Rainwater Harvesting Calculator
Estimate how many litres of rainwater you can collect annually from your roof. Ideal for homeowners and gardeners planning rainwater tanks, irrigation systems, or off-grid water supply.
About this calculator
Rainwater harvesting volume is calculated using the formula: Volume (m³) = roofArea × rainfall × efficiency ÷ 1000. Roof area is measured in m², annual rainfall in millimetres, and efficiency as a decimal (e.g., 0.85 for 85%). Dividing by 1000 converts from litres-equivalent (mm × m² = litres) to cubic metres. Collection efficiency accounts for losses from evaporation off the roof surface, first-flush diversions, gutter overflow, and filter losses. Typical efficiency values range from 0.75 to 0.90 for metal or tile roofs with properly maintained gutters. The result tells you the maximum harvestable volume per year, which you can compare against your household or irrigation demand to size a tank appropriately.
How to use
Imagine your roof has a catchment area of 120 m², your region receives 600 mm of annual rainfall, and your collection system has 80% efficiency (0.80). Volume = 120 × 600 × 0.80 ÷ 1000 = 57,600 ÷ 1000 = 57.6 m³ per year. That equals 57,600 litres — enough to fill a standard 5,000 L tank more than 11 times. If you increase efficiency to 0.90, the yield rises to 64.8 m³, showing how important well-maintained gutters and filters are.
Frequently asked questions
What collection efficiency should I use for my rainwater harvesting system?
Most domestic rooftop systems achieve 75–90% efficiency. Metal (Colorbond or zinc) roofs typically sit around 85–90%, while concrete tiles may be lower at 75–80% due to greater surface absorption. First-flush diverters, which discard the initial dirty runoff, can slightly reduce total yield but significantly improve water quality. Start with 0.80 as a conservative estimate and refine it based on your roof material and guttering condition.
How do I calculate the roof area for rainwater harvesting?
Use the plan (horizontal footprint) area of the roof, not the actual slope area. Rainfall is measured vertically, so only the horizontal projection matters for catchment calculations. Multiply the length by the width of your building's footprint. For complex roof shapes, divide the roof into rectangles, calculate each section separately, and add them together. Skylights and areas draining away from your gutters should be excluded.
How large a rainwater tank do I need for my household?
Tank size depends on how much water you want to store relative to your demand and the dry periods between rain events. A common rule of thumb is to size the tank to hold at least one month's expected usage. For garden irrigation of around 200 L/day, a 6,000 L tank provides roughly 30 days of storage. Compare your calculated annual harvest volume with your annual demand to check whether harvesting can realistically meet your needs.