Rainwater Harvesting Calculator

A Roof Is a Collection Surface, Whether You Use It or Not

Every rainfall event that lands on a roof either runs off unused or can be captured for irrigation, washing, or non-potable household use — the only question is how much. That volume scales directly with roof area and rainfall depth, adjusted downward slightly for the water lost to evaporation, first-flush diversion, and gutter overflow before it reaches a storage tank.

The Formula

The 0.623 constant is a standard conversion factor — it's the number of gallons collected per square foot of roof area per inch of rainfall, assuming 100% capture. The runoff coefficient (default 0.85) accounts for real-world collection losses; a coefficient of 1.0 would represent perfect capture, which no real system achieves.

Where This Calculation Applies

• Sizing a storage tank — knowing expected volume from a typical rainfall event is the starting point for choosing a cistern or rain barrel large enough to capture it without overflow.
• Irrigation planning — gardeners and small farms use harvested volume estimates to judge how much of their irrigation needs rainwater could realistically offset.
• Stormwater management — some municipalities offer incentives for rainwater harvesting specifically because it reduces runoff volume entering storm drains during heavy rain events.

Worked Examples

Figures computed directly from the calculator's own formula and the standard 0.623 gallons/sq ft/inch conversion constant.

How to Use This Calculator

1. Enter your Roof Area in square feet.
2. Enter the Rainfall amount in inches for the event or period you're estimating.
3. Optionally adjust the Runoff Coefficient (defaults to 0.85, a typical value for a standard roof).
4. Select Calculate to get the total gallons of water that could be collected.

Related Calculations

Compare harvested volume against what your household actually uses with the Water Usage Calculator.