Rainfall Calculator
Rainfall Volume
Calculate total water volume from rainfall on an area
Metric (mm & m²)
Calculate rainfall volume in metric units
Roof Collection Potential
Estimate harvestable rainwater from your roof
Formula
Volume (gallons) = Area (sq ft) × Rainfall (inches) / 12 × 7.481 gal/ft³
Frequently Asked Questions
How much water does 1 inch of rain produce?
One inch of rain on 1,000 square feet produces about 623 gallons of water. On an average 1,500 sq ft roof, that is roughly 935 gallons per inch of rainfall.
How do I measure rainfall at home?
Use a rain gauge — a graduated cylinder placed in an open area away from buildings and trees. Measure the depth of water collected after each rain event. Alternatively, use a flat-bottomed container and a ruler.
What is considered heavy rainfall?
The NWS classifies rainfall rates as: light (< 0.10 in/hr), moderate (0.10-0.30 in/hr), and heavy (> 0.30 in/hr). Rainfall exceeding 1 inch per hour is considered very heavy and can cause flash flooding.
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