Rainwater Collection
Use the Rainwater Collection for plumbing planning. Enter Roof area (ft²) and Rainfall (in) to get a clear result, formula explanation, and practical checks.
Rainwater Collection measurements
Enter your values, then calculate.
Result
How to calculate rainwater collection
This calculator estimates total rainwater collected from a roof catchment area, rainfall amount, and system collection efficiency.
How the calculation works
Collected water = Roof area × Rainfall × 0.623 × (Efficiency ÷ 100). The 0.623 constant converts square-foot-inches of rainfall to gallons.
Example
1,500 sq ft roof area, 1 inch of rain, 85% collection efficiency: 1,500×1×0.623×0.85 ≈ 794 gallons.
Frequently asked questions
How is Result calculated?
Result = [Roof area] × [Rainfall] × 0.623 × ([Collection efficiency] ÷ 100).
Is the Rainwater Collection free to use?
Yes — every calculator on Simple Calculator Tools is free, runs in your browser, and does not require an account.
Rainwater Collection
Collected water = Roof area × Rainfall × 0.623 × (Efficiency ÷ 100). The 0.623 constant converts square-foot-inches of rainfall to gallons.
Let's understand your rainwater collection result.
Calculate a result above and this guide will help you interpret it using this calculator's own formula and explanation.
Pro Tips for Rainwater Collection
- Collection efficiency accounts for losses from evaporation, first-flush diversion (discarding the initial dirty runoff), and system leaks — 80-90% is a commonly cited realistic range.
- Use only the roof area that actually drains to your collection system, not total roof area, if only part of the roof feeds your gutters and barrels.
Common Rainwater Collection Mistakes to Avoid
- Using total roof area when only a portion actually drains into the collection system, which would overstate real collected water.
When to Use This Calculator
This calculator estimates total rainwater collected from a roof catchment area, rainfall amount, and system collection efficiency.