Rainwater Harvesting Calculator
Estimate rainwater harvesting for your project with our free calculator. Get accurate material quantities, costs, and specifications.
Formula
Annual Harvest (gal) = Roof Area (sq ft) x Rainfall (in) x 0.623 x Runoff Coefficient
Multiply your roof catchment area by the annual rainfall in inches and the conversion factor of 0.623 gallons per square foot per inch of rain. Then apply the runoff coefficient for your roof type to account for losses from absorption, evaporation, and splash.
Worked Examples
Example 1: Suburban Home Garden Irrigation
Problem: A 1,500 sq ft roof area in a region with 40 inches of annual rainfall. The roof is asphalt shingle (coefficient 0.85). Daily garden usage is 50 gallons.
Solution: Annual harvest = 1,500 x 40 x 0.623 x 0.85 = 31,773 gallons\nMonthly average = 31,773 / 12 = 2,648 gallons\nDaily harvest = 31,773 / 365 = 87 gallons\nUsage coverage = (87 / 50) x 100 = 100%
Result: 31,773 gallons per year, fully covers 50 gal/day usage
Example 2: Metal Roof Workshop
Problem: A 800 sq ft metal roof shop in an area with 30 inches of annual rainfall. Coefficient 0.95, daily usage 20 gallons for cleaning.
Solution: Annual harvest = 800 x 30 x 0.623 x 0.95 = 14,205 gallons\nDaily harvest = 14,205 / 365 = 38.9 gallons\nUsage coverage = (38.9 / 20) x 100 = 100%
Result: 14,205 gallons annually, easily covers cleaning needs
Frequently Asked Questions
How much rainwater can I collect from my roof?
The collection potential depends on your roof area, local rainfall, and roof material. A 1,000 sq ft roof in an area receiving 40 inches of rain annually can harvest roughly 21,000 gallons per year with a standard asphalt shingle runoff coefficient of 0.85. Metal roofs have higher coefficients around 0.95, meaning they shed more water into your collection system.
What size rainwater tank do I need?
Tank sizing depends on your monthly rainfall pattern and intended usage. A good rule of thumb is to size your tank to hold at least one month of expected collection. For a household supplementing garden irrigation at 50 gallons per day, a 1,500 to 2,500 gallon tank provides reasonable buffer during dry spells. Consider your local dry season length when choosing capacity.
What is a runoff coefficient for rainwater harvesting?
The runoff coefficient represents the fraction of rainfall that actually reaches your collection tank. Metal roofs have coefficients of 0.90 to 0.95, asphalt shingles range from 0.80 to 0.85, and clay tiles are around 0.75 to 0.80. Losses come from absorption, evaporation, splash, and first-flush diverters that discard the initial dirty runoff from each storm event.
Is collected rainwater safe to drink?
Raw rainwater is generally not safe for drinking without treatment. It can contain bacteria, parasites, dust, and pollutants from roof surfaces. For potable use, you need a multi-stage filtration system including sediment filters, activated carbon, and UV sterilization or chlorination. Many homeowners use harvested rainwater for irrigation, toilet flushing, and laundry, which require minimal or no treatment.
Can I share or bookmark my calculation?
You can bookmark the calculator page in your browser. Many calculators also display a shareable result summary you can copy. The page URL stays the same so returning to it will bring you back to the same tool.
How accurate are the results from Rainwater Harvesting Calculator?
All calculations use established mathematical formulas and are performed with high-precision arithmetic. Results are accurate to the precision shown. For critical decisions in finance, medicine, or engineering, always verify results with a qualified professional.