Rainwater Harvesting Calculator
The rainwater harvesting estimator converts roof area and rainfall depth into a theoretical collection volume after runoff and collection losses.
How to use this calculator
- Enter contributing roof area, rainfall, a runoff coefficient between zero and one, and collection efficiency.
- Check the units and definitions before comparing results with another source.
- Change one input at a time when comparing scenarios, and keep the reporting period and assumptions consistent.
Formula used by the rainwater harvesting calculator
Collected litres ≈ roof area (m²) × rainfall (mm) × runoff coefficient × collection efficiency
- Collected litres ≈ roof area (m²) × rainfall (mm) × runoff coefficient × collection efficiency
- Real yield depends on roof material, rainfall intensity, first-flush diversion, leaks, tank capacity and local rules about rainwater use.
- Inputs are checked for required values and valid ranges; displayed results are rounded for readability.
Worked examples
Worked example
A 120 m² roof receives 20 mm rainfall, with 0.85 runoff and 90% collection efficiency.
120×20 = 2,400 litres gross; after the stated factors the estimate is 1,836 litres.
120×20 = 2,400 litres gross; after the stated factors the estimate is 1,836 litres.
Understanding the result
The rainwater harvesting estimator converts roof area and rainfall depth into a theoretical collection volume after runoff and collection losses.
Real yield depends on roof material, rainfall intensity, first-flush diversion, leaks, tank capacity and local rules about rainwater use.
Important notes
Real yield depends on roof material, rainfall intensity, first-flush diversion, leaks, tank capacity and local rules about rainwater use.
The result applies to the numbers and assumptions entered here; verify specifications for safety-critical or regulated uses.
Frequently asked questions
Why does millimetres times square metres equal litres?
One millimetre of water over one square metre is one litre, before runoff and collection losses.
Does this size a storage tank?
It estimates event yield only; tank sizing also needs demand, rainfall patterns and desired autonomy.
How do I use this calculator?
Enter contributing roof area, rainfall, a runoff coefficient between zero and one, and collection efficiency.
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Related explanations
Learn the method behind the number or compare the terminology before using the calculator.
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This page provides a calculation tool plus explanatory information so you can understand what the result means and when the underlying method may not fit your situation.