Rainwater Harvesting Calculator

About the Rainwater Harvesting Calculator

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 page turns roof area, rainfall, and efficiency into a storage-sizing number that is easier to compare with seasonal demand.

When This Page Helps

Harvesting only matters if the captured volume is large enough to change purchased or pumped water needs. This page helps test that quickly.

How to Use the Inputs

1. Enter the total catchment (roof) area in square feet.
2. Enter the annual rainfall in inches.
3. Enter the collection efficiency as a percentage (typically 75–90%).
4. Read the annual capture volume in gallons.
5. Size your storage tank to hold 1–3 months of expected demand.

Example Calculation

Tips & Best Practices

• Metal roofs have higher collection efficiency (85–95%) than shingle roofs (75–85%).
• Install a first-flush diverter to discard the initial dirty runoff.
• Size tanks for 1–3 months of demand to bridge dry spells.
• Screen gutters and tank inlets to keep debris and mosquitoes out.
• Use dark or opaque tanks to prevent algae growth.
• Check local regulations; some states require permits or restrict rainwater collection.

System Components

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.

Matching Supply to Demand

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.

Cost-Benefit Analysis

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.

Frequently Asked Questions

• What is the 0.623 factor?

  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.

• What collection efficiency should I use?

  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.

• Can I use rainwater for irrigation?

  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.

• How big a tank do I need?

  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.

• Is rainwater harvesting legal?

  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.

• Can I drink harvested rainwater?

  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.