Gutter capacity depends on how hard it rains where you are. The same 5-inch K-style that copes in a drizzle climate overflows every summer on the Gulf Coast.
Enter the plan area the gutter drains. Pitch is applied as a multiplier because a steep roof delivers water faster.
This is the input everyone leaves out. Design rainfall is a 5-minute peak rate, not an annual average — gutters fail in the downpour, not the drizzle.
Almost every gutter calculator asks for roof area and stops. Roof area tells you how much water arrives; it says nothing about how fast.
Design rainfall intensity is a short-duration peak rate — the five-minute burst in a heavy storm, not the annual average. A gutter that never overflows in Portland can overflow several times a summer in Houston on an identical roof, because the storms deliver several times the rate.
The same 5″ K-style gutter handles around 5,500 sq ft at 2 in/hr and under 2,000 at 7. That is the whole reason this page exists.
A steep roof delivers its water to the eave faster than a shallow one, and it catches more wind-driven rain. Standard practice applies a multiplier — modest at 4/12, more significant at 12/12.
A gutter is a channel; the downspout is the drain. Undersize the outlets and the gutter backs up and overflows regardless of its own capacity.
| Downspout | Area | Drains about |
|---|---|---|
| 2×3 | 6 sq in | 600 sq ft at 4 in/hr |
| 3×4 | 12 sq in | 1,200 sq ft at 4 in/hr |
| 4″ round | 12.6 sq in | 1,250 sq ft at 4 in/hr |
Doubling the nominal size roughly doubles the capacity, and the larger section is far less prone to blocking on a single wad of leaves — which is the failure mode that matters in autumn.
About 1/16″ per foot toward each outlet — half an inch over eight feet. Flat gutters hold standing water, which is where debris settles and mosquitoes breed. Steeper looks visibly wrong against a level fascia, and people notice.
On long runs, slope from the centre to outlets at both ends rather than running the whole length one way.
NOAA publishes rainfall frequency data by location, and local codes usually specify a design figure. The dropdown here is a regional approximation for comparison.
Most do, some substantially, because water has to get through the guard before it gets into the gutter. In heavy rain a fine mesh can shed water straight over the front. Worth sizing up if you are fitting them.
Away from the building. Discharging at the foundation causes far more expensive problems than an occasional overflow, and extensions or buried drainage cost very little by comparison.