💧 Rainfall Harvest Calculator — Rainwater Collection
Calculate rainwater collection volume from roof and catchment areas. Plan tank sizing for household, stock and garden use.
Quick answer: Litres = Area (m²) × Rainfall (mm) × 0.9. 200m² roof × 500mm rain = 90,000L/year. Tank size = Annual harvest ÷ 2 = 45,000L. Enough for household use.
💧 Calculate Rainwater Harvest
📐 How Rainwater Harvest Is Calculated
Annual harvest (L) = Area (m²) × Rainfall (mm) × Runoff coefficient
Tank size (L) = Annual harvest ÷ 2 (balance storage and usage)
📊 Worked Example
Shed roof 200m², 500mm annual rain, roof coefficient 0.9.
Harvest = 200 × 500 × 0.9 = 90,000 L/year
Tank = 90,000 ÷ 2 = 45,000 L (45 kL)
90 kL harvest = household use (500L/day) for 180 days. Tank stores 45 kL = 90 days supply without rain.
❓ Frequently Asked Questions
Litres = Roof area (m²) × Rainfall (mm) × 0.9. Example: 200m² roof × 500mm = 90,000 litres per year.
Tank size = Annual harvest ÷ 2. Example: 90 kL harvest ÷ 2 = 45 kL (45,000L) tank. Household only: 20-30 kL typical. Livestock: 50-100 kL.
Each 1,000L tank needs approx 20m² roof in 500mm rainfall zone. For 100 cattle, dam or bore is usually better than roof collection.
Colorblind steel (best), Zincalume, painted steel, galvanised iron. Concrete/tiles (ok). Avoid asbestos. First flush diverter improves quality.