WaterCalcLabRain Barrel Calculator
Estimate how many empty barrels could contain one specified rainfall event.
Your result
Bars compare only results in the same unit group. They are normalized, not a time-series forecast.
Planning result based on your inputs.
How this tool works
Estimate how many empty barrels could contain one specified rainfall event.
N = ceil(event yield ÷ barrel capacity)Example calculation
The prefilled form is a worked example. Its base-unit inputs and expected outputs are shown below; use the reset button to restore it.
| Input | Example |
|---|---|
| Area | 100 |
| Rainfall for the selected period | 25 |
| Roof runoff coefficient | 0.85 |
| System efficiency (separate losses) | 0.9 |
| One barrel capacity | 200 |
Frequently asked questions
How should I choose the inputs?
Use measurements and local records for the stated period. All preset values are illustrative examples, not recommendations.
What does this result leave out?
Use event rainfall, not annual rainfall. Connected barrels may not fill evenly; overflow routes and mosquito-proof covers are necessary.
Continue your plan
Rainwater Harvesting Calculator
Estimate catchment yield from horizontal roof area and period rainfall. One millimetre on one square metre is one litre.
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Measure the horizontal roof footprint draining to the connected downpipes, not the sloping roof surface.
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Sources & method references
Method version 1.0 · Last reviewed: 2026-10-10