Gutter sizing by rainfall

Whichever size carries the roof draining into it at your rainfall intensity. Sorted heaviest first, so the states where the national five-inch default stops working are at the top.

Why intensity and not annual rainfall

Seattle receives far more rain in a year than Houston and needs much smaller gutters, because the two places get it differently. Pacific north-west rain arrives gently over days; Gulf coast rain arrives in a quarter of an hour.

A gutter has no memory. It does not care how much fell last month — only how much is arriving right now, in the worst ten minutes of the year. That is what design intensity measures, and it is the only rainfall figure that belongs in this calculation.

Reading the spread

The ratio between the top and bottom of this table is more than three to one. A 5 inch gutter that comfortably drains a 2,200 square foot roof in Oregon manages barely 600 on the Gulf coast — and the same five-inch default gets fitted in both places because it is what the truck carries.

This is the single most useful thing on the page: if your state is in the top third, the standard gutter on a standard house is probably undersized, and stepping up one size costs about fifteen per cent more for forty per cent more capacity.

Downspouts are usually the constraint

The last two columns are the ones that bite. A 2 × 3 downspout has a cross-section smaller than a fist and is asked to drain half a house; in heavy rainfall it runs out of capacity long before the trough does. A 3 × 4 carries roughly twice as much and blocks far less readily, for a few dollars more per foot.

Whatever the flow arithmetic says, put one every 35 feet of run regardless — water has to travel along the trough to reach an outlet, and a long run with one downspout overflows at the far end.

Using it

The gutter calculator applies these figures from your state, adds a pitch factor for wind-driven rain, counts the downspouts by both flow and spacing, and works out the 0.25 in per 10 ft of fall each run needs.