Rainfall Calculator

Rainfall depth measures the height of water that would accumulate on a flat, impermeable surface. One inch of rain over one square foot equals 1/12 of a cubic foot of water, or about 0.623 gallons. Scaling this to larger areas reveals surprising volumes. One inch of rain falling on a 1,000 square foot roof produces approximately 623 gallons of water. This is enough to fill about 12 standard bathtubs. Over an acre (43,560 sq ft), one inch of rain produces about 27,154 gallons, or roughly 102.8 cubic meters. Rainwater harvesting captures this water for later use. The collectible volume depends on the collection efficiency, which accounts for losses from evaporation, splash, and gutter overflow. A typical residential system captures 75-90% of the theoretical volume. For stormwater management, engineers calculate the runoff volume to size drainage systems, retention ponds, and storm sewers. The runoff coefficient varies by surface type: roofs and pavement have high runoff (0.85-0.95), lawns have moderate runoff (0.10-0.35), and forests have low runoff (0.05-0.25). Rainfall intensity (inches per hour) is as important as total depth for flood risk assessment. A slow, steady 2-inch rainfall over 24 hours is easily absorbed, while 2 inches in 30 minutes can cause flash flooding because the ground cannot absorb water fast enough.