Rainfall Volume Calculator
Calculate rainwater volume collected from roof area, rainfall depth, and runoff coefficient for harvesting estimates.
What Is Rainfall Volume?
Rainfall volume is the amount of water that falls over a catchment area during a storm or rainfall event. Multiply the area of land (or roof) by the depth of rain to estimate how much water you can collect, route as runoff, or store in a tank.
Rainfall Volume Formula
$$V_{\text{rainfall}} = A_{\text{catchment}} \times d_{\text{rainfall}} \times C$$\(A\) is catchment area, \(d\) is rainfall depth, and \(C\) is the runoff coefficient (0 to 1). A coefficient of 1 means all rain is collected; lower values account for absorption, evaporation, or spillover.
Example: a 50 m² roof receiving 5 cm of rain with a runoff coefficient of 1.0 collects \(50 \times 0.05 \times 1 = 2.5\) m³, or 2,500 liters.
Why Calculate Rainfall Volume?
Homeowners use this to size rain barrels and cisterns. Farmers estimate pond recharge. Engineers model stormwater runoff for drainage design. One inch of rain on one acre equals about 27,154 gallons.
Related tools: Rainwater Collection Calculator, Rain to Snow Calculator, and Volume Calculator.
Frequently Asked Questions
How do I convert rainfall depth units?
1 inch equals 25.4 mm or 2.54 cm. This calculator converts mm, cm, and inches to meters automatically before computing volume.
What runoff coefficient should I use?
Use 0.9–1.0 for smooth metal roofs, 0.8–0.9 for asphalt shingles, and 0.1–0.3 for lawns or forested land where much water infiltrates.
Does this include snow?
No. Enter liquid-equivalent rainfall depth. Use the Rain to Snow Calculator to convert snowfall amounts at a given temperature.
How much is one inch of rain on one acre?
About 27,154 gallons (roughly 102,800 liters or 0.103 acre-feet). The exact value depends on unit conversions.
Can I use length and width instead of area?
Multiply length by width to get area first, then enter that value. A 10 m by 5 m roof is 50 m².
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