Rain Barrel Collection Calculator — Rainwater Captured from a Roof
Estimate how much rainwater your roof or catchment area captures from a rainfall event. Enter the catchment area, the rainfall amount, and a collection efficiency (the fraction that actually reaches the barrel). Efficiency is user-entered — a typical assumption is 0.8–0.9.
What this calculator determines
It estimates how many gallons of rainwater your catchment area captures from a rainfall event, after applying a collection efficiency you enter.
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Examples
1000 sq ft roof, 1 in rain, 0.85 efficiency
1000 sq ft, 1 in, 85% efficient
Calculation
- Gross: 1000 × 1 × 0.623 = 623 gal
- Captured: 623 × 0.85 = 530 gal
~530 gallons captured
Assumptions & Conventions
- Collection efficiency
- The default 85% efficiency allows for collection losses. Actual efficiency varies with roof material, gutters, downspouts, first-flush diversion, overflow, and other system conditions.
- Catchment area
- Use the horizontal footprint area, not the sloped roof surface — rain falls vertically.
- Conversion factor
- Gross gallons = rainfall × area × 0.623; captured = gross × efficiency.