Downspout Sizing Calculator

How It Works

Downspouts are the vertical pipes that carry water from gutters to the ground. A standard 2x3-inch downspout handles about 600 sq ft of roof area at 1 inch/hour rainfall. In the Southeast US where 4-6 in/hr is common, you need more or larger downspouts. The rule of thumb: one 2x3 downspout per 600 sq ft, or one 3x4 per 1,200 sq ft. Always direct downspout water at least 4-6 feet away from the foundation via extensions or splash blocks. Underground drain pipes connecting to a dry well or storm drain is the ideal setup for heavy-rain areas.