WaterCalcLabRainwater Harvesting Calculator
Estimate catchment yield from horizontal roof area and period rainfall. One millimetre on one square metre is one litre.
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 catchment yield from horizontal roof area and period rainfall. One millimetre on one square metre is one litre.
V = A × P; Vusable = V × C × ηExample 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 | 600 |
| Roof material (context only) | Tile |
| Roof runoff coefficient | 0.85 |
| System efficiency (separate losses) | 0.9 |
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?
Annual or event yield is not the volume actually used. Overflow, timing, first-flush and water quality need separate assessment. Roof and downstream losses must not overlap. Roof material does not automatically set a universal coefficient.
Continue your plan
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Measure the horizontal roof footprint draining to the connected downpipes, not the sloping roof surface.
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A collection system needs somewhere for incoming water to go after the tank reaches its operating limit. Plan that route from the start rather than treating every overflow as a malfunction. Distinguish a full-tank overflow from water escaping at a blocked gutter or inlet; the latter can occur even while substantial storage space remains available.
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An efficiency value needs a beginning and an end. Roof runoff relative to rainfall is different from tank inflow relative to roof runoff, and both differ from useful withdrawals relative to annual rainfall. Write the boundary beside every coefficient. Without that description, two apparently conflicting percentages may simply describe different stages of the same system.
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A rainwater system is easiest to understand as a route with checkpoints. Identify the connected roof, follow each gutter to its downpipe, locate the inlet and then trace the outlet to the watering point. A large roof disconnected from the tank contributes nothing. Draw this route before comparing storage products; it often exposes the missing connection that changes a project.
Read guide ↗Sources & method references
Method version 1.0 · Last reviewed: 2026-10-10