Rainwater Harvesting Calculator
Estimate collectible rainwater from roof catchment area, rainfall and collection efficiency
Collectible water
How we calculated it
About the Rainwater Harvesting Calculator
Rainwater Harvesting Calculator estimates how much water can be collected from a roof or other catchment area from a given rainfall depth. It is useful for sizing storage and understanding the potential yield of a harvesting system.
Not all rainfall becomes stored water. First-flush diversion, gutter losses, splash, evaporation and overflow reduce practical collection, so a runoff or collection efficiency factor is important.
How to Use the Rainwater Harvesting Calculator
- Enter catchment area.
- Enter rainfall depth for the event or period.
- Enter a collection-efficiency factor when supported.
- Calculate potential captured volume and compare it with available tank capacity.
How the Calculation Works
Gross rainfall volume = catchment area × rainfall depth after units are converted to a common volume basis.
Collected volume = gross rainfall volume × collection efficiency. In U.S. units, 1 inch of rain on 1 ft² is about 0.623 gallons before losses.
Worked example
Rainwater Harvesting Calculator Worked Example
1,000 ft² roof, 1 inch rain, 80% efficiency
Gross volume ≈ 1,000×0.623 = 623 gallons. Collected volume ≈ 623×0.80 = 498 gallons.
Result guide
Understanding Your Rainwater Harvesting Calculator Result
The result is potential harvest for the specified rainfall and efficiency. Actual stored water is also limited by tank capacity and pre-existing tank level.
Before you rely on the answer
Assumptions and Limitations
- Catchment area is measured as the effective horizontal collection area.
- Rainfall depth represents the site.
- Efficiency reasonably reflects system losses.
Practical tip
Using This Result in Practice
When sizing storage, use rainfall patterns over time rather than one large storm alone. A huge theoretical event yield is not useful if the tank is already full during the wet season.
Related calculations
Useful Calculations to Continue With
If you need to calculate the storage vessel’s capacity, use the Tank Volume Calculator.
If you are planning a smaller residential collection setup, use the Rain Barrel Calculator.
Common questions
Rainwater Harvesting Calculator FAQ
How much water does 1 inch of rain produce?
About 0.623 U.S. gallons per square foot of catchment before collection losses.
Why use an efficiency factor?
Some rainfall is lost through first flush, splash, gutters, evaporation and system inefficiency.
Can the tank collect more than its capacity?
No. Excess potential collection becomes overflow once available storage is full.