This calculator estimates the size of a rainwater storage tank you would need based on your average monthly rainfall, roof area, runoff efficiency, and water usage. It first calculates how much rainwater you can collect during a typical month, then compares that to your expected water needs.
The result shows how many gallons of storage you’d need to make up the difference — helping you plan for dry months and right-size your system.
Enter your ZIP code below to view the total monthly and annual rainfall for the past five full calendar years in your area. This tool retrieves daily precipitation data from the Visual Crossing Weather API. Please allow time for the data to be retrieved and displayed. This may take 10 to 20 seconds.
Once you have your area’s average rainfall, use the calculator below to help you size your system.
You can use the month’s average that you prefer. Use the driest month for the worst-case scenario, or use more of an average month for a general estimate.