How much rain can you catch off a 2,000 sq ft roof?

Civil & DrainageWater Harvesting Sizing

Rainwater Harvesting & Cistern Sizing Calculator

Calculate total harvestable rainwater gallons based on roof footprint area, average precipitation, and catchment runoff efficiency.

Civil & Drainage Field StandardsWater Harvesting Sizing

Engineering formulations calibrated to IPC, UPC & NEC specificationsSub-Zero Cold Climate Tolerances

Quick Summary & Key Answer

To calculate rainwater harvesting yield: multiply roof catchment square feet by rainfall in inches, multiply by 0.623, and multiply by the runoff efficiency factor (0.80-0.95). A 2,000 sq ft roof receiving 32 inches of annual rain yields roughly 38,000 gallons of harvestable water per year.

1 inch of rain on a 1,000 sq ft roof yields approximately 623 gallons of water.
Smooth standing seam metal roofs provide the highest water purity and 95% runoff efficiency.
A first-flush diverter removes initial dust and bird droppings before tank entry.

Interactive Calculator

Adjust inputs for real-time results

Catchment Roof Footprint Areasq ft

sq ft

Average Annual Rainfallinches/year

inches/year

Roof Catchment Material

Calculated OutputWater Catchment Calculated

Total Annual Harvestable Water

37,878 Galgallons/year

Average Monthly Yield: 3,157 gal/mo
Recommended Cistern Tank Size: 2,367 Gallons
Water Yield per 1 Inch of Rain: 1,184 Gallons

Note: Formula: Gallons = Catchment Area (sq ft) × Rainfall (in) × 0.623 × Runoff Coefficient. Install first-flush diverters to discard roof debris.

First-Flush Filtration Importance

First-flush diverters discard the first 1 to 2 gallons per 100 sq ft of roof area to prevent organic sediment contamination.

Frequently Asked Questions

Common questions regarding this calculation and code compliance

Every 1 inch of rain yields roughly 1,180 to 1,200 gallons of collectable water from a 2,000 sq ft roof.