Rainwater harvesting calculator
How much water a roof collects from a given amount of rain.
Fill in the fields and the result will appear here automatically.
Convert rainfall depth over the roof’s horizontal plan into potential volume:1 mm on 1 m² gives 1 litre before losses. For 60 m² and 25 mm that is 1500 L; an entered collection fraction 0.9 gives 1350 L. The fraction is your assumption, not a material standard. Separate 200-litre barrel equivalents do not account for available capacity, overflow or water quality.
How it works
Formula and logic
V=A×d×c litres: A is horizontal collection area m², d is rainfall depth mm and 0<c≤1 is the share reaching storage. 1 m²×1 mm=0.001 m³=1 L. Losses=A×d×(1−c);200 L barrels=ceil(V/200). This is potential volume for selected c; overflow, storage capacity, separate first flush and water suitability are not calculated.
Example
A 60 m² roof under 25 mm of rain with a coefficient of 0.9 collects 1350 litres. 10 m²,20 mm and c=1 yield 200 L, one barrel and zero loss; actual storage capacity is not checked.
Fields and units
- Roof area in plan, m² — m²
- Rainfall depth, mm — mm
- Runoff coefficient — unitless
How to use
- — Use the horizontal roof plan; vertical rain is a model assumption.
- — Enter millimetres for the selected event, not intensity without duration.
- — Enter your collection fraction;0.9 is an example, not a material standard.
- — Compare volume with available storage;200-litre barrels are a rounded-up equivalent.
Method and limitations
- Calculation method
- Formula and logic
- Data or methodology source
- CDC, 2024: rainwater, microbes, chemicals and use-specific requirements
- Limitation
- This is potential volume for selected c; overflow, storage capacity, separate first flush and water suitability are not calculated.
FAQ
Why the area in plan rather than the slope?
This model assumes vertical rain, so use horizontal plan area. Wind and orientation may change actual capture and are not modelled. Convert sloped surface area to its corresponding plan first.
What is the runoff coefficient?
The fraction of initial volume reaching collection. An entered 0.9 means 10% combined losses, without guaranteeing that value for metal or tile. Use your measurements and allow for wetting, first flush and conveyance losses; storage overflow is not separately calculated.
How much does ordinary rain give?
Use measured or forecast depth for a defined event. Millimetres alone without duration cannot classify intensity as light or heavy. For example, selected 25 mm over 60 m² at 0.9 gives 1350 L regardless of a verbal rain category.
Is the water drinkable?
Volume does not establish drinking safety. Roof runoff may contain microbes and chemical contaminants; filtration and disinfection do not guarantee removal of everything. Testing and treatment must match the intended use and local requirements, including use on food crops.