and Soakaway Sizing Calculator.