Hong Life CalculatorsUnderstanding the rainwater harvesting calculator — concept and principle
This calculator estimates how much rainwater a roof or other catchment can collect from a given rainfall, allowing for collection losses, and how many times it would fill a tank. It works in metric or US units. Homeowners, gardeners and designers of rainwater systems use it to size tanks.
How it is calculated
Metric: volume (L) = area (m²) × rainfall (mm) × efficiency
US: volume (gal) = area (ft²) × rainfall (in) × 0.623 × efficiency
Tank fills = collected volume ÷ tank capacity
- Use the horizontal footprint of the roof, not its sloped area.
The principle
One millimetre of rain falling on one square metre is exactly one litre of water. So the rain landing on a roof is its footprint area times the rainfall depth. Not all of it reaches the tank: some splashes off, wets the surface, evaporates or is diverted as the dirty "first flush". An efficiency of around 80% is a common planning value for roofs. Rain falls vertically, so a steep roof catches no more than a flat one with the same footprint.
Worked example
Default inputs: metric, 100 m², 25 mm of rain, 80% efficiency, tank 2,000 L.
- Rain on the roof = 100 × 25 = 2,500 L.
- Collected = 2,500 × 0.80 = 2,000 L = 2 m³ = 528 US gal.
- Tank fills = 2,000 ÷ 2,000 = 1.
| Roof or catchment area (horizontal footprint) | Water collected (L) |
|---|---|
| 25 | 500 |
| 50 | 1,000 |
| 75 | 1,500 |
| 100 | 2,000 |
| 125 | 2,500 |
| 150 | 3,000 |
| 200 | 4,000 |
How to read the result
A single heavy rain can fill a modest tank, as here. For a year's supply, enter annual rainfall; for tank sizing, compare collection in wet and dry months with your water use. The Texas Water Development Board notes roughly 1,000 gallons per inch of rain on a 2,000 ft² roof at about 80% efficiency.
Common mistakes
- Using the sloped roof area instead of the footprint.
- Assuming 100% collection.
- Sizing a tank from annual rainfall without considering dry spells.
Frequently asked questions
How much water does 1 mm of rain on 1 m² give?
Exactly 1 litre.
What efficiency should I use?
About 80% is a common planning value; smooth metal roofs collect more, gravel or green roofs less.
Does roof slope matter?
Not for the amount; use the horizontal footprint.
What is a first-flush diverter?
A device that discards the first, dirtiest runoff from each rain.
Sources: Texas Water Development Board, "Rainwater volumes from roof runoff"