Enter your roof length, roof width, and rainfall amount to find out how many gallons of rainwater you can collect. The Rain Barrel Calculator shows your total water harvested, the number of rain barrels needed, and the estimated water savings — so you can size your collection system before buying a single barrel.
Rainwater Collected
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Roof Catchment Area
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Rain Barrels Needed
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Estimated Water Savings
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Est. Annual Collection (50" rain/yr)
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The calculator multiplies your roof's catchment area (length × width) by the rainfall depth, then applies a runoff coefficient to account for losses from evaporation and absorption. The formula is: Gallons = Roof Area (sq ft) × Rainfall (in) × 0.623 × Runoff Coefficient. The 0.623 factor converts cubic feet to gallons.
Measure only the section of your roof that drains into the downspout connected to your rain barrel. If your downspout collects from half the roof, use half the roof's length or width. A single standard downspout typically serves 200–400 sq ft of roof area.
It depends on how much rain falls in a single event and your storage goals. A 1-inch rainfall on a 800 sq ft roof can produce over 400 gallons, far exceeding a single 55-gallon barrel. Linking multiple barrels in series or using a larger cistern helps you capture more of each rain event.
The runoff coefficient represents how efficiently your roof surface captures rainfall — some water is lost to evaporation, absorption, or splash. Metal roofs are the most efficient at ~95%, while flat or gravel roofs may only capture 70%. Asphalt shingles, the most common residential roofing, typically rate around 85%.
Rainwater harvesting laws vary by state and country. Most US states allow residential rain barrel collection, and many actively encourage it with rebates. A few states have historically restricted it, though most have since relaxed those rules. Always check your local regulations before installing a collection system.
Collected rainwater is ideal for garden irrigation, lawn watering, washing cars, and topping off ponds or water features. Plants especially benefit because rainwater is naturally soft and free of chlorine and fluoride. For potable uses, the water would need additional filtration and treatment.
Savings depend on your local water rates and how much you collect. While a single barrel saves modest amounts per event, consistent use throughout the growing season can save hundreds of gallons — and real dollars off your water bill. In drought-prone areas or regions with tiered water pricing, savings multiply quickly.
Standard residential rain barrels range from 50–100 gallons and work well for small gardens. If your roof area is large or you want to capture significant rain events, consider a 200–500 gallon cistern or linking multiple barrels. Use this calculator's 'Barrels Needed' output to match your storage to your typical rainfall event size.