Rain Barrel / Cistern Sizing Calculator

How this calculator works

One inch of rain on one square foot of roof yields 0.623 gallons — a fixed unit conversion (1 cubic foot = 7.48 gallons, divided by 12 for one inch of a 12-inch cubic foot). Multiplying that by your roof area and rainfall gives the raw yield; the efficiency percentage then accounts for real-world losses.

Where these numbers come from

• 0.623 gallons per sq ft per inch of rain: standard unit conversion, confirmed consistently across New Mexico Office of the State Engineer, University of Maryland Extension, and Washington State conservation district guidance
• Typical collection efficiency (80–90%): widely used planning assumption across the same sources

For sizing a permanent cistern rather than a barrel, plan around your area's typical rainfall pattern (frequency and gaps between rain events), not just single-storm totals.