The four questions behind rainwater searches
How much can I collect?
Roof size and rainfall create the gross opportunity. Collection efficiency and first-flush diversion reduce what reaches storage.
Will my barrel overflow?
The answer depends on water already in the tank. A small barrel can fill during a fraction of a storm, so overflow routing is part of system sizing.
How much storage do I need?
Storage should be linked to actual non-potable demand and the dry period you want to cover, not only to annual rainfall.
Will it save money?
Savings depend on how much captured water is actually used, the local combined rate, maintenance and system cost.
Formula and assumptions
gallons = roof area (sq ft) × rainfall (in) × 0.623
The 0.623 factor converts one inch of water over one square foot into US gallons. The planner then applies your efficiency and first-flush assumptions.
A 1,000-square-foot roof receiving one inch of rain produces 623 gross gallons before losses. A 55-gallon barrel therefore captures only a small share unless the water is used between storms or the system has multiple tanks.
Water quality and local rules
This tool defaults to non-potable outdoor reuse. EPA notes that roof runoff can pick up bacteria from animals and chemicals from roofing materials. Drinking-water use, edible-crop use, indoor plumbing, backflow protection, treatment and permits require location-specific guidance.
- Check state, provincial and local collection rules.
- Check roof material and previous container contents.
- Use a screened inlet, secure cover and controlled overflow.
- Route overflow away from structures and unstable ground.
- Follow local health guidance before potable or edible-crop use.
What the calculator does not certify
The full-tank load is a planning number, not a structural approval. Soil, blocks, platforms, decks, racks, trailers and elevated stands require their own rated design. Water weighs about 8.34 lb per US gallon before the tank, pump and fittings are added.
The payback result is also a simple estimate. It does not model rainfall timing, evaporation, leakage, drought restrictions, rebates, financing or future utility-rate changes.
Sources and further checks
- U.S. EPA: Soak Up the Rain — Rain Barrels explains roof-runoff collection, landscape reuse, cisterns and roof-pollutant concerns.
- Better Homes & Gardens: How to Harvest Rainwater at Home presents the 0.623 gallon per square-foot-inch calculation and common 55-gallon/IBC choices.
- Use local rainfall records, water rates, rebates and legal guidance rather than national averages.