Three numbers decide the size
Gutter sizing is an area problem with a rate attached. You need the plan area of roof draining into the run, a factor for how steep that roof is, and the rainfall intensity the system is meant to survive. Multiply the first two to get an adjusted drainage area, then compare it against what the gutter can carry at your intensity.
The plan area is the horizontal projection, not the sloped surface. This trips people who have just done a shingle calculation, because there the sloped area is the one that matters. Here it does not: a square foot of rain falls on a square foot of ground regardless of the angle of the roof it lands on. The slope multiplier belongs to the roofing calculation and has no business in this one.
What the pitch does affect is throw. A steep plane launches water off the edge harder, and it presents more face to wind-driven rain, so a factor above 1 is applied. The common set of factors used in the trade steps up gently: 1.00 up to 3/12, 1.05 to 5/12, 1.10 to 8/12, 1.20 to 11/12 and 1.30 above that. Those are a long way short of the slope multiplier, which at 12/12 would be 1.41, and they are doing a completely different job.
Rainfall intensity is the number nobody looks up
Everything on this page scales linearly with intensity. Double the design rainfall and every capacity figure halves. A 5 inch K-style gutter that comfortably drains a small house in a mild maritime climate is undersized on the same house in a place that sees short, violent summer storms.
The design figure comes from local rainfall records, usually as a short-duration intensity for a given return period, and it is published by the meteorological service and reproduced in the drainage provisions of local codes. It is not a national number and there is no sensible default. The 4 inches per hour in the field above is a placeholder to make the page do something on first load, and it should be the first thing you change. Ask the building department what intensity they design to, or find the local rainfall data. If you cannot get either, ask a gutter installer who works in your town what they size on, because they will know.
Downspouts, and why they are the real constraint
A useful way to think about leader capacity is one square inch of downspout section per 100 square feet of adjusted drainage area, per inch per hour of rainfall. A 2 by 3 rectangular leader is 6 square inches, so it handles 600 square feet at 1 inch per hour, or 150 square feet at 4. A 3 by 4 is 12 square inches and does twice that. Going from 2 by 3 to 3 by 4 is the single cheapest capacity upgrade available on a residential system, and it also stops the leaf that would have jammed the small one.
| Leader | Section | Area at 1 in/hr | At 4 in/hr | At 8 in/hr |
|---|---|---|---|---|
| 2 x 3 rectangular | 6.0 sq in | 600 sq ft | 150 sq ft | 75 sq ft |
| 3 in round | 7.1 sq in | 707 sq ft | 177 sq ft | 88 sq ft |
| 3 x 4 rectangular | 12.0 sq in | 1,200 sq ft | 300 sq ft | 150 sq ft |
| 4 in round | 12.6 sq in | 1,257 sq ft | 314 sq ft | 157 sq ft |
Then there is spacing, which usually decides the count before flow does. A gutter is hung with a small fall toward the outlet, and over a long run that fall either runs out of fascia or gets lost to a sagging hanger. Somewhere around 30 to 40 feet per downspout is where most installers stop, and putting outlets at the ends of a run rather than in the middle of it means the water never has far to travel. If the count from spacing is higher than the count from flow, use the spacing count.
Where the water goes after the leader
Sizing the gutter is the part with arithmetic in it, and it is not the part that damages houses. What does the damage is a correctly sized system discharging four hundred gallons at the base of a foundation wall over the course of a storm. A downspout that ends in a splash block eighteen inches from the wall is putting the water back into the soil you were trying to keep dry. Extensions, buried drain lines to daylight, and dry wells all cost more than the gutter and matter more than whether you went 5 inch or 6 inch. If you are sizing gutters because the basement is damp, start at the discharge end and work backwards.
Questions people ask
Do I need 5 inch or 6 inch gutters?
Five inch K-style handles most single-family houses in most climates, and six inch is for large roof areas, steep pitches, high rainfall intensity, or long runs with few outlets. The honest answer is that it depends on all three inputs at once: a 900 square foot roof plane at 6/12 in a place designing to 3 inches per hour is comfortably inside a 5 inch gutter, and the same roof where the design figure is 8 is not. Run your own numbers rather than picking by house size, and remember that going up a downspout size is often the better spend.
How many downspouts do I need?
Take the higher of two counts. The flow count is the adjusted drainage area divided by what one leader carries at your rainfall intensity, using roughly one square inch of leader per 100 square feet per inch per hour. The spacing count is the gutter run divided by a working maximum of about 30 to 40 feet. On most houses the spacing count is the larger of the two, which is why a 40 foot eave usually gets two downspouts even when one would technically pass the flow check.
Does roof pitch change the gutter size?
A little, and less than people expect. A steeper roof throws water further off the edge and intercepts more wind-driven rain, so a factor between 1.00 and 1.30 is applied to the plan area depending on pitch. What it does not do is increase the amount of rain landing on the building, which is set by the plan area alone. The much bigger effect of a steep roof is on gutter position and the size of the splash: on a 12/12 roof in a hard rain, a gutter hung by the book can still be overshot at the valleys.
What rainfall intensity should I use?
The design figure for your specific location, taken from local rainfall data or from the drainage provisions your building department works to. It commonly ranges from around 2 inches per hour in mild maritime climates to 9 or more on the US Gulf Coast, and it can differ between towns in the same state. There is no national number worth using, and this calculator ships with a placeholder specifically so you replace it. If you are not sure, call the building department and ask what they size roof drainage on.
Will gutter guards change the sizing?
Yes, downward, and by an amount nobody publishes honestly. Every screen, guard and reverse-curve product reduces the rate at which water can enter the gutter, and most of them do it more in a heavy downpour than in a light one, which is exactly backwards from what you want. If you are fitting guards, treat the calculated size as a minimum rather than an answer, and expect to size up or add an outlet. The same applies to a gutter that will collect pine needles or maple keys, guard or not.