Calculating your water volume is straightforward once you know the Rainfall Volume 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 Rainfall Volume Calculator.
What is Rainfall Volume?
The Rainfall Volume calculation tells you your water volume from a few simple inputs. The figure you are solving for here is the water volume.
The Rainfall Volume formula
The core formula is:
Water volume = Catchment area × Rainfall depth × Collection efficiency ÷ 100Here is what each input means:
- Catchment area — a value measured in m². Example: 100 m².
- Rainfall depth — a value measured in mm. Example: 25 mm.
- Collection efficiency — a percentage, such as an annual rate. Example: 1%.
How to calculate it step by step
- Write down the catchment area (for example, 100 m²).
- Write down the rainfall depth (for example, 25 mm).
- Write down the collection efficiency (for example, 1%).
- Apply the formula above to get your water volume.
- Double-check the result with the Rainfall Volume Calculator.
Worked examples
Example 1
| Input / Output | Value |
|---|---|
| Catchment area | 100 m² |
| Rainfall depth | 25 mm |
| Collection efficiency | 1% |
| Water volume | 2,500.0 |
| In cubic metres | 2.500 |
With catchment area of 100 m², rainfall depth of 25 mm and collection efficiency of 1%, the water volume works out to 2,500.0.
Example 2
With catchment area of 200 m², rainfall depth of 25 mm and collection efficiency of 1%, the water volume works out to 5,000.0.
| Result | Value |
|---|---|
| Water volume | 5,000.0 |
| In cubic metres | 5.000 |
Example 3
With catchment area of 50 m², rainfall depth of 25 mm and collection efficiency of 1%, the water volume works out to 1,250.0.
| Result | Value |
|---|---|
| Water volume | 1,250.0 |
| In cubic metres | 1.250 |
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 Rainfall Volume Calculator does it instantly, for free, with the formula and a worked example built in.
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