Gutter Size Calculator

A five inch K-style gutter handling around 1,200 square feet is a genuinely useful figure, and it is what falls out of the code capacities at a five inch per hour design storm - the baseline for most of the continental United States. As a starting point it is fine.

The problem is that rainfall intensity is not a constant. The design rate on the Gulf Coast is seven inches an hour or more; in the Pacific Northwest, where it rains constantly but rarely hard, it is around four. That is a spread of nearly two to one, and the rule of thumb travels across the country unchanged while the number underneath it does not.

This matters because gutters are sized for the worst few minutes of the worst storm, not for the average. A Seattle gutter deals with far more total water a year than a Houston one and needs far less capacity to do it, because the Houston storm arrives all at once.

A gutter is a channel, and a channel only works if what is at the end of it can take the flow. The most common cause of a gutter overflowing is not that the gutter is too small; it is that there are not enough outlets, or that the ones there are too small.

The standard residential downspout is 2 by 3 inches, which is six square inches of cross-section and carries around 30 gallons a minute. A 3 by 4 is twelve square inches and carries three times as much - capacity goes with area, not with width, so a downspout half again as wide moves three times the water. It also blocks far less readily, which under trees is the more important property.

Placement matters as much as size. Water has to travel along the gutter to reach an outlet, and a gutter running 70 feet to a single downspout will be running deep at the far end long before it reaches its calculated capacity. Outlets roughly every 30 to 40 feet, with one near each end rather than both in the middle, is a better rule than any capacity figure.

A gutter can be correctly sized and still fail, and it is worth separating the modes. Overshoot happens on steep roofs, where water arrives with enough velocity to fly straight over the front lip - most visible at valleys, where two planes converge and concentrate the flow into a narrow band. The answer is a deeper profile, hanging the gutter slightly lower so the roof plane projects further over it, or a splash guard at the valley.

Ice is the winter version. A gutter full of ice has no capacity at all, and meltwater from the roof simply runs over it - which is why ice dams and gutter overflow appear together and why neither is really a gutter problem. Gutters do not cause ice dams and removing them does not prevent them.

And then there is fall. A gutter hung dead level holds standing water, silts up, and grows things. A quarter of an inch of fall over ten feet is enough, and it should run consistently towards the outlets rather than sagging between hangers. Most gutters that overflow in ordinary rain are either blocked or hung badly, and no amount of extra size fixes either.