What Is the Rainwater Harvesting Calculator?
This calculator estimates how much rainwater you can capture from a roof during a rainfall event or over a period. It uses three inputs — the horizontal roof catchment area, the depth of rainfall, and a runoff coefficient that accounts for losses from evaporation, splash, and the roof material — and returns the harvestable volume in US gallons, liters, and cubic feet.
How to Use It
Enter your roof's footprint (the flat, horizontal area it covers, in square feet), the rainfall depth in inches (use a single storm total or an annual average), and a runoff coefficient between 0 and 1. Metal and tile roofs typically have a coefficient of 0.85–0.95, while rough or porous surfaces may be lower (0.7–0.8). The calculator multiplies these together with the constant 0.623.
The Formula Explained
The constant 0.623 converts square-feet × inches into US gallons: one square foot of roof collecting one inch of rain holds 0.623 gallons of water. So the full equation is
$$\text{Gallons} = \text{Area} \times \text{Rainfall} \times \text{Runoff} \times 0.623$$The runoff coefficient reduces the theoretical maximum to a realistic, collectible amount.
Worked Example
A 1,000 sq ft roof receives 1 inch of rain with a runoff coefficient of 0.85:
$$1000 \times 1 \times 0.85 \times 0.623 = 529.55 \text{ gallons}$$which equals about 2,004 liters or 70.8 cubic feet.
FAQ
Should I use the roof's slope area or footprint? Use the horizontal footprint (the area the roof shadows on the ground). Rain falls vertically, so the slope does not increase the catchment.
What runoff coefficient should I pick? A common default is 0.85 for typical pitched roofs. Use higher (0.9–0.95) for smooth metal and lower (0.75) for textured or absorbent materials.
Can I estimate annual harvest? Yes — enter your area's annual rainfall total in inches to get a yearly estimate, then size your storage tank accordingly.
