Rainwater Harvesting Calculator Skip to main content
Municipal Free Pro Features Available

Rainwater Harvesting 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, facility teams, and homeowners. Enter roof catchment area, monthly rainfall prompts, roof surface, planned demand, autonomy days, water cost, and installed-cost prompt to review a 12-month collection-versus-demand balance. The output shows source-gap tank-autonomy prompts, next standard tank row, first-flush prompt, pump-flow prompt, annual collection coverage, and avoided-water-cost payback. Potable use, indoor nonpotable plumbing, backflow protection, cross-connection control, treatment, water testing, stormwater credit, water rights, permits, product selection, and AHJ acceptance remain outside the calculator.

Pro Tip: Replace the built-in city rainfall rows with the nearest NOAA station normals or measured site data before using the balance. Monthly averages do not show storm intensity, gutter overflow, dry-spell duration, first-flush event loss, climate variability, or seasonal demand, which are the details that drive real storage and overflow decisions.

Check pump energy-cost prompts for cistern pressurization

Pump Energy Cost Calculator →

Review water storage tank prompts with source boundaries

Water Tower Storage Sizing Calculator →

Calculate detention time for treatment systems

Detention Time Calculator →

Check irrigation pump and pipe prompts before product review

Irrigation Pump & Pipe Sizing Calculator →

PREVIEW All Pro features are currently free for a limited time. No license key required.

Rainwater Harvesting Calculator
Pop Out

How It Works

  1. Enter Catchment and Roof Prompts

    Enter the horizontal roof catchment area and select a roof-surface efficiency prompt. Verify actual roof materials, contaminants, coatings, gutter capacity, debris, screens, first-flush configuration, and maintenance outside this calculator.

  2. Enter Rainfall Prompts

    Choose a built-in city row or enter monthly rainfall. Replace built-in rows with current NOAA station normals, measured site data, or a project hydrology record before design, procurement, or stormwater review.

  3. Define Demand Prompts

    Select irrigation, indoor nonpotable, household total-use, or custom demand. Potable and indoor nonpotable uses require local health, plumbing, treatment, testing, backflow, cross-connection, and AHJ review.

  4. Review Balance and Source Gaps

    Review annual collection, annual demand, coverage, autonomy tank prompt, next standard tank row, first-flush prompt, pump-flow prompt, and avoided-water-cost payback. Treat the result as an estimate, not final design or approval.

Built For

  • Homeowners screening rainwater collection and storage prompts before local water-right, health, and plumbing review
  • Architects documenting rainfall, demand, tank, treatment, and certification source gaps before green-building or owner review
  • Civil engineers reviewing whether a rainwater reuse concept needs separate stormwater, overflow, and permitting analysis
  • Agricultural producers estimating roof-catchment supply prompts before livestock, water-quality, and drought-supply review
  • Project planners screening roof-catchment storage concepts before treatment, public-health, product, and local-authority review

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

  • ICC IPC 2024 Chapter 13 Nonpotable Water Systems source pointer
  • CSA B805-22 / ICC 805-2022 Rainwater Harvesting Systems source pointer
  • ARCSA/ASPE/ANSI 63-2020 Rainwater Catchment Systems source pointer
  • CDC, Collecting Rainwater and Your Health
  • EPA, Rainwater Harvesting: Conservation, Credit, Codes, and Cost
  • Texas Water Development Board, The Texas Manual on Rainwater Harvesting, 3rd Edition
  • NOAA NCEI U.S. Climate Normals

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.
This calculator gives an autonomy tank prompt and next standard tank row from the entered demand, autonomy days, and safety factor. Actual tank capacity depends on local rainfall records, dry spells, seasonal demand, dead storage, overflow, tank product, foundation, code, maintenance, and qualified review.
Treat that as a local source question. State water-rights rules, adopted plumbing/building code, stormwater or overflow requirements, indoor-use rules, HOA/owner requirements, and AHJ interpretation can all change the answer.
This calculator does not determine drinking-water safety. Rainwater can contain biological and chemical contaminants. Potable or bathing uses require current local health department rules, treatment design, water testing, component listings, cross-connection control, and qualified review.
Disclaimer: This screen provides preliminary source-aware rainwater balance prompts only. It is not potable-water approval, nonpotable plumbing approval, tank product selection, stormwater credit, water-rights review, treatment design, pump design, permit approval, or installation authorization. Verify current local code, health department, water-quality, product, utility, environmental, owner, insurer, and AHJ requirements with qualified reviewers.

Learn More

Municipal

Rainwater Harvesting System Design and Cistern Sizing

Review rainwater collection, monthly supply-demand balance, first-flush, treatment, tank, plumbing, health, and local code source gaps.

Read Guide →

Related Tools

Municipal Live

Chemical Dosing Calculator

Calculate gallons per day or pounds per day of chemical feed for any water or wastewater treatment process. Supports chlorine, alum, ferric chloride, polymer, lime, caustic, permanganate, and fluoride with automatic unit conversions and metering pump settings.

Launch Tool →
Municipal Live

Detention Time Calculator

Calculate hydraulic detention time for any basin, tank, or lagoon and check against regulatory minimums. Supports rectangular and circular tanks with dead zone correction for actual vs theoretical retention time.

Launch Tool →
Municipal Live

Disinfection CT Value Calculator

Calculate CT values for chlorine disinfection and verify EPA Surface Water Treatment Rule compliance. Check Giardia and virus log inactivation credits based on residual, contact time, temperature, and pH.

Launch Tool →