Gutter Length, Downspouts & Hangers for Your Roof

A gutter run is easy: it equals the length of the eaves it follows. The harder question is how many downspouts drain it, and that depends on two limits working together.

The first limit is drainage area. Each downspout can carry the runoff from only so much roof before it backs up in a storm. A small 2 by 3 inch outlet handles roughly 600 square feet; a larger 3 by 4 inch outlet about 1,200. A common planning figure is one downspout per 1,000 square feet of roof, and the calculator lets you pick the outlet size to tighten that up.

The second limit is run length. Even if the area is small, a gutter should not run more than about 35 to 40 feet to a single outlet, or water pools at the far end and overflows on the low side. So the calculator computes downspouts two ways — by roof area and by gutter length — and takes whichever is greater. A short gutter over a big roof is area-limited; a long gutter over a small roof is length-limited.

Hangers hold the gutter to the fascia. Spaced about every 24 inches, the count is the gutter length divided by two feet. Closer spacing — 18 inches — is wise in snow country where ice loads the gutter. End caps, outlets, and elbows are separate small parts; this tool sizes the gutter, the downspouts, and the hangers.