RAINWATER CALCULATOR
How Much Rainwater Can Your Roof Collect?
Enter your roof area and rainfall amount to estimate how many gallons of rainwater your roof can collect.
Calculate Your Rainwater Collection
Enter the roof collection area and rainfall amount below.
Let's Calculate It.
To calculate the area of your roof, multiply the length in feet by the width in feet for a square or rectangular roof. The resulting figure represents the roof collection area in square feet.
For this basic rainwater collection estimate, use the horizontal footprint of the roof. You generally do not need to calculate the additional surface area created by the roof slope.
Have More Than One Roof Section?
If the roof contains several rectangular collection areas, calculate the length × width of each section separately and add the square-foot areas together. Enter the combined roof area into the rainwater calculator above.
Formulas & Equations
Once you know the roof collection area, estimating rainwater harvesting potential is straightforward.
Actual Collection Can Be Lower.
The standard 0.623 formula estimates the theoretical volume of rainfall available from the roof collection area. Actual water reaching a storage tank can be lower because of first-flush diversion, splash, overflow, evaporation and other system losses. Use the collection-efficiency setting in the calculator when estimating these losses.
Calculate Your Rainwater Harvesting Potential
Use the All-Tex Irrigation & Supply rainwater calculator to estimate rainwater collection based on roof area, rainfall and collection efficiency. Understanding potential roof runoff can help when planning rainwater harvesting systems, rainwater storage tanks, cisterns, pumps, filtration and water distribution equipment.