About the Rainwater Harvesting Calculator
This rainwater harvesting calculator estimates how much water you can collect from a roof, how big a tank or how many rain barrels you need to catch a typical storm, and how much of your outdoor or non-potable water demand the harvest can cover. Enter the roof footprint, your annual rainfall, the roof material and your daily water use.
It is useful for gardeners planning rain barrels, homeowners considering a cistern for irrigation or toilet flushing, and off-grid builders sizing storage. Every inch of rain on 1,000 square feet of roof yields about 623 gallons before losses.
Rainfall is treated as an annual total; real harvests depend on how rain is spread across the year, so size storage generously in climates with long dry seasons. Use the roof’s horizontal footprint, not its sloped surface area, and check local rules — some areas regulate or incentivise rainwater collection.
With the default inputs, the rainwater collected per year is 28,781 gal. Change any value above to recalculate instantly.
How to use the rainwater harvesting calculator
- 1Measure the roof footprint (length × width) that drains to your gutters.
- 2Look up your average annual rainfall in inches.
- 3Select the roof surface and allow for system losses.
- 4Enter the storm size you want to capture and your daily water demand.
- 5Read the annual harvest, tank size and the per-storm table.
Formula and method
One inch of rain falling on one square foot is 144 cubic inches of water, and a US gallon is 231 cubic inches, so each square foot yields 0.623 gallons per inch of rain. Multiplying by the roof footprint and rainfall gives the theoretical volume.
The runoff coefficient C reflects how much water the roof surface actually sheds (metal about 0.95, shingles about 0.9, gravel 0.8, planted roofs 0.5). A further allowance covers first-flush diverters, gutter overflow in heavy storms and filter losses. In metric, 1 mm of rain on 1 m² gives 1 litre before the same adjustments.
- Area
- Horizontal roof footprint in square feet
- Rain
- Rainfall depth in inches
- 0.623
- US gallons per square foot per inch of rain
- C
- Runoff coefficient of the roof surface
Worked examples
1,500 sq ft shingle roof, 38 in of rain
Each inch of rain gives 1,500 × 0.623 × 0.9 × 0.9 ≈ 757 gallons, so 38 inches yields about 28,781 gallons a year. Capturing a 1-inch storm needs a 757-gallon tank or 14 rain barrels. That covers about 79% of a 100 gal/day demand and saves about $345 a year.
Large metal roof in a dry climate
A 2,400 sq ft metal roof sheds about 1,279 gallons per inch, so even 15 inches of rain yields about 19,188 gallons — more than a 50 gal/day demand (18,250 gallons). Savings are limited to the water you actually use: 18,250 gallons at $12 per 1,000 = $219.
Flat roof sized for a 2-inch storm
A 1,000 sq ft gravel roof with 15% losses collects about 424 gallons per inch, or 21,195 gallons from 50 inches of rain. Holding a 2-inch storm needs about 848 gallons (16 barrels), and the harvest covers about 73% of an 80 gal/day demand.
Frequently asked questions
How much rainwater can I collect from my roof?+
About 0.623 gallons per square foot per inch of rain before losses — roughly 623 gallons per inch from a 1,000 sq ft roof. After typical runoff and system losses, expect around 75–85% of that.
What size rainwater tank do I need?+
A common approach is to size storage to capture one typical storm (often 1 inch of rain), or to hold several weeks of your demand if rain falls unevenly. Many gardens start with one or two 55-gallon barrels per downspout, while irrigation or toilet supply usually needs 1,000+ gallon cisterns.
Should I use the roof’s sloped area?+
No. Rain falls vertically, so the catchment is the horizontal footprint of the roof — its length × width as seen from above, including overhangs. A steep roof does not collect more rain than a flat roof of the same footprint.
Is rainwater safe to drink?+
Untreated roof rainwater can carry bacteria, bird droppings and roof-material residues, so it is best used for irrigation, toilets and laundry. Drinking it requires proper filtration and disinfection and may be regulated by local health codes.
Is rainwater harvesting legal?+
Collecting rainwater is legal in most places, and some US states and cities offer rebates for rain barrels. A few jurisdictions limit the volume you can store or how it may be used, so check your state and local rules before installing a large cistern.