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Rainwater Harvesting / Cistern Sizing Calculator

Calculate monthly collection volume, cistern storage sizing, and payback period based on local rainfall and demand

Free rainwater harvesting calculator for civil engineers, architects, landscape designers, and homeowners. Enter the roof catchment area, monthly rainfall data, roofing material, planned water demand (irrigation, non-potable indoor, or full household), and days of autonomy to calculate the monthly collection volume, recommended cistern size, and simple payback period based on avoided water utility costs. The calculator runs a 12-month mass balance model (collection minus demand) and sizes the cistern from daily demand times days of autonomy times a safety factor. Shows which months produce a surplus and which create a deficit, along with first-flush diversion, pump sizing, and the maximum single-month deficit.

Pro Tip: Collection efficiency is the biggest source of error in rainwater calculations. The theoretical collection (rainfall × roof area) is reduced by first-flush diversion (discarding the first 1-2 gallons per 100 sq ft to wash off debris), gutter overflow during intense storms, absorption by roofing material, and evaporation. Use 75-85% for metal roofs, 70-80% for asphalt shingles, and 60-70% for flat or gravel roofs. Most online calculators use 100%, which oversizes the system by 15-30%.

Calculate pump energy cost for cistern pressurization

Pump Energy Cost Calculator →

Size water storage tanks and elevated towers

Water Tower Sizing Calculator →

Calculate detention time for treatment systems

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Size irrigation pumps and piping from the cistern

Irrigation Pump & Pipe Calculator →

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Rainwater Harvesting Calculator
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How It Works

  1. Enter Catchment Area

    Enter the roof footprint area in square feet (horizontal projection, not the sloped roof area). For complex rooflines, use the building footprint as a close approximation. Select the roofing material to set the default collection efficiency.

  2. Enter Rainfall Data

    Enter the average monthly rainfall or the annual total (which the calculator distributes by month using regional patterns). For more accurate results, use NOAA monthly normals for your specific location. The calculator converts rainfall inches to gallons collected per month.

  3. Define Water Demand

    Select the end use, irrigation only, non-potable indoor (toilets + laundry), or full potable supply, and enter the monthly demand or let the calculator estimate based on household size. Irrigation demand varies seasonally; indoor demand is relatively constant year-round.

  4. Review Cistern Sizing and ROI

    The calculator sizes the cistern based on daily demand times days of autonomy times your safety factor, then rounds up to the next standard tank size. It shows the recommended and next-standard cistern volume, annual collection versus demand, the maximum single-month deficit, and simple payback period based on avoided water utility costs.

Built For

  • Homeowners in drought-prone regions sizing rainwater cisterns for landscape irrigation supplementation
  • Architects designing LEED-certified buildings with rainwater harvesting for WE credit qualification
  • Civil engineers designing stormwater management systems that incorporate rainwater reuse to reduce runoff volume
  • Agricultural producers sizing collection systems for livestock watering in areas without municipal water service
  • Developing-world project planners calculating cistern sizes for roof catchment systems providing community water supply

Assumptions

  • Monthly rainfall is based on averages, actual monthly totals vary significantly year to year, especially in arid and semi-arid climates.
  • Collection efficiency is a fixed percentage, actual efficiency varies with rainfall intensity (heavy storms overflow gutters, reducing effective collection).
  • Demand is entered as a monthly average, actual demand fluctuates with weather, occupancy, and seasonal irrigation needs.
  • The cistern is initially empty at the start of the analysis, steady-state performance may differ from first-year performance.

References

  • ARCSA/ASPE/ANSI 63-2013, Rainwater Catchment Systems
  • Texas Water Development Board, The Texas Manual on Rainwater Harvesting, 3rd Edition
  • NOAA, Monthly Climate Normals (precipitation data by station)
  • LEED v4.1 BD+C, Water Efficiency Credits: Outdoor Water Use Reduction and Indoor Water Use Reduction

Frequently Asked Questions

The formula is: gallons = roof area (sq ft) × rainfall (inches) × 0.623 × collection efficiency. One inch of rain on 1,000 sq ft of roof yields 623 gallons at 100% efficiency. With a realistic 80% efficiency (metal roof with first-flush diversion): 623 × 0.80 = about 498 gallons per inch of rain per 1,000 sq ft of roof. A 2,000 sq ft roof in an area receiving 40 inches of rain per year can collect roughly 40,000 gallons annually.
Cistern size depends on the mismatch between collection (rain) and demand (usage) patterns. If rain and demand are in sync (rain falls when you need water), a smaller tank works. If they're offset (summer demand with winter rainfall), you need a larger tank to bridge the dry months. The calculator runs a month-by-month balance to find the minimum size. As a rough starting point: for irrigation-only systems, 500-1,500 gallons per 1,000 sq ft of roof area covers most of the US.
It depends on your state and local jurisdiction. Most US states explicitly allow rainwater harvesting, Texas, Virginia, and several others actively encourage it with tax incentives. Colorado historically restricted it but now allows up to two 110-gallon barrels for residential use without a permit. A few states require permits for larger systems or systems connected to indoor plumbing. Check your state water rights laws and local building codes before installing a system.
Yes, with proper treatment. Untreated roof runoff can contain bird droppings, dust, pollen, and leached roofing chemicals. For potable use, the ARCSA/ASPE/ANSI 63-2013 standard recommends a treatment train of: first-flush diversion, screening/filtration (minimum 5 micron), UV disinfection or chlorination, and activated carbon for taste/odor. Metal roofs are preferred for potable systems, avoid roofs with lead flashing, treated wood, or asbestos shingles. Many jurisdictions require a building permit and plumbing inspection for indoor potable rainwater systems.
Disclaimer: This calculator provides rainwater harvesting estimates based on average rainfall data and simplified monthly mass balance modeling. Actual collection volumes depend on rainfall intensity distribution, roof condition, gutter sizing, first-flush diverter capacity, and actual demand patterns. Potable rainwater systems must comply with local building codes and health regulations. Treatment system design should be performed by a qualified professional.

Learn More

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Rainwater Harvesting System Design and Cistern Sizing

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