Calculating your water harvested is straightforward once you know the Rainwater Harvesting formula and what each input means. This guide explains the method in plain language, walks through a manual calculation, and gives worked examples you can follow — then you can do it instantly with the Rainwater Harvesting Calculator.
What is Rainwater Harvesting?
The Rainwater Harvesting calculation tells you your water harvested from a few simple inputs. The figure you are solving for here is the water harvested.
The Rainwater Harvesting formula
The core formula is:
Water harvested = Roof catchment area × Rainfall × Runoff coefficientHere is what each input means:
- Roof catchment area — a value measured in m². Example: 100 m².
- Rainfall — a value measured in mm. Example: 50 mm.
- Runoff coefficient — a number. Example: 0.85.
How to calculate it step by step
- Write down the roof catchment area (for example, 100 m²).
- Write down the rainfall (for example, 50 mm).
- Write down the runoff coefficient (for example, 0.85).
- Apply the formula above to get your water harvested.
- Double-check the result with the Rainwater Harvesting Calculator.
Worked examples
Example 1
| Input / Output | Value |
|---|---|
| Roof catchment area | 100 m² |
| Rainfall | 50 mm |
| Runoff coefficient | 0.85 |
| Water harvested | 4,250 |
With roof catchment area of 100 m², rainfall of 50 mm and runoff coefficient of 0.85, the water harvested works out to 4,250.
Example 2
With roof catchment area of 200 m², rainfall of 50 mm and runoff coefficient of 0.85, the water harvested works out to 8,500.
| Result | Value |
|---|---|
| Water harvested | 8,500 |
Example 3
With roof catchment area of 50 m², rainfall of 50 mm and runoff coefficient of 0.85, the water harvested works out to 2,125.
| Result | Value |
|---|---|
| Water harvested | 2,125 |
Tips for an accurate result
- Keep your units consistent — mixing, say, months with years or grams with kilograms is the most common source of error.
- Round only at the very end. Rounding inputs early can shift the final answer noticeably.
- Re-run the numbers whenever an input changes, rather than estimating from an old result.
Prefer not to do the maths by hand? — the Rainwater Harvesting Calculator does it instantly, for free, with the formula and a worked example built in.
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