Rainwater harvesting calculator

What is a rainwater harvesting calculator?

A rainwater harvesting calculator estimates how much water a roof (or any catchment surface) can collect from a given amount of rain. Rain that lands on a roof does not soak away — it runs off along the slope, into the gutters, down the downpipe and, if you plan for it, into a tank or barrel instead of the storm drain. This calculator turns three numbers you can measure or look up — the catchment area, the rainfall depth and a runoff efficiency — into the volume of water you could realistically store.

It is the first sizing step for a rain barrel, a garden irrigation store or a whole-house harvesting system: knowing that a typical roof yields hundreds of gallons from a single inch of rain is what tells you whether a 55-gallon barrel or a 2,500-gallon cistern is the right scale.

How does the calculator work?

Enter the three inputs and the calculator returns the harvested volume:

• Roof catchment area — the horizontal (plan) footprint the rain falls on, switchable between ft², m² and yd². Use the building footprint the roof covers, not the sloped roof surface: a pitched roof collects the same rain as its flat footprint, because rain falls vertically.
• Rainfall depth — how much rain has fallen, in inches, millimetres or centimetres. Use a single storm, a monthly total or an annual average depending on what you want to size.
• Runoff efficiency — the fraction of that rain that actually reaches storage after splash, evaporation, wetting of the roof, first-flush diversion and overflow, entered as a percentage. A well-guttered metal or tile roof runs at about 80–95%; a rough or absorbent surface loses more. The default is 90%.

The result, harvested water, is shown in US gallons and can be switched to litres or cubic metres.

Formula

The harvested volume uses the standard collection factor of 0.623 US gallons per square foot per inch of rain. That factor is just a unit conversion: one cubic foot of water is 7.48 US gallons, spread over 12 inches of depth, so $7.48 \div 12 = 0.623$ gallons for each square foot covered by one inch of rain. With catchment area $A$ (ft²), rainfall depth $D$ (in) and runoff efficiency $E$ (%), the harvested volume $V$ in US gallons is:

$$V = A \times D \times 0.623 \times \frac{E}{100}$$

In metric units the same idea is even simpler, because one millimetre of rain on one square metre is exactly one litre:

$$V_{\text{L}} = A_{\text{m}^2} \times D_{\text{mm}} \times \frac{E}{100}$$

The calculator computes the area in ft² and the rainfall in inches internally, so you get the same answer whichever units you type in.

Worked examples

Example 1: one inch of rain on a 1,000 ft² roof

A roof with a 1,000 ft² footprint receives 1 inch of rain, and the gutters and tank capture essentially all of it (100% efficiency):

$$V = 1000 \times 1 \times 0.623 \times \frac{100}{100} = 623 \, \text{gal}$$

That single inch of rain yields 623 gallons — about 2,358 litres. It is a useful benchmark: at 100% efficiency, every 1,000 ft² of roof harvests roughly 623 gallons per inch of rain.

Example 2: a heavier storm at a realistic efficiency

The same 1,000 ft² roof, but 2 inches of rain and a more realistic 90% runoff efficiency:

$$V = 1000 \times 2 \times 0.623 \times \frac{90}{100} = 1121.4 \, \text{gal}$$

So the storm delivers 1,121.4 gallons to storage — enough to overflow several rain barrels, which is why a downpipe diverter and an overflow route matter.

Example 3: the same calculation in metric units

A 92.903 m² roof (the metric equivalent of 1,000 ft²) receives 25.4 mm of rain (1 inch) at 100% efficiency. Because 1 mm on 1 m² is 1 litre:

$$V_{\text{L}} = 92.903 \times 25.4 \times \frac{100}{100} \approx 2360 \, \text{L}$$

That is about 2,358 litres, or 623 gallons — the same result as Example 1, as it must be.

Practical notes

• Measure the footprint, not the slope. A pitched roof collects the rain that falls on its plan area, so use the building’s outline. If you know the roof pitch you can check the sloped area with the roof pitch calculator, but the catchment for harvesting is still the horizontal footprint.
• Pick the rainfall period that matches your question. Use a single design storm to size gutters and overflow, a dry-month total to check how long a tank lasts, or the annual average to estimate a season’s supply.
• Runoff efficiency is where estimates go wrong. Metal and glazed tile roofs are the most efficient; asphalt shingle, concrete tile and green roofs lose more to absorption and evaporation. Dropping the efficiency below 100% is what turns a theoretical maximum into a usable figure.
• Size the tank to the catch, not the other way round. Once you know the harvested volume, the tank volume calculator helps you pick a cistern that holds a meaningful share of it; anything above the tank’s capacity simply overflows.
• Rain, snow and mixed precipitation differ. Melting snow also runs off a roof — if your totals are given as snowfall, convert them first with the rain-to-snow calculator so the depth you enter here is liquid-equivalent rainfall.
• Check local rules. Rainwater harvesting is encouraged in many places and restricted in a few; confirm what is allowed, and never use roof-collected water as drinking water without proper treatment.