Rainwater Harvest Calculator

How to use this calculator

1. Enter roof catchment area in square feet.
2. Enter rainfall depth in inches for the storm/event.
3. Set collection efficiency to account for losses, then review gallons collected.

Inputs explained

Roof area

Horizontal projection of roof catchment in square feet.

Rainfall

Depth of rainfall in inches for the event/month you’re estimating.

Collection efficiency

Adjust for first flush, gutter losses, and absorption (e.g., 75–90%).

When to use it

• Sizing rain barrels or cisterns for irrigation.
• Estimating harvest from a given storm or monthly rainfall.
• Comparing collection potential across roof areas and efficiencies.

Tips & cautions

• Use average monthly rainfall to estimate typical yields; sum over months for annual potential.
• First-flush diverters and clean gutters can improve effective collection.
• Account for material runoff quality (roof material) if using water beyond irrigation.
• Does not include storage sizing or overflow management.
• Assumes uniform rainfall over the roof; wind effects not modeled.
• Water quality considerations are outside scope.

Worked examples

Deep dive

FAQs

What is 0.623?

It converts 1 square foot × 1 inch of rain to gallons (1 cu ft = 7.48 gal; 1 inch = 1/12 ft → 7.48/12 ≈ 0.623).

How do I size a tank?

Use the gallons output as demand input; choose a tank that balances typical rainfall and usage with overflow handling.

Does roof pitch matter?

Catchment uses the horizontal area; pitch doesn’t change the projected area materially for this estimate.

Can I use monthly rainfall?

Yes. Enter total inches for the month to estimate monthly collection.

What about water quality?

Not modeled. Roofing material and pre-filtration affect quality—follow local guidelines if using beyond irrigation.

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