The Homestead Calculator

Rainwater Collection Estimator

Calculate how many gallons you can collect from your roof, what tank size you need, and how far that water goes for your garden, animals, and household.

Roof Catchment Area (sq ft)

Annual Rainfall (inches)

Tip: US average is ~38". Look up your area at weather.gov.

Roof Material

Primary Use

Rainfall Pattern

Garden Size (sq ft)

Annual Collection

—

gallons per year

Per Month (avg)

—

gallons / month

Per Day (avg)

—

gallons

/ day

Garden Coverage

—

sq ft irrigated

Recommended Tank Sizing

Monthly Collection Estimate

Month	Rainfall (in)	Gallons Collected	Relative Amount

The Formula


      Gallons = Roof Area (sq ft)

      × Rainfall (in) × 0.623

      × Efficiency Factor

The 0.623 constant converts inches of rain on square feet into US gallons. A perfectly efficient 1,000 sq ft roof in 1 inch of rain would collect 623 gallons — but real roofs lose some to evaporation, splash, and first-flush diversion.

Legality Note Rainwater harvesting laws vary by state. Most states allow it freely; a few (historically Colorado) have had restrictions. Always check your local regulations before installing a large collection system.

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Frequently Asked Questions

How many gallons of rainwater can I collect from my roof?

The standard formula is: roof area (sq ft) × rainfall (inches) × 0.623 × efficiency factor. A 1,500 sq ft roof in an area with 40 inches of annual rainfall can collect roughly 33,000 gallons per year at 90% efficiency.

What size rain tank do I need?

Size your tank to hold 1–2 times your peak monthly collection. For most homesteads, a 1,500–5,000 gallon tank covers garden irrigation needs. If you want to bridge a dry season, size for your driest 2–3 months of usage.

Is rainwater collection legal?

Rainwater harvesting is legal in most US states. Most states allow it freely; a few have historically had restrictions (though Colorado has since relaxed its rules). Always check your local regulations before installing a large system.