Rainwater Harvesting Calculator

Background.

A storm that drops two and a half inches of rain on a 1,200 square foot house sends about 1,870 gallons off the roof. People who put a tank under a downspout are usually surprised by that number, and then surprised again by how little of it they keep. Some of the rain never reaches the gutter, the first-flush diverter dumps the dirtiest runoff, and what is left, about 1,570 gallons here, arrives at a 1,000-gallon tank that was already holding 200. It can take 800 more. Everything else goes out of the overflow.

That is the usual pattern: the roof is rarely the limit, the tank is. A common 55-gallon rain barrel under the same roof fills from empty with less than an eighth of an inch of rain once the diverter has done its work, so one ordinary shower tops it up and the rest of the storm runs to waste. A larger tank only pays off if the water is used between storms, which is why the calculator asks for the starting level and the demand as well as the rain. Enter a garden's watering need for a week of dry weather and the tank either carries it or comes up short, and you see by how much.

Keep the period consistent. A single storm, a month or a dry spell all work, provided the rainfall, the demand and the starting level describe the same stretch of time. A whole year in one go does not: the model fills the tank once and drains it once, while a real tank fills and empties many times, so annual totals produce absurd overflow and shortfall figures. Work month by month, carrying each month's ending volume forward as the next month's starting level.

What is rainwater harvesting calculator?

Rainwater harvesting is collecting the runoff from a roof or other hard surface and storing it for later use, typically for watering gardens, washing and, with the right plumbing, flushing toilets. The yield of a catchment is set by its area and the depth of rain: one inch falling on one square foot is a twelfth of a cubic foot, or about 0.6234 US gallons, so every 1,000 square feet of roof sheds about 623 gallons per inch. In metric units the relationship is exactly one litre per millimetre per square metre. Real systems collect less than that gross volume. Rain splashes past gutters in wind, wets the roof and evaporates in light showers, and leaks or overflows at joints and screens; a first-flush diverter deliberately discards the opening runoff, which carries most of the dust, pollen, leaves and droppings. What reaches the tank then has to fit. The Texas Water Development Board's rainwater harvesting manual is a widely used practical reference for these systems, and NOAA's climate normals give long-term monthly rainfall for US weather stations.

How to use this calculator.

1. 01Enter the catchment roof area in square feet: the footprint the roof covers seen from above, overhangs included, not the area along the slope. Count only the roof sections whose gutters actually drain to this tank.
2. 02Enter the rainfall depth in inches for the period you are modelling: a storm total from a forecast or gauge, or a monthly normal for planning.
3. 03Set the collection efficiency. The 85 percent default is a planning assumption; use less for rough or absorbent roofing and neglected gutters, more for a smooth metal roof with clean gutters, and 100 with no fixed loss if you want the theoretical maximum.
4. 04Enter the first-flush or fixed loss in gallons: what your diverter discards each storm. If the period covers several storms, multiply by their number, because the loss is subtracted only once.
5. 05Enter the tank capacity, how much is in it at the start (it cannot exceed the capacity), and the water you expect to draw over the same period.
6. 06Read the collectable rainfall first, then the overflow, the ending volume and any shortfall to see whether the tank or the demand is the limit.

The formula.

Gross volume is G = A × R × 7.48052 ⁄ 12, with the roof area A in square feet and the rainfall R in inches. Dividing by 12 turns inches into feet, so A × R ⁄ 12 is cubic feet of water, and 7.48052 is the number of US gallons in a cubic foot, from the NIST unit tables. Together they make the 0.6234 gallons per square foot per inch usually rounded to 0.623.

Collectable volume is C = max(0, G × e − f), where e is the collection efficiency as a fraction and f the fixed first-flush loss in gallons. The two losses behave differently on purpose. Efficiency scales with the storm, while the diverter takes the same volume from a drizzle as from a downpour, so small events lose a large share and very small ones yield nothing at all. The capture ratio reported with the result is C ⁄ G × 100, the share of the gross rainfall that reaches the tank inlet.

The tank balance then runs in a fixed order. Starting volume S plus C is compared with capacity T; anything above T is overflow. The tank is capped at T, the period's demand D is drawn from it, the ending volume is max(0, min(T, S + C) − D), and any demand the tank could not meet is the shortfall.

Filling the tank before any water is drawn is a simplification. If you water the garden between showers within the same period, that use frees space before the next rain arrives, so a real system overflows somewhat less and stores somewhat more than this model shows. For a single storm the difference is small; across a long, mixed period it grows, which is why shorter periods give better answers.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Texas Water Development Board (2005). "The Texas Manual on Rainwater Harvesting, 3rd edition." TWDB.
2. [2]U.S. Environmental Protection Agency (2024). "Soak Up the Rain: Rain Barrels." EPA.
3. [3]Centers for Disease Control and Prevention (2024). "Collecting Rainwater and Your Health: An Overview." CDC.
4. [4]National Oceanic and Atmospheric Administration (n.d.). "U.S. Climate Normals." NOAA National Centers for Environmental Information.
5. [5]National Institute of Standards and Technology (2026). "NIST Handbook 133, Appendix E: General Tables of Units of Measurement." U.S. Department of Commerce.