Rainwater Harvesting Calculator
Estimate how many gallons and liters of rainwater a roof can capture. Enter the roof footprint in square feet, the rainfall depth in inches and a collection efficiency, and the calculator multiplies them by 0.623 gallons per square foot per inch. Real systems keep about 75 to 90 percent, so set the efficiency to match your roof and gutters.
How to estimate rainwater harvest
- Enter your roof catchment area in square feet, using the flat footprint seen from above.
- Enter the rainfall depth for the storm or period, in inches.
- Set a collection efficiency between 0 and 1, often about 0.75 to 0.9 in practice.
- Read the captured volume in the Gallons and Liters result cards.
- Click Copy estimate to save the summary line for later.
Examples
1000 sq ft roof, 1 inch of rain, ideal capture
Roof 1000 sq ft, rain 1 in, efficiency 1
623 gallons, about 2358.31 liters captured
Same roof at a realistic 0.85 efficiency
Roof 1000 sq ft, rain 1 in, efficiency 0.85
529.55 gallons, about 2004.56 liters captured
Larger roof and a 2 inch storm at 0.9
Roof 2000 sq ft, rain 2 in, efficiency 0.9
2242.8 gallons, about 8489.92 liters captured
Frequently asked questions
How much water comes off a roof per inch of rain?
About 0.623 gallons per square foot per inch. One inch over a square foot is one twelfth of a cubic foot, and a cubic foot holds roughly 7.48 gallons.
Should I use the roof area or the footprint?
Use the footprint, the area the roof covers as seen from above. A steeper pitch does not catch more rain, since rain falls onto the projected, flat area.
What collection efficiency should I assume?
Around 0.75 to 0.9 is realistic. Water is lost to splash, evaporation and first-flush diversion that discards the dirty initial runoff. Smooth metal roofs do best.
How do the gallons and liters figures relate?
The calculator works in US gallons, then multiplies by 3.785412 to give liters. Note that a US gallon is smaller than an imperial gallon, so British readers should treat the gallon figure as US gallons.
Does the roof material change the yield?
Yes, through the efficiency you set. Smooth sheet metal sheds water cleanly and runs high, near 0.9, while rough tiles, gravel and green roofs absorb and lose more, so use a lower value for them.
How big a tank do I need?
Size it to your typical storm and your use between rains. Multiply your roof area, a common rainfall and the efficiency to see how fast a tank fills, then compare that to your daily demand.
Is harvested rainwater safe to drink?
Not without treatment. Roof runoff can carry debris, bird droppings and contaminants. It is fine for irrigation, toilets and laundry, but drinking needs filtration and disinfection.
Can I enter metric roof area or millimetres of rain?
The fields expect square feet and inches. Convert first: multiply square metres by 10.7639 for square feet, and multiply millimetres by 0.03937 for inches, then type those numbers in.
Do my numbers get uploaded anywhere?
No. The calculation runs entirely in your browser and nothing is sent to a server, so your roof size and location details stay on your device.
Learn more
- Sizing a rainwater harvesting system
How catchment area, runoff coefficient, first flush and tank size decide how much roof rainwater you can actually store and use.
Related tools
Lawn Watering Calculator
Find how much to water your lawn. Enter area and inches per week to get gallons per week, plus sprinkler run time when you add gallons per minute.
Pond Liner Calculator
Find the liner size for a garden pond. Enter the pond length, width, depth and overlap to get the liner dimensions, area and an estimated water volume.
Pool Volume Calculator
Calculate your swimming pool water volume in gallons and liters. Works for rectangular, circular and oval pools with shallow and deep end depths.
Appliance Energy Calculator
Find what an appliance costs to run from its wattage. Enter watts, hours per day and your rate to see kWh and cost per day, month and year.
Aquarium Volume Calculator
Calculate your fish tank volume in gallons and liters from inside dimensions in inches. Set a fill level to estimate the actual water it holds.
Baseboard Calculator
Estimate linear feet and pieces of baseboard or trim for a room. Enter length and width, subtract doorways, set piece length and add waste.