Rainwater Harvesting Calculator
Estimate how many gallons of rainwater you can collect annually from your roof, capped by your tank size. Use it when designing a rain barrel system or planning irrigation needs.
About this calculator
Rainwater collection potential is governed by roof area, rainfall depth, and the efficiency of the collection surface. The core formula is: Annual Collection (gal) = roofArea (sq ft) × monthlyRainfall (in) × 0.623 × runoffCoefficient × 12 months. The factor 0.623 converts inches of rainfall over one square foot into gallons (1 inch of rain on 1 sq ft ≈ 0.623 gallons). The runoff coefficient accounts for losses to absorption and splash — metal roofs score ~0.95, while asphalt shingles score ~0.85 and gravel roofs ~0.70. Multiplying by 12 annualizes a monthly average. If the calculated annual volume exceeds your storage tank capacity, the usable total is capped at the tank size, since excess water overflows. Comparing annual collection to monthly demand helps size the tank correctly.
How to use
You have a 1,500 sq ft asphalt shingle roof (runoff coefficient 0.85), average monthly rainfall of 3 inches, and a 2,000-gallon storage tank. Annual potential = 1,500 × 3 × 0.623 × 0.85 × 12 = 1,500 × 3 × 0.5296 × 12 = 28,591 gallons. Since 28,591 gallons far exceeds the 2,000-gallon tank, usable collection is capped at 2,000 gallons. This means the tank fills quickly — roughly every 3–4 weeks — so a larger tank or multiple tanks would let you capture significantly more rainwater.
Frequently asked questions
How much rainwater can I collect from my roof per inch of rainfall?
For every 1 inch of rain falling on 1,000 square feet of roof, you can collect approximately 623 gallons before accounting for losses. Multiplying by your roof's runoff coefficient (0.70–0.95 depending on material) gives realistic yield. A 2,000 sq ft metal roof in a region receiving 2 inches of monthly rain could theoretically yield over 2,200 gallons per month.
What size rainwater storage tank do I need for irrigation use?
Tank sizing depends on the gap between rainfall events and your water demand between collections. A common rule of thumb is to size the tank to store at least one month's worth of usable collection or one month's demand, whichever is smaller. For a typical suburban garden requiring 500 gallons per month, a 500–1,000 gallon tank is a practical starting point; larger irrigated plots may need 2,500+ gallons.
Is harvested rainwater safe to use for drinking or gardening?
Rainwater is generally safe for garden irrigation and toilet flushing without treatment, though it can pick up pollutants from roofing materials and atmospheric dust. For potable use, the CDC recommends filtering and disinfecting collected rainwater with a multi-stage system including sediment filters, activated carbon, and UV sterilization. Regulations on rainwater harvesting for drinking vary by state, so always check local health codes before using harvested water for human consumption.