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Rainfall Calculator
Convert rainfall in inches or millimeters to gallons, liters, or cubic feet for any surface area. Calculate how much rainwater your roof can collect per inch of rain — essential for rain barrel sizing, garden irrigation planning, and stormwater management.
Knowing how much rain falls on your property helps you plan rainwater harvesting systems, size rain barrels, calculate garden irrigation, and understand flood risk. Just 1 inch of rain on a 1,000 square foot roof produces 623 gallons of collectible rainwater.
Understanding rainfall volumes helps homeowners design rain harvesting systems, gardeners plan irrigation, engineers design stormwater systems, and farmers manage water resources. The conversion factor of 0.623 gallons per square foot per inch of rainfall comes from the density of water and unit conversions.
Rainfall Reference Table
Rainfall Type
Rate
Description
Trace
<0.01"
Barely measurable
Light rain
<0.1" / hr
Typical drizzle or light shower
Moderate rain
0.1-0.3" / hr
Steady rainfall
Heavy rain
0.3-2" / hr
Downpour
Extreme rain
>2" / hr
Flooding possible
1" of rain on 1,000 sq ft
623 gallons
~2.36 m³ water
Rain Barrel and Cistern Sizing
A standard rain barrel holds 50-75 gallons. For a 1,500 sq ft roof, 1 inch of rain produces 935 gallons — enough to fill 12-18 standard rain barrels. Cisterns for serious rainwater harvesting: 500-2,500 gallons for residential use. Calculate your average monthly rainfall × roof area to size your storage system appropriately for local precipitation patterns.
Collection Efficiency Factors
First-flush contamination (first rain carries rooftop pollutants) should be diverted — typically 0.01 inch of rain per 100 sq ft of roof. Collection efficiency: metal or tile roofs 90-95%, asphalt shingles 80-90%, gutters and downspouts add 5-10% loss. Evaporation from collection surface: negligible. Net collection efficiency: typically 75-85% of calculated theoretical volume.
Frequently Asked Questions
Gallons = Rainfall (inches) × Area (sq ft) × 0.6233. For 1 inch of rain on 1,000 sq ft: 1 × 1,000 × 0.6233 = 623 gallons. For metric: Liters = Rainfall (mm) × Area (m²). For 25mm of rain on 100 m²: 25 × 100 = 2,500 liters (2.5 cubic meters).
Collectible water = Roof area (sq ft) × Rainfall (inches) × 0.623 × Collection efficiency (0.80-0.90). For a 2,000 sq ft roof in a 30-inch annual rainfall area: 2,000 × 30 × 0.623 × 0.85 = 31,773 gallons per year — enough for significant landscape irrigation.
1 inch of rain falling on 1 acre (43,560 sq ft) equals 27,154 gallons or 102.8 cubic meters (102,800 liters) of water. On a typical suburban lot of 8,000 sq ft: 1 inch of rain = 4,986 gallons. On a football field (57,600 sq ft): 1 inch = 35,877 gallons.
A rain gauge is a calibrated cylindrical container that collects rainfall. Standard NWS rain gauges have a funnel directing water into a narrow inner cylinder — the funnel opening is 10× the cylinder area, so each 0.01 inch of rain in the funnel produces 0.1 inch in the measuring tube, improving accuracy for small amounts.
Rainwater collected from rooftops is generally not safe to drink without treatment. Contaminants include: bird and animal droppings, atmospheric pollutants, roofing materials (especially older asphalt shingles), and algae growth in storage. For potable use, rainwater needs filtration (sediment filter + activated carbon) and UV or chemical disinfection. It is safe for irrigation without treatment.
Flash flood risk typically begins when rainfall exceeds 1 inch per hour. Urban areas flood faster than rural areas because impervious surfaces (roads, parking lots, roofs) prevent absorption. A 1-inch/hour rainfall on 1 acre generates 27,154 gallons/hour flowing into storm drains. FEMA 100-year flood maps are based on 1% annual probability rainfall events.