Rainfall Volume from Area Calculator
A rainfall depth, like 1 inch from your rain gauge, becomes a useful planning number once you multiply it by the area it fell on. This calculator takes the rainfall depth in inches and a catchment area in square feet (a roof, lawn, or parking lot), converts to cubic feet, then reports the gross water volume in US gallons and liters. Use it to size rain barrels, estimate roof runoff for rainwater harvesting, or judge stormwater load. The result is gross volume before absorption and collection losses, so apply your own efficiency factor for a net figure.
Rainfall volume formula
Cubic feet = (depth in inches / 12) * area in square feet
US gallons = cubic feet * 7.480519
Liters = cubic feet * 28.316847
Weight (US tons) = US gallons * 8.345404 / 2000
One US gallon of water weighs about 8.345 pounds, so the weight in US tons follows directly from the gallon volume. All conversions use exact NIST-published unit factors.
Rainfall volume context
- 1 inch of rain on 1,000 square feet is roughly 623 US gallons of water.
- Roof catchment area is the building footprint (plan area), not the sloped roof surface, for collecting runoff.
- The National Weather Service standard rain gauge has an 8-inch diameter collector and measures depth directly.
- Gross volume ignores absorption, splash, and evaporation; real harvested volume is lower.
- Water weighs about 8.345 pounds per US gallon, so volumes translate quickly into structural load estimates.
Rainfall volume: frequently asked questions
How do I turn inches of rain into gallons of water?
Rainfall depth times the area it falls on equals a volume. Convert the depth to feet (inches divided by 12), multiply by the area in square feet to get cubic feet, then multiply cubic feet by 7.480519 to get US gallons. For example, 1 inch of rain on 1,000 square feet is about 623 gallons.
What is the formula for rainfall volume?
Volume equals depth multiplied by area, with consistent units. This calculator takes depth in inches and area in square feet, computes cubic feet (depth in feet times area), then converts to US gallons (times 7.480519) and to liters (cubic feet times 28.3168).
Why is rainfall useful to measure by volume?
Knowing total volume helps size rain barrels, cisterns, drainage systems, and stormwater capacity. It also lets you estimate roof runoff for rainwater harvesting. A roof area times the rainfall depth gives the harvestable volume before losses.
Does this account for runoff losses or absorption?
No. The calculation gives the gross volume of rain that landed on the area. Real harvested or runoff volume is lower because of surface absorption, evaporation, splash, and collection efficiency. Apply your own efficiency factor to the gross volume if you need a net figure.
How is a rain gauge depth measured?
A rain gauge reports depth, the height the water would reach if it stayed where it fell and did not drain or evaporate. The National Weather Service standard 8-inch gauge measures depth directly, and that depth applies uniformly across any flat area exposed to the same rain.
Official sources
- NOAA National Weather Service: National Weather Service.
- NIST: Fundamental physical constants and units.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.