How to use the rainwater harvesting calculator
A rainwater harvesting system collects water from a roof, filters it and stores it in a tank for toilets, laundry and irrigation. Before choosing a tank you need two numbers: how much water the roof can deliver in a year, and how much the building will use. This calculator works out both and suggests a tank size by two common methods, so you can test options at concept stage or check a supplier's proposal.
- Choose units. Metric mode uses square metres, millimetres of rain and litres. US mode uses square feet, inches and US gallons.
- Enter the roof plan area that drains to the tank. Measure the footprint from above, not the sloped surface.
- Enter the long-term average annual rainfall for the site.
- Pick the roof surface. It sets the runoff coefficient, the share of rain that reaches the gutters.
- Set the filter efficiency, usually about 90% for a gutter or downpipe filter, and the daily use you want to supply.
- Enter the longest dry spell you want to bridge, for a second tank size based on storage days.
Rainwater yield and tank size formulas
Annual yield (L) = roof area (m²) × rainfall (mm) × runoff coefficient × filter efficiency
Annual yield (gal) = roof area (ft²) × rainfall (in) × 0.623 × runoff coefficient × filter efficiency
Tank, 5% rule = 0.05 × lesser of (annual yield, annual demand)
Tank, dry-spell method = lesser of (daily use, average daily yield) × dry days
The 5% rule is the simplified approach in BS 8515 and BS EN 16941-1, the British and European standards for rainwater systems. It gives roughly 18 days of storage, which suits temperate climates where it rains regularly. The dry-spell method suits climates with long dry seasons and irrigation-heavy demand. The calculator recommends the larger of the two; a detailed design would run a month-by-month water balance.
| Roof surface | Typical runoff coefficient |
|---|---|
| Metal sheet | 0.90–0.95 |
| Clay or concrete tiles, slate, asphalt shingles | 0.75–0.90 |
| Flat roof, smooth membrane | 0.80 |
| Flat roof with gravel | 0.60 |
| Green roof | 0.30–0.50 |
Worked examples
A family house in metric
A house has 100 m² of tiled roof draining to the tank, in a climate with 800 mm of rain a year. The yield is 100 × 800 × 0.85 × 0.9 = 61,200 litres a year, or about 168 litres a day on average. The family uses 200 litres a day for toilets, laundry and garden, 73,000 litres a year, so rainwater covers about 84% of that use. The 5% rule gives 0.05 × 61,200 = 3,060 litres. Bridging a 21-day dry spell at 168 litres a day needs about 3,520 litres, so a 3,500–4,000 litre tank is sensible.
An irrigation tank in US units
A 2,000 ft² metal roof in a 36-inch rainfall area yields 2,000 × 36 × 0.623 × 0.95 × 0.9 = about 38,350 gallons a year. The owner wants 100 gallons a day for irrigation, 36,500 gallons a year. The 5% rule suggests 1,825 gallons, but a 30-day dry summer spell at 100 gallons a day needs 3,000 gallons, so the larger tank is the safer choice.
Who this calculator helps
- Architects sizing tanks and plant space early and showing water savings in sustainability statements.
- Landscape designers checking whether a roof can supply an irrigation scheme through the summer.
- Plumbing and drainage designers doing a quick check before a monthly water balance.
- Self-builders and homeowners deciding between a water butt, a garden tank and an underground system.
Practical tips
- Measure the plan area with the square footage calculator, and size the gutters and downpipes that feed the tank with the gutter size calculator.
- Check the roof slope with the roof pitch calculator; very low slopes on gravel or green roofs return much less water.
- An underground tank needs digging, base and backfill. Estimate the hole with the excavation calculator.
- Fit a first-flush diverter or filter, an overflow to a soakaway or sewer, and a mains top-up so toilets still work in a drought.
- If rainfall is very seasonal, check the driest months separately; the annual average can hide a long empty spell.
Showing a client a rain garden or new planting fed by the tank? Visualise it on a photo of their yard with AI Landscape Design.
