Rain Barrel / Rainwater Harvesting Calculator
Calculate rainwater collection from your roof
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Rain Barrel / Rainwater Harvesting Calculator is built for calculating rainwater collection from your roof — fast, free, and private. Fill in Roof Area (sq ft), Monthly Rainfall (inches) and Barrel Size (gallons) on Rain Barrel / Rainwater Harvesting and read your answer immediately. Rain Barrel / Rainwater Harvesting displays each intermediate figure as well. Rain Barrel / Rainwater Harvesting shines when speed matters. No sign-up is needed for Rain Barrel / Rainwater Harvesting. The math runs on your device. Looking for rain barrel / rainwater harvesting calculator or free online rain barrel / rainwater harvesting calculator? Rain Barrel / Rainwater Harvesting covers both. Rain Barrel / Rainwater Harvesting stays bookmarked for next time.
How much rainwater can my roof collect?
A roof harvests its footprint area multiplied by the rainfall, discounted by how much water actually reaches the barrel. Enter the roof area, the rainfall and the collection efficiency to see the litres or gallons available, and the barrel size that makes sense for it.
At 55 gallons a barrel that is 28 barrels, or roughly 50 gallons a day across the month.
The formula
Gallons per inch = roof area in sq ft × 0.623
Monthly harvest = gallons per inch × rainfall × 0.8
Barrels = ceil(monthly harvest ÷ barrel size)
The 0.623 is how many gallons one square foot of catchment holds at one inch of depth, and 0.8 is a runoff coefficient: about 20% is lost to splashing, first-flush diversion, gutter overflow and absorption before it reaches the barrel.
Worked examples
1,000 sq ft × 3 in → 1,495 gal, 28 barrels
1,000 sq ft × 1 in → 498 gal, 10 barrels
2,000 sq ft × 3 in → 2,990 gal, 55 barrels
500 sq ft × 2 in → 498 gal, 10 barrels
1,000 sq ft × 3 in with 50 gal barrels → 30 barrels
1,000 sq ft × 6 in → 2,990 gal, 55 barrels
Roof area is footprint, not surface
Use the horizontal footprint the roof covers, not the sloped area of the tiles. A pitched roof has more surface but intercepts exactly the rain that falls on its plan area, so the footprint is the correct input. For a simple gable, that is length times width of the building plus overhangs.
If only part of the roof drains to the barrel, use that part. A downpipe serving one 300 sq ft section of a larger roof will collect 30% of the calculated volume, which is the most common reason real barrels overflow while the maths says they should not.
Sizing storage against supply
Monthly totals hide the peaks. Three inches may arrive as one storm that fills every barrel in an hour and then nothing for three weeks, so storage sized to the monthly average is either overflowing or empty. Practical systems size storage to the dry spell they need to cover: a 500 sq ft garden using 30 gallons a day through a two-week dry spell needs about 420 gallons, which is eight 55-gallon barrels.
Include a first-flush diverter, which discards the first 10 to 20 gallons from each roof section to keep bird droppings and dust out, and an overflow route that moves water away from the foundation. Rainwater is fine for irrigation and washing; it is not treated drinking water, and stored water should be covered to stop mosquitoes breeding.
Frequently Asked Questions
How much rain can a 1,000 sq ft roof collect?
About 498 gallons per inch of rain, or 1,495 gallons from 3 inches after allowing for 20% runoff loss.
How many 55 gallon barrels do I need?
Twenty-eight for 1,495 gallons a month at the defaults, and more in practice because storage must cover dry spells rather than the monthly average.
Should I use roof footprint or sloped area?
The horizontal footprint. A pitched roof intercepts only the rain falling on its plan area, so sloped tile area overstates the harvest.
What is the 0.8 factor for?
Runoff losses: splashing, gutter overflow, absorption and the first-flush discard. About 80% of the rain on the footprint reaches storage.
Can I drink harvested rainwater?
Not without treatment. It is suitable for irrigation and washing, and stored water needs a cover to prevent mosquitoes and a first-flush diverter to keep debris out.
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