Calculate rainwater capture volume from roof area, annual rainfall, and collection efficiency. Size cisterns and plan water storage for farm use.
Rainwater harvesting captures precipitation from roof surfaces, stores it in tanks or cisterns, and uses it for irrigation, livestock watering, or other non-potable farm purposes. In water-scarce regions, it supplements well water and reduces dependence on limited groundwater or surface water rights.
The volume captured depends on three factors: the catchment area (roof or impervious surface), the depth of rainfall, and the collection efficiency (which accounts for first-flush loss, gutter overflow, evaporation, and other system losses).
This calculator estimates annual and monthly capture volumes, helping you size storage tanks and plan water use throughout the dry season. Whether you are a beginner or experienced professional, this free online tool provides instant, reliable results without manual computation. By automating the calculation, you save time and reduce the risk of costly errors in your planning and decision-making process. This tool handles all the complex arithmetic so you can focus on interpreting results and making informed decisions based on accurate data.
Rainwater is free once the collection system is built. Knowing how much you can capture helps you right-size storage and decide whether harvesting meaningfully supplements your water supply. Having a precise figure at your fingertips empowers better planning and more confident decisions. Manual calculations are error-prone and time-consuming; this tool delivers verified results in seconds so you can focus on strategy.
Capture (gal) = Roof Area (sq ft) × Rainfall (in) × 0.623 × Efficiency Where 0.623 = gallons per sq ft per inch of rain 1 inch of rain on 1 sq ft = 0.623 gallons
Result: Capture = 74,760 gal/yr
Capture = 5,000 × 30 × 0.623 × 0.80 = 74,760 gallons per year. That's about 0.23 ac-ft — enough to irrigate a large garden or water 10–15 head of cattle for the year.
A rainwater harvesting system includes: catchment surface (roof), gutters and downspouts, first-flush diverter, storage tank (cistern, poly tank, or ferro-cement), overflow outlet, and a pump for distribution. Costs range from $1–$3 per gallon of storage capacity.
Create a monthly water budget comparing expected rainfall capture to monthly demand. In months where demand exceeds capture, you draw down the tank. In wet months, the tank refills. The tank must be large enough to bridge the longest deficit period.
Compare the annualized cost of the collection system (tank + gutters + pump) to the cost of well water or purchased water. In areas with expensive water or limited wells, rainwater harvesting can have a payback of 5–10 years. NRCS EQIP cost-share programs sometimes cover 50–75% of installation costs.
One inch of rain falling on one square foot of surface produces 0.623 gallons (144 cu in / 231 cu in per gallon). This is the standard conversion factor for rainwater harvesting calculations.
Use 80–90% for well-designed systems with metal roofs and screened gutters. Use 70–80% for older shingle roofs or systems without first-flush diverters.
Yes. Rainwater is excellent for irrigation because it is naturally soft, slightly acidic, and free of chlorine. It's ideal for drip systems and greenhouses.
Match tank size to your dry-spell duration and daily use. If you need 200 gal/day for livestock and the longest dry spell is 30 days, you need at least 6,000 gallons of storage.
In most U.S. states, yes. Colorado and a few others historically restricted it but now allow collection from residential rooftops. Check your state's current regulations.
With proper filtration and disinfection (UV or chlorination), rainwater can be made potable. However, most farm systems use it for non-potable purposes without treatment.