Rainfall Volume Calculator
Convert rainfall over an area into theoretical and collectible water volume
Collectible water
How we calculated it
About the Rainfall Volume Calculator
Rainfall Volume Calculator converts a rainfall depth over a catchment area into total water volume. It is useful for roofs, gardens, ponds and stormwater planning because even a small depth of rain can represent a large volume across a broad area.
The calculation gives gross rainfall volume. Actual harvested or runoff water is usually lower because of first-flush diversion, leaks, evaporation, infiltration and surface losses.
How to Use the Rainfall Volume Calculator
- Enter rainfall depth.
- Enter the catchment area.
- Convert both measurements to compatible units.
- Calculate gross rainfall volume and apply a runoff or collection efficiency separately if you want usable captured water.
How the Calculation Works
Rainfall volume = area × rainfall depth.
In metric units, 1 mm of rainfall over 1 m² equals 1 litre. If a collection efficiency e is applied, usable volume = gross rainfall volume × e.
Worked example
Rainfall Volume Calculator Worked Example
25 mm rain on a 120 m² roof
Gross volume = 25×120 = 3,000 litres because each millimetre over one square metre equals one litre.
Result guide
Understanding Your Rainfall Volume Calculator Result
Three thousand litres is the theoretical water falling on the roof. The amount reaching a tank will be lower if collection efficiency is below 100%. For stormwater planning, short intense rainfall can create drainage problems even when total event volume is modest. This calculator estimates volume only; peak runoff rate requires rainfall intensity, runoff timing and drainage characteristics.
Before you rely on the answer
Assumptions and Limitations
- Rainfall depth is representative across the catchment.
- Catchment area is measured in plan area appropriate to rainfall collection.
- Losses are excluded from gross volume unless an efficiency factor is applied.
Practical tip
Using This Result in Practice
For rainwater harvesting, use roof plan area rather than sloped roof surface area unless the method specifically says otherwise. Rainfall depth is defined relative to horizontal ground area.
Related calculations
Useful Calculations to Continue With
If you want to compare captured rainfall with storage capacity, use the Rain Barrel Calculator.
If you want a broader collection estimate including catchment and efficiency, use the Rainwater Harvesting Calculator.
Common questions
Rainfall Volume Calculator FAQ
How much water is 1 mm of rain on 1 m²?
Exactly 1 litre.
Should sloped roof surface area be used?
Rainfall collection is generally based on horizontal projected roof area.
Why is captured water less than gross rainfall volume?
First flush, leaks, overflow and other losses reduce collection efficiency.