Water Resources Tool

Rainwater Harvesting & Cistern Sizing Calculator

Calculate your roof catchment water yield in gallons, estimate first-flush diversion deductions, and determine the optimal storage cistern capacity needed to survive extended dry spells.

1. Catchment & Weather Data

Use building footprint area (length × width of building including eaves), not sloped roof area.
US average: ~30 inches/year. Pacific NW / Southeast: 45–60 in. Southwest: 8–18 in.
Conservation-minded off-grid homes typically use 40–80 gallons/day for a 2-person household.

2. Harvesting Potential & Tank Sizing

Net Annual Harvesting Potential
39,141 gal/yr
Calculated using the standard hydrology formula (0.6233 gal/sq ft/inch).
Monthly Average Yield
3,262 gal/mo
Actual monthly collection fluctuates with local seasonal rainfall patterns.
Annual First-Flush Diversion Deducted
750 gal
Diverts roof pollen, dust, and bird contaminants to keep cistern clean.
Recommended Minimum Cistern Capacity
3,375 gal
Includes a 25% safety reserve buffer for dry spell demand.
Estimated Storage Tank & Pre-Filtration Cost
$3,518 - $5,756
Polyethylene food-grade tank, leaf screen, first-flush diverter, and sediment pre-filter.

Hydrology Principles & Sizing Equations

Rainwater harvesting calculation relies on physical water density and volumetric geometry. To convert inches of rainfall over a square-foot area into gallons, engineers apply the constant conversion factor 0.6233:

Catchment Yield (Gallons) = Roof Area (sq ft) × Rainfall (inches) × 0.6233 × Runoff Coefficient (C)

Runoff Coefficients by Roofing Material

Not all water hitting a roof reaches the storage tank. Water is lost to absorption, evaporation, wind drift, and surface roughness:

Roof Material Runoff Coefficient (C) Suitability for Potable Domestic Use
Standing Seam Metal (Steel / Aluminum) 0.95 (95%) Excellent. Non-porous, smooth, does not leach toxic chemicals.
Glazed Ceramic Tile / Slate 0.90 (90%) Very Good. High durability, minimal particulate shed.
Asphalt Fiberglass Shingle 0.75 – 0.80 (80%) Non-Potable Only. Leaches petro-hydrocarbons and sheds mineral grit.
Flat Gravel / Built-Up Bitumen 0.65 – 0.70 (70%) Poor. High water retention, ponding, high organic contamination.

First-Flush Diversion Standards

According to the American Rainwater Catchment Systems Association (ARCSA) guidelines, the initial runoff from a rain event carries between 85% and 95% of airborne particulate matter, pollen, bird droppings, and roof grit. Sizing a first-flush diverter to discard the first 1 to 2 gallons per 100 square feet of catchment area prevents sediment buildup and biological fouling inside the primary cistern.