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Rainwater Harvesting Tank Size Calculator
Optimal tank size and annual water savings from rainwater collection.
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How much water can you collect?
The formula: 0.623 gallons per square foot per inch of rain. A 1,200 sq ft roof with 40" annual rainfall = ~30,000 gallons potential (before efficiency). That's enough for significant irrigation or outdoor use.
Rainwater harvesting legal status
Legal in all US states as of 2026 (Texas, Colorado, and others previously restricted have since changed laws). Many states offer rebates of $100–1,000 for tank installation. Check your local utility for incentives.
⚡ CalcWolf Insight
A 1,000 square foot roof receiving 30 inches of annual rainfall can capture approximately 18,700 gallons of water per year.
Frequently asked questions
How big should my rainwater tank be?▾
Rule of thumb: size for 2–4 weeks of your typical use. For garden irrigation at 500 gal/month: 250–1,000 gallon tank. Larger tanks have diminishing returns as they'll be full most of the time.
Is it legal to collect rainwater?▾
In most US states, yes. Colorado, Utah, and a few others have restrictions on volume or require permits. Most states actively encourage rainwater harvesting with tax credits or rebates. Check your local and state regulations before installing a system. HOA rules may also apply.
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