How Many Downspouts You Need, Plus Drop Length

A downspout can only carry the runoff from a limited patch of roof before it overwhelms in a downpour, so the number you need scales with drainage area. Divide the roof area by the area one downspout handles and round up.

How much one downspout handles depends on its outlet size and your local rainfall intensity. As a planning rule, a 2 by 3 inch outlet drains about 600 square feet, a 3 by 4 inch outlet about 1,200, and a round 3-inch pipe falls in between. This calculator uses a midpoint of about 1,000 square feet per downspout by default and lets you switch to the smaller or larger allowance to match your outlets.

The tool also estimates the pipe you will run down the wall. Each downspout drops from the gutter to grade, so the total downspout length is the number of downspouts times the wall height, plus a little for the elbows and the kick-out at the bottom. That gives you a length to price alongside the count.

This is a rule-of-thumb estimate for planning. In a region with intense storms, or on a roof with valleys that concentrate flow, err toward more and larger downspouts, and consider a proper sizing that accounts for your rainfall rate.