Land, water & building research
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Rainwater harvesting calculator

By Published October 10, 2026

How much water can your roof actually supply, and how does that compare with what the household uses? Start with a state average, then replace it with rainfall from your own site.

Every inch of rain on one square foot of roof is about 0.623 gallons. Gutters, first-flush diverters, evaporation and overflow lose part of that, so the calculator applies a collection efficiency you can change. It then compares annual yield with annual household use. Annual totals can hide long dry spells; our water supply guide explains why storage has to be sized for the driest stretch, not the average year.

Estimate the yield

Annual roof yield––
Annual household demand––
Roof needed for full supply–At this rainfall and efficiency
Storage for the dry spell–Demand × dry days, before reserve

How the numbers are calculated

Yield (gallons) = roof area (sq ft) × rainfall (inches) × 0.623 × efficiency. A 1,500 sq ft roof in 20 inches of rain at 80% efficiency yields about 14,950 gallons a year, roughly 41 gallons a day averaged across the year.

Demand (gallons) = people × gallons per person per day × 365. EPA’s WaterSense program puts the average American’s home use at about 82 gallons a day. Off-grid households often use far less, but the number is a household decision that should be measured, not assumed.

Dry-spell storage (gallons) = daily demand × days without useful rain. This is a minimum before any reserve for fire, livestock, a failed pump or a drier-than-normal year. Look at monthly records for the site, not the annual average used here.

The state averages come from NOAA NCEI Climate at a Glance (1991–2020 statewide averages). Statewide averages blend very different places: Oregon’s coast and its high desert share one figure. Replace it with a nearby station before relying on the result.

Before relying on rainwater

  • Check the rules. Most states allow roof collection, but some cap volumes, require registration or limit potable use. Colorado, for example, limits most households to two barrels. See the 50-state rainwater table.
  • Ask whether rain can be the legal primary supply. A building or health department may require an approved potable source, such as a permitted well, before issuing a certificate of occupancy.
  • Plan treatment and testing. Roof water can carry bacteria, debris and roofing residues. Filtration, disinfection and routine testing are part of a drinking-water system.
  • Size for failure. Decide how you will haul or buy water when storage runs low, and where a delivery truck can reach.

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