Rainwater Harvest Calculator

Frequently Asked Questions

How much rainwater can I collect from a roof?

Gallons = catchment area (ft²) × rainfall (in) × 0.623 × efficiency. A 1,000 ft² roof in 1 in of rain at 85% efficiency yields 1,000 × 1 × 0.623 × 0.85 ≈ 530 gallons.

Where does the 0.623 factor come from?

One inch of rain over one square foot is 144 cubic inches ≈ 0.623 gallons. Multiplying area in ft² by inches of rain by 0.623 converts directly to gallons.

Why include a collection efficiency?

Some water is lost to evaporation, splash, first-flush diversion, and gutter overflow, so real systems capture about 75–90% of the theoretical amount. Apply an efficiency factor for a realistic estimate.

How big a tank do I need?

Size the tank to the largest expected single storm or your usage between rains. For the 530-gallon example, a 550-gallon tank captures it, but pair it with overflow routing for bigger storms.

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Important Disclaimer: Estimates for informational purposes only.

This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How This Calculator Works

Every inch of rain that lands on your roof is water you could be storing instead of sending down the storm drain, and the amount is surprisingly large - a single inch on a modest roof yields hundreds of gallons. This calculator multiplies your roof's catchment footprint (the flat plan area, not the sloped surface) by the rainfall depth and the industry conversion factor of 0.623 gallons per square foot per inch, then trims the result by a collection efficiency that accounts for the water lost to gutter splash, first-flush diversion, evaporation, and overshoot. It reports the catch from one rainfall event, the expected annual yield from your average yearly rainfall, and a suggested tank size so you can store a worthwhile fraction of what falls without buying a cistern far larger than you will ever fill.

Formula / Method

Catchment area is the building footprint - length × width of the plan view - because rain falls vertically, so a steep roof catches no more rain than its flat shadow. Gallons per event = area (sq ft) × rainfall (in) × 0.623 × efficiency. The constant 0.623 comes from 1 inch of water over 1 square foot = 144 cubic inches ≈ 0.623 US gallons. Efficiency is entered as a percent (typically 75–90%) and divided by 100. Annual yield uses your average yearly rainfall in place of the single-event depth. The suggested tank is the larger of one big-storm capture or about 5% of the annual yield, rounded up to the nearest 50 gallons. Inputs are validated - non-positive dimensions, negative rainfall, or an out-of-range efficiency skip the calculation rather than returning NaN.

Worked Example

A 40 ft × 30 ft roof footprint = 1,200 sq ft. A 1-inch storm at 85% efficiency captures 1,200 × 1 × 0.623 × 0.85 ≈ 635 gallons from a single rain. Over a year with 38 inches of average rainfall the same roof yields 1,200 × 38 × 0.623 × 0.85 ≈ 24,140 gallons - enough to make a real dent in summer irrigation. The suggested tank rounds the single-event catch (or 5% of annual, whichever is larger) up to a stock size, landing around 1,200 gallons, a realistic backyard cistern that fills in a couple of good storms.

Common Mistakes

Using the sloped roof surface area instead of the flat footprint - rain falls vertically, so only the plan area counts.

Assuming 100% efficiency; first-flush diverters, gutter overflow, and splash routinely cost 10–25% of the theoretical catch.

Sizing a tank for the annual total rather than for realistic storm-by-storm capture - a tank only holds what fits before the next dry spell.

Forgetting that a tank that is always full overflows and wastes the next storm; storage should match how fast you use the water.

Ignoring local regulations - a few jurisdictions restrict rainwater harvesting or require permits for large cisterns.

Tips & FAQ

Why only the footprint? Imagine the roof's shadow at noon on the equinox - that flat area is exactly what intercepts vertical rain, so a 6/12 pitch catches no more than a flat roof of the same footprint. What efficiency should I use? A clean metal roof with good gutters and a first-flush diverter runs 80–90%; an older asphalt-shingle roof with leaky gutters may be 65–75%. Lower the number if in doubt - under-promising storage is cheaper than overflow. How big a tank do I really need? Match storage to use, not to total rainfall: if you irrigate a garden through dry weeks, you want enough to bridge the longest dry gap, which is usually a few hundred to a couple thousand gallons for a home garden. Is the water safe? Roof runoff is fine for irrigation but is not potable without filtration and disinfection - keep harvested rainwater for plants, washing, and flushing unless you install a treatment system. First flush? Always divert the first gallon or two of each storm, which carries roof dust, bird droppings, and debris; a simple first-flush device dramatically improves stored-water quality. Screen the tank inlet to keep out mosquitoes and leaves, keep the cistern opaque to suppress algae, and remember that a single big storm can fill a small tank in minutes - plan an overflow route that drains safely away from the foundation.

Rainwater Harvest Calculator

Frequently Asked Questions

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