What it calculates
Estimate gallons captured from roof area and rainfall.
Rainwater Collection Calculator
Estimate rainwater captured from roof area and rainfall.
Planning and estimating aid only. Verify project requirements, manufacturer instructions, adopted codes, permits, and field conditions before performing regulated work.
Plumbing planning
How to Use This Calculator
Use the field units shown below, then compare your result with the worked example. Use the relevant catchment area and the rainfall depth for the event being considered.
How the calculation works
Approximate gallons = roof area × rainfall in inches × 0.623 × collection efficiency.
What the result means
The efficiency factor accounts for losses such as first flush, splash and incomplete collection.
Estimate one rainfall event, not annual supply
For 1,000 square feet of catchment, 1 inch of rain and 80% collection efficiency, this model gives 1,000 × 1 × 0.623 × 0.80 = 498.4 U.S. gallons, displayed as 498 gallons.
Contractor tip: Use the relevant catchment area and the rainfall depth for the event being considered. A result for a single inch of rain is not a promise of a dependable daily water supply.
Verify the final quantity
The efficiency input is an assumption covering collection losses. The tool does not size storage, model overflow, forecast rainfall or evaluate water quality, treatment, roof suitability or permitted uses.
Good practice: Compare the calculated quantity with plans, specifications, product packaging and current field conditions before purchasing material.