Rainfall Calculator
Find out how much water actually falls on a given area during a rain event, converted into a usable volume.
How much water does rainfall actually produce?
Rainfall is usually measured as a depth (like 1 inch), but that depth falls over an entire area, which means even a modest rain event can produce a large volume of water. This calculator converts rainfall depth and catchment area into a real water volume.
The formula
The calculator converts rainfall depth from inches to feet, multiplies it by the catchment area in square feet to get a volume in cubic feet, then converts that to gallons (1 cubic foot equals about 7.48 gallons). A collection efficiency percentage accounts for real-world losses from evaporation, splash-out, gutter overflow, or filtration in a rainwater harvesting system.
Use this for general planning of rain barrels, cisterns, or rooftop rainwater harvesting systems. Actual collected volume can vary with roof material, gutter condition, and storm intensity.