Estimate how much rainwater you can collect from a roof or surface area. Enter rainfall depth, collection area, system efficiency, and storage tank size to calculate collected gallons, liters, overflow, and tank fill percentage.
Rainfall depth:
Enter how much rain fell, such as 1 inch or 25 millimeters.
Collection area:
Enter the roof or surface area that drains into your rain barrel, cistern, or tank.
Collection efficiency:
This adjusts the estimate for losses from gutters, filters, first flush, splash, leaks, or overflow.
Tank size:
The calculator compares collected water with your storage capacity to estimate tank fill and overflow.
A rainwater collection calculator is useful for rain barrels, cistern sizing, garden watering, roof collection planning, stormwater management, lawn care, homesteads, and irrigation estimates.
This gives a planning estimate. Actual collected water can vary based on roof material, gutter layout, rainfall intensity, wind, first-flush diverters, tank plumbing, and storage overflow.
Your result shows how much rainwater could be collected after applying the efficiency percentage. It also shows how full the tank would be, how much water could overflow, and how much unused tank space remains.
One inch of rain over 1,000 square feet produces about 623 gallons before collection losses.
Multiply rainfall depth by collection area, then multiply by collection efficiency.
Many simple estimates use 75% to 95%, depending on roof material, gutters, screens, first-flush loss, and system condition.
Yes. Enter your rainfall, roof area, efficiency, and barrel capacity to estimate tank fill and overflow.