Rainwater Harvesting Calculator
Use this rainwater harvesting calculator to estimate how much water you could collect from your roof. Enter the roof's footprint area, the rainfall over a period, and the roof material, and it returns the harvestable volume in litres and gallons — useful for sizing a water butt or tank and planning garden irrigation.
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Estimate your rainwater collection
Enter values above and press Calculate to see your result.
Formula used
Harvestable water is the area times the rainfall depth times a runoff coefficient for losses:
Metric: Litres = Area (m²) × Rainfall (mm) × Runoff
US: Gallons = Area (ft²) × Rainfall (in) × 0.623 × Runoff
The neat part of the metric version: 1 mm of rain on 1 m² of roof is exactly 1 litre. The runoff coefficient (0.5–0.9) accounts for splash, evaporation, first-flush diversion and absorption — smoother roofs collect more.
Worked examples
Typical house. A 100 m² roof with 800 mm annual rain and a 0.85 coefficient collects about 68,000 L a year.
One rainy month. The same roof in a 60 mm month yields about 5,100 L.
US example. 1,000 ft² of roof with 30 in of rain (0.85) ≈ 15,900 gallons.
How to use this calculator
- Choose metric or US units.
- Enter the roof's footprint area — the area it covers when viewed from above.
- Enter rainfall for the period you care about (a month, a season, or a year).
- Select the roof material for the runoff coefficient.
- Press Calculate for the harvestable volume.
Runoff coefficients by roof type
| Roof material | Coefficient | Notes |
|---|---|---|
| Metal, glazed tile | 0.90 | Smooth, low absorption — best collection |
| Asphalt shingle, concrete | 0.85 | Common residential roofing |
| Clay/terracotta tile | 0.75 | Porous, more loss |
| Gravel ballast / green roof | 0.40–0.60 | High absorption and retention |
Use the roof's plan area: a steeper pitch doesn't collect more rain than its footprint.
Who should use this calculator
Homeowners and gardeners planning a water butt or rainwater tank, allotment holders, and anyone wanting to cut mains water use for irrigation. It's also useful for sizing storage so you capture the wettest months without overflowing.
Why roof footprint, not slope
Rain falls vertically, so a roof collects rain equal to its horizontal footprint, regardless of pitch. A steep roof has more surface but catches the same rainfall as its shadow on the ground. Always use the plan area — length × width of the area the roof covers.
Sizing your storage
- Match storage to demand and rainfall pattern — a tank that fills and overflows in the first storm wastes the rest.
- Add a first-flush diverter to discard the dirty initial runoff.
- Plan an overflow route away from foundations.
Limitations of this calculator
This is an estimate. Real collection varies with gutter efficiency, debris, first-flush losses, splash, and how evenly rain falls. Harvested rainwater is generally for non-potable uses (gardens, flushing, washing) unless properly filtered and treated. Check local regulations, which in some areas restrict or require permits for rainwater harvesting.
Frequently asked questions
How much rainwater can I collect from my roof?
Multiply roof footprint by rainfall and a runoff coefficient. In metric, 1 mm of rain on 1 m² gives 1 litre, so a 100 m² roof in 800 mm of rain yields roughly 68,000 L a year at 0.85 efficiency.
Do I use the sloped roof area or the footprint?
The footprint (plan area). Rain falls vertically, so pitch doesn't increase collection beyond the area the roof covers.
What is a runoff coefficient?
A factor (0.5–0.9) for real-world losses — splash, evaporation, absorption, first-flush. Smooth metal roofs are around 0.9; porous tiles lower.
Can I drink harvested rainwater?
Not without proper filtration and treatment. Most home systems use rainwater for gardens, flushing and washing. Treat and test before any potable use.
Is rainwater harvesting legal where I live?
In most places yes, but some regions regulate or require permits. Check your local rules before installing a system.