Rainwater Harvesting Calculator
Created by: Ethan Brooks
Last updated:
Calculate how much rainwater you can collect from your roof. Enter roof dimensions, material type, and local rainfall to get annual collection estimates, tank sizing recommendations, and system planning guidance.
Rainwater Harvesting Calculator
HomesteadingCalculate collection potential and tank sizing for your roof.
Enter roof dimensions OR total square footage:
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What is a Homesteading Rainwater Harvesting Calculator?
A homesteading rainwater harvesting calculator helps you determine how much water you can collect from your roof and what size storage tank you need. Rainwater harvesting captures and stores precipitation for later use, reducing dependence on wells, municipal water, or trucked-in supplies. For off-grid homesteads, it can provide complete water independence.
The Rainwater Collection Formula
Every inch of rain on one square foot of roof yields 0.623 gallons. A modest 1,000 sq ft roof collecting 30 inches of rain annually produces about 18,700 gallons — enough to irrigate a large garden. The runoff coefficient adjusts for absorption: metal roofs capture 95%, while asphalt shingles capture about 85%.
Storage Tank Considerations
Tank sizing balances cost, space, and dry-season needs. Larger tanks cost more but provide security during droughts. Common options include food-grade IBC totes (275-330 gallons), polyethylene tanks (500-10,000+ gallons), concrete cisterns, and ferrocement tanks. Underground tanks save space but require pumping.
Water Quality and Treatment
For irrigation, minimal treatment is needed — just leaf screens and first-flush diverters. For household use, add sediment filtration (5 micron), carbon filtration (taste/odor), and UV or chlorine disinfection. Test water annually for bacteria. Covered, dark tanks prevent algae growth.
Frequently Asked Questions
How much rainwater can I collect from my roof?
Use the formula: Gallons = Roof Area (sq ft) × Rainfall (inches) × 0.623 × Runoff Coefficient. A 1,000 sq ft roof in an area with 40 inches annual rainfall can collect approximately 21,000 gallons per year with a metal roof (95% runoff coefficient).
What size storage tank do I need?
Tank size depends on your water usage and dry season length. For garden irrigation, plan for 1-2 months of storage. For whole-house use, calculate your daily usage × longest dry spell in days. A family of 4 using 50 gallons/day needs 1,500 gallons for a 30-day dry period.
Is rainwater safe for drinking?
Rainwater can be potable with proper treatment. First-flush diverters remove initial dirty water. Filtration (sediment, carbon) and UV or chlorine disinfection make it safe. Many off-grid homes use rainwater as their primary drinking source with appropriate treatment systems.
What is a first-flush diverter?
A first-flush diverter captures the first 10 gallons per 1,000 sq ft of roof, which contains the most contaminants (dust, bird droppings, pollen). After this dirty water is diverted, clean water flows to your tank. Essential for potable or garden use.
Do I need gutters on my entire roof?
You only need gutters on the sections draining to your collection system. Calculate each section's area separately. North-facing roofs stay cleaner (less sun = less algae). Consider overflow routing for excess water during heavy rains.
Sources and References
- Texas Water Development Board, "Texas Manual on Rainwater Harvesting"
- American Rainwater Catchment Systems Association (ARCSA)
- University of Arizona Cooperative Extension, "Harvesting Rainwater"