Rainwater Harvesting Calculator
Estimate how much water your roof can collect from local rainfall, and whether your tank and demand line up across the dry season.
Your catchment
The roof area that feeds your gutters (footprint × pitch factor).
Check your local met service (e.g. NIWA, BOM, NOAA).
Not sure? Use the water demand estimator.
Results
- Annual harvest
- — L/yr
- Average per month
- — L
- Demand covered
- —
- harvest vs annual household use
- Tank autonomy
- —
- days of supply if it stops raining
How this calculator works
Rainwater harvesting is simple arithmetic once you know two things: how much falls, and how much of it your roof catches. The core rule:
1 mm of rain on 1 m² of roof ≈ 1 litre collected.
So the annual harvest is:
- Harvest:
roof area (m²) × annual rainfall (mm) × runoff coefficient→ litres/year. - Monthly average: annual harvest ÷ 12 (note: real rainfall is seasonal — see caveats).
- Demand coverage:
harvest ÷ (daily use × 365). - Tank autonomy:
tank size ÷ daily use→ days of supply without rain.
The runoff coefficient accounts for what the roof loses to evaporation, splashing and first-flush diversion: ~0.9 for clean metal, down to ~0.3 for a planted roof.
Caveats
- Rain is seasonal — size your tank for the dry season (e.g. longest period without meaningful rain), not the annual average.
- A first-flush diverter and leaf screens reduce contamination but also divert a little water.
- For drinking, add filtration + purification; roof water can carry bird droppings and dust.
- Check local rules — rainwater harvesting is encouraged in most places but regulated (or restricted) in some regions.