Rainwater Harvesting Tank Size Calculator

Enter the roof area as its horizontal footprint, not the sloped surface. Rain falls vertically, so a steep roof catches no more than a flat roof covering the same ground. Measure the length and width of the area that drains to the gutters feeding the tank. If only one side of the roof drains to it, enter only that side.

Enter annual rainfall as a long-term average for your location, then pick the roof material. The calculator applies a runoff coefficient of 0.95 for metal, 0.90 for asphalt shingle or concrete, and 0.80 for tar and gravel. On the same 1,500 sq ft roof with 40 inches of rain and 85 percent efficiency, that choice alone moves the harvest from 30,184 gallons on metal to 28,596 on asphalt and 25,418 on tar and gravel.

Set system efficiency to reflect how well the gutters, screens and diverter perform. 85 percent sits in the middle of the Texas manual's 75-90 percent range. The example yields 30,184 gallons a year.

Enter daily demand and buffer days. At 20 gallons a day, annual demand is 7,300 gallons and the roof could supply 413 percent of it. Fourteen days of buffer gives a 280-gallon tank, the volume that keeps 20 gallons a day flowing through two weeks without rain.

Read the percentage first. Above 100 percent, the question is storage. Below 100 percent, no tank closes the gap, and rainwater is a supplement to another supply.

Annual harvest (gallons) = A × R × 0.623 × C × E, where A is the catchment footprint in square feet, R is annual rainfall in inches, C is the runoff coefficient for the roof material and E is system efficiency as a decimal (0-100 percent).

The metric version needs no conversion constant: Litres = Roof area (m²) × Rainfall (mm) × C × E, because 1 mm of rain on 1 m² is exactly one litre. A 100 m² roof in an 800 mm climate with a 0.90 coefficient delivers 72,000 litres before system losses. The 1,500 sq ft roof in the example is 139.4 m², and 40 inches is 1,016 mm.

Runoff coefficients used: metal 0.95, asphalt shingle or concrete 0.90, tar and gravel 0.80. Smooth, non-absorbent surfaces shed more of each storm; rough, porous surfaces hold water that later evaporates. These are planning values, and a real roof can sit a few points either side.

Monthly average harvest = Annual harvest ÷ 12. It is an average only. The calculator does not model rainfall month by month.

Annual demand = Daily demand × 365.

Percent of demand met = Annual harvest ÷ Annual demand × 100.

Recommended tank size = Daily demand × Buffer days, with buffer days floored at one. The tank is sized to carry demand through a dry spell, not to store the year's harvest.

A dry-summer climate. A 2,000 sq ft asphalt roof, 15 inches of rain a year falling mostly in winter, 85 percent efficiency, and a garden using 50 gallons a day through a 90-day dry season. The roof collects 14,298 gallons a year against demand of 18,250 gallons, so it covers only 78 percent. The recommended tank is 4,500 gallons: almost a third of the entire year's harvest has to be stored and carried into summer. This is the hard case: a large, expensive tank, and still a shortfall that must come from mains water or a smaller garden.

A wet climate with frequent rain. A 1,200 sq ft metal roof, 55 inches of rain spread through the year, 85 percent efficiency, 40 gallons a day and a 7-day buffer. The smaller roof collects 33,203 gallons, covers 227 percent of the 14,600-gallon demand, and needs only a 280-gallon tank because it rarely goes a week without refilling.

The instructive part is that the smaller roof with the smaller tank is the more reliable system. Tank size is driven by the length of the dry spell, and roof size by the ratio of rain to demand. The dry climate needs sixteen times the storage despite having the bigger roof.

The first rain off a roof carries the dust, pollen, leaf debris and bird droppings that settled since the last storm. A first-flush diverter fills a small chamber with that water and sends only the cleaner runoff after it to the tank. The *Texas Manual on Rainwater Harvesting* gives one rule of thumb of diverting at least 10 gallons for every 1,000 square feet of collection surface, and cites recommendations of one to two gallons per 100 square feet. For the 1,500 sq ft example roof, the first rule means at least 15 gallons per storm.

That diverted water is a loss, and it is one reason the efficiency input exists. Many small storms lose a larger share to first flush than a few big ones, so a climate of frequent light showers justifies a lower efficiency figure than the same annual rainfall arriving in downpours, which in turn lose more to gutter overshoot.

As for the tank, dividing annual harvest by twelve assumes rain arrives evenly, which it almost never does. In the dry-summer example the monthly average is 1,191 gallons, but for three months the real figure is close to zero. The practical question is how many consecutive days the tank must cover, which is why buffer days, not the monthly average, set the size.

This is a planning estimate, not an engineering design. It uses a long-term annual average, so a below-average year collects less, exactly the case the buffer is meant to cover. It does not simulate daily rainfall against daily use, which is the method a designer would use to test whether a given tank actually runs dry, and it does not check gutter or downspout capacity.

The buffer-days tank size assumes the tank starts the dry spell full. If the rainy season is short or weak, it may not fill, and the harvest percentage will overstate what reaches the garden.

Many states and municipalities regulate rainwater harvesting, particularly for potable use, and some water-rights jurisdictions restrict how much can be collected. Check local codes before installing, especially if the water will be used indoors or for drinking, which requires filtration and disinfection.

If you're weighing rainwater harvesting against other water-saving options, the rainwater harvesting calculator covers the broader collection-system planning question, while the water conservation calculator and greywater recycling calculator quantify indoor fixture and reuse savings that can reduce your overall demand and shrink the tank size you need. To see your full household water picture before sizing a system, start with the water footprint calculator.

A smaller tank goes further when the landscape needs less. The Xeriscaping Water Savings Calculator estimates how much a turf conversion cuts annual irrigation demand. In the dry-summer example above, halving garden demand would halve the 4,500-gallon tank.