What this calculator does
Find out how much rainwater your roof can collect, and how big a tank you'd need. A roof is a large, clean surface for catching rain. Every inch of rain that falls on it is a sheet of water an inch deep over the whole roof. Most of that runs into the gutters and downspouts. Collecting it in barrels or a tank gives you water for gardens, washing and other non-drinking uses. It also eases the load on storm drains.
The calculator multiplies the roof's footprint (its area seen from above) by the rainfall. That gives the amount of water that falls on it. Then it applies a collection efficiency, the share that actually reaches your tank. Some water is lost to splash, evaporation, overflow and first-flush diversion. Enter a yearly rainfall for a year's supply, or a monthly figure for one month. It also sizes a tank to hold the runoff from one large storm. That's a practical starting point for choosing barrels or a cistern (a large storage tank).
Where it's used
Why this math matters outside the classroom:
- Rain barrels. Gardeners with a 1,000 ft² roof find 1,000 × 0.6234 ≈ 623 gallons per inch of rain before losses.
- Unit rates. Students show that 1 in of rain on 1 ft² is 144 in³, and 144 ÷ 231 in³ per gallon ≈ 0.62 gallons.
- Metric roofs. A 140 m² roof under 900 mm of yearly rain at 80% collects 140 × 900 × 0.8 = 100,800 liters.
- Tank sizing. Homeowners sizing a cistern for a 1 in storm on a 1,500 ft² roof at 80% plan for about 750 gallons.
- Storm-water planning. School groups estimate how much runoff their building's roof sends to storm drains in a year of average rain.
Grade level and standards: Grades 6–7; Common Core State Standards for Mathematics 6.RP.A.3.d. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.
How to use it
- Choose US or metric units.
- Enter the roof area that drains to your collection point, measured flat as if seen from above.
- Enter the rainfall for the period you care about. Then enter the rain from one large storm for the tank size.
- Set a collection efficiency. Then read the water collected and the tank size.
What the inputs mean
- Roof area that drains to the tank (measured flat, from above)
- Rain falls straight down, so a steep roof catches no more than a flat one with the same footprint. Measure the roof's length and width from above, including overhangs. Only count the sections whose gutters feed your tank.
- Rainfall over the period
- Use long-term averages for where you live, from a national weather service or climate records. Keep in mind that dry years can bring much less.
- Rain from one large storm (for tank size)
- A tank that holds one large storm's runoff without overflowing is a common starting size. A bigger tank stores water through dry spells. A smaller one overflows more often.
- Collection efficiency
- Metal and tile roofs shed water well. Rough surfaces, gravel and green (planted) roofs hold back more. First-flush diverters are devices that throw away the first, dirtiest runoff of each storm on purpose.
Formula and method
US: gallons = roof area (ft²) × rainfall (in) × 0.6234 × efficiency 0.6234 = 144 in³ per ft²·in ÷ 231 in³ per gallon metric: liters = roof area (m²) × rainfall (mm) × efficiency tank = roof area × one storm's rainfall × the same factors
One inch of rain over one square foot is 12 × 12 × 1 = 144 cubic inches (in³) of water. A US gallon is 231 cubic inches. So each square foot gives 144 ÷ 231 ≈ 0.6234 gallons per inch of rain. Efficiency is the share that reaches the tank, such as 0.8 for 80%.
In metric, the numbers line up neatly. One millimeter of rain on one square meter is 0.001 m³, which is exactly one liter.
Worked example
A 1,500 ft² roof footprint in a place with 36 in of rain a year, 80% collection efficiency, and a tank sized for a 1 in storm.
- Yield per square foot per inch: 144 ÷ 231 = 0.623377 gal.
- Water collected: 1,500 × 36 × 0.623377 × 0.8 = 26,929.8701 gal.
- Tank for a 1 in storm: 1,500 × 1 × 0.623377 × 0.8 = 748.0519 gal.
Result: The roof could supply about 26,930 gallons a year, and a tank of about 750 gallons would hold the runoff from a 1 in storm.
Understanding the result
The yearly figure is what the roof can deliver, not what you'll use. Real savings depend on whether the tank has room when it rains. They also depend on whether you use water up between storms.
Compare the tank size with your space and budget. Several linked barrels can add up to a larger volume. An overflow should lead water safely away from the foundation.
Assumptions and limitations
- Rain falls evenly over the roof area you enter.
- The efficiency covers all losses between the roof and the tank.
- It doesn't model supply and use month by month, which decides how big a tank is truly useful.
- Collected rainwater isn't safe to drink without proper treatment. Some places regulate or limit rainwater collection, so check local rules before installing a system.
- Rainfall varies from year to year. Averages hide droughts and wet spells.
Frequently asked questions
How many gallons of water come off a roof per inch of rain?
About 0.62 gallons per square foot of roof footprint before losses. That's roughly 623 gallons per 1,000 ft² for each inch of rain. With 80% efficiency, it's about 500 gallons.
Does roof pitch change how much water I collect?
No. The rain a roof catches depends on its footprint, the area it covers on the ground. The sloped surface area doesn't matter. A steep roof and a flat roof over the same footprint catch about the same rain.
How big should my rain barrel or tank be?
A common approach is to size it to hold one typical large storm, which this calculator gives. A 1,000 ft² roof sheds about 500 gallons in a 1 in storm at 80% efficiency. That's far more than one 55 gallon barrel holds.
Can I drink harvested rainwater?
Not without treatment. Roof runoff picks up dust, droppings and bits of the roof itself. Use it for gardens and other non-drinking purposes. The exception is a system designed and treated for drinking water, where that's allowed.
How this calculator is checked
Method: Volume of rain on a plan area: 144 in³ per ft² per inch ÷ 231 in³ per US gallon (exact); 1 mm on 1 m² = 1 L; times a user-entered collection efficiency. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.
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