Rainwater Harvesting Yield & Cistern Tank Sizing Calculator

Hydrological Engineering & Sustainable Living: Rainwater Harvesting Principles

Rainwater harvesting captures and stores rooftop precipitation for landscape irrigation, toilet flushing, clothes washing, and potable water filtration:

• The 0.623 Gallon Rule: One inch of rainfall on one square foot of roof surface yields exactly 0.6233 US Gallons (2.359 Liters) of water before factoring in material absorption and filter diversion losses.

• Runoff Coefficients (C):

• Metal Standing Seam (0.90 – 0.95): Smooth, non-porous surface offering the highest collection efficiency and cleanest water.

• Clay / Concrete Tiles (0.80 – 0.85): Good yield with slight absorption into porous ceramics.

• Asphalt Fiberglass Shingles (0.75 – 0.80): Texture retains granular grit and releases minor petroleum hydrocarbons, requiring first-flush filtration.

• The First-Flush Principle: The first 1 to 2 millimeters of rainfall washes airborne particulate dust, bird droppings, and leaves off the roof; mechanical first-flush diverters discard this initial volume before routing clean water into the cistern.

How to Use This Calculator

Enter your Catchment Roof Surface Area (Square Feet) (e.g. 2,500), Roof Surface Material & Runoff Coefficient (C) — choose from Metal Standing Seam / Corrugated (C = 0.95 — Optimal), Asphalt Fiberglass Shingles (C = 0.80), Clay / Concrete Tile (C = 0.85), Annual Local Rainfall (Inches / Year) (e.g. 36 (US average: 30 to 45 inches/year)), First-Flush Diverter & Filter Efficiency (%) (e.g. 90 (Accounts for first 1–2mm roof wash diversion)), Daily Non-Potable Water Demand (Gallons / Day) (e.g. 80 (Toilets, laundry, garden irrigation)), and Drought Dry-Period Storage Reserve (Days of Autonomy) (e.g. 21 (Typically 14 to 30 dry days)). Results update instantly as you type — adjust any value to compare scenarios.

The Formula

Hydrological Rainwater Harvesting \& Catchment equations:

Annual Harvestable Water (Gallons) = Roof Area (ft^2) × Rainfall (in) × C × η_filter × 0.623305

Runoff Coefficients (C): [Metal: 0.95, Clay Tile: 0.85, Asphalt Shingle: 0.80]

Recommended Cistern Tank Size (Gallons) = Daily Water Demand (Gal/day) × Drought Autonomy Days

Demand Coverage (\%) = (100\%, (Annual Harvestable Gallons) / (Daily Demand × 365) × 100\%)

Annual Utility Water Savings = (Harvested Gallons, Annual Demand) × Municipal Water Rate (\/Gal)

Worked Example

For example, harvesting rainfall from a 2,500 ft^2 metal roof) in a region with 36.0 inches of annual rainfall, 90.0\% filter efficiency, and 21 days of drought autonomy: The annual harvest yield is . The recommended cistern tank capacity is . This satisfies 100% of the 29,200 gal/year household demand, generating \262.80/year in utility savings.

Understanding Your Result

Date and time results depend on the reference timezone — all calculations here use the local system clock. For legal or contractual purposes, "days between" dates should be clarified as calendar days (counting the end date) vs. business days (excluding weekends and public holidays). Age calculations use full completed years — someone born on December 31 is the same age as someone born on January 1 of the same year for most legal purposes until their actual birthday.