Rainwater Harvesting Calculator

Estimate how many gallons or liters your roof can collect, how much of your water use a tank covers month by month, overflow and shortfall, and the tank size beyond which extra storage adds little.

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Results will appear here once you enter your values.

How much water a roof really collects

One inch of rain on one square foot of roof is 144 cubic inches of water, or 0.623 US gallons. A 1,500-square-foot roof therefore sheds about 935 gallons per inch of rain before losses, and a place with 36 inches a year offers roughly 33,600 gallons. In metric the rule is even simpler: 1 millimeter on 1 square meter is 1 liter.

Not all of that reaches the tank. The Texas Manual on Rainwater Harvesting notes that water is lost to first-flush diversion, evaporation, splash-out and gutter overshoot during hard rains, and that most practitioners assume a collection efficiency of 75 to 90 percent. Smooth metal sheds water best; rough or gravel surfaces hold water in pores, where it evaporates. This calculator applies that loss as a runoff coefficient: 0.95 for metal, 0.90 for shingle, 0.85 for tile, and 0.70 for gravel flat roofs.

Measure the footprint that drains to the gutters feeding your tank, not the sloped roof surface. Rain falls vertically, so a steep roof catches no more than a flat one with the same outline. The manual's own Dallas example uses 1.97 inches on a 2,500-square-foot roof at 85 percent efficiency: 1.97 x 2,500 x 0.62 x 0.85 = 2,595 gallons for January.

Sizing the tank with a monthly water balance

Annual totals hide the real constraint, which is the dry season. A garden in a Mediterranean climate may need the most water in July and August, exactly when the roof collects almost nothing. The storage has to carry winter and spring rain forward. That is why this tool runs a month-by-month balance: water in storage plus water captured, minus water used, with anything above tank capacity counted as overflow.

The model repeats the year until the tank level at the start of January stops changing, so the result reflects a typical year instead of a brand-new empty tank. It then searches for the smallest tank that delivers at least 99 percent of what an unlimited tank could. Past that point, a bigger cistern mostly stores air.

  • Enter monthly normals for your nearest station for a realistic size; an even annual spread will make almost any tank look adequate.
  • A monthly step smooths out single storms and dry spells, so real systems overflow more in cloudbursts and run short sooner in long droughts. Add a margin for drinking-water systems.
  • Check state and local rules. Most U.S. states allow rooftop collection, but a few regulate volumes, and potable use needs filtration and disinfection.

Common sizing mistakes

Most oversized or undersized systems trace back to one of these assumptions.

  • Measuring the sloped roof. Measuring along the pitch overstates the catchment. Use the outline seen from above, and only the sections whose gutters feed the tank.
  • Sizing from annual rainfall alone. A roof can collect twice what you use in a year and still leave the tank empty for three summer months. Use monthly normals.
  • Forgetting the first flush. The Texas Manual describes diverting the first, dirtiest runoff of each storm, commonly about 10 gallons per 1,000 square feet of roof. In climates with many small storms, that diverted water adds up and lowers real yield.
  • Overestimating demand savings. A garden watered twice a week does not use water every day. Estimate daily demand by averaging real use across the month, and remember that toilet flushing is steady while irrigation is seasonal.

Frequently Asked Questions

Common questions about the Rainwater Harvesting Calculator

About 0.623 gallons per square foot of roof footprint per inch of rain before losses. A 1,000-square-foot roof yields about 623 gallons per inch, or around 560 gallons after a typical 90 percent collection efficiency.

Sources & References

The Texas Manual on Rainwater Harvesting, Third Edition

Source for the 0.62 gallons per square foot per inch rule, 75 to 90 percent collection efficiency, and the monthly storage balance method.
→Source: Texas Water Development Board (2005)

Soak Up the Rain: Rain Barrels

Guidance on residential rain barrels, uses for collected water, and stormwater benefits.
→Source: U.S. EPA

U.S. Climate Normals

1991-2020 monthly precipitation normals by station for entering realistic monthly rainfall.
→Source: NOAA National Centers for Environmental Information

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