Gutter Size Calculator: What Size Gutters and How Many Downspouts?
Enter a gutter run, the roof that drains to it and your state to see whether 5 or 6 inch gutters can keep up, and how many downspouts you need.
Gutters fail in two ways: the trough overflows because it can’t carry the water to an outlet fast enough, or the outlets can’t drain it as fast as it arrives. Either way, water ends up against the foundation. This calculator checks both for one gutter run, using rainfall intensity from NOAA for your state, and tells you how many downspouts you need for each gutter size.
How it works
Work one gutter run at a time: a straight stretch of gutter and the part of the roof that drains into it. On a simple gable, that’s one eave and the slope above it, from the gutter up to the ridge.
catch area = length × depth + ½ × length × rise (rise = depth × pitch ÷ 12)
flow (gpm) = catch area × rainfall (in/h) × 0.01039
gutter = width × √(g × yc³), yc = water depth ÷ √3
outlet = 0.6 × outlet area × √(2 × g × water depth)
downspouts = enough for the outlet and the gutter, and ≤ 40 ft apartRainfall intensity. We use the 5-minute intensity from NOAA Atlas 14, the federal standard for rainfall frequency, for a major city in each state. Pick the 10-year storm for normal homes or the 100-year storm where overflow would cause damage. Atlas 14 doesn’t cover Oregon and Washington, so there you enter your own value. Intensity also changes within a state, so if you’re far from the reference city, look up your address on NOAA’s data server and enter that number.
Gutter capacity. We treat each profile as a level channel and use the classic result from the National Bureau of Standards’ 1934 gutter study. Water flows to a free outlet, so the depth at the upstream end is about √3 times the critical depth at the outlet. A 5-inch K-style gutter, modeled as a 4 × 3 inch channel, carries about 46 gpm to an outlet from one side.
Downspout capacity. The outlet acts like an orifice with the gutter’s water depth pushing on it. A 2 × 3 inch outlet in a 5-inch gutter passes about 45 gpm, and a 3 × 4 inch outlet about 90 gpm.
Conservative choices. The gutter is level, and each outlet takes all its water from one side, as at a corner downspout. With an outlet in the middle of a run, the gutter only carries half the flow each way. That gives you some extra capacity, but leaves are the bigger risk in practice.
Worked example
Take a gable version of our ranch in Houston, where the NOAA 10-year, 5-minute intensity is 10.1 in/h. The back eave has a 55 ft gutter run, the roof above it is 16.5 ft deep to the ridge (including the 18-inch overhang), and the pitch is 5/12.
- Plan area: 55 × 16.5 = 907.5 sq ft
- Rise: 16.5 × 5 ÷ 12 = 6.9 ft. Half the vertical projection, 0.5 × 55 × 6.9, adds 189 sq ft, for a catch area of 1,097 sq ft
- Flow: 1,097 × 10.1 × 0.01039 = 115 gpm
- 5-inch K-style with 2 × 3 downspouts: each outlet takes 45 gpm, so you need 3 downspouts, about 18 ft apart
- 6-inch K-style with 3 × 4 downspouts: each outlet takes about 99 gpm and the gutter about 76 gpm, so 2 downspouts at the ends do it
The same run in Billings, Montana, at 6.12 in/h, needs 70 gpm, so a 5-inch gutter with two 2 × 3 downspouts is enough.
Gutter capacity reference table
Roof catch area one downspout can serve, including the wind allowance for sloped roofs. Divide your run’s catch area by the number in your column and round up.
| Gutter and downspout | 4 in/h | 6 in/h | 8 in/h | 10 in/h |
|---|---|---|---|---|
| 5 in K-style, 2 × 3 in rectangular | 1,080 | 720 | 540 | 430 |
| 5 in K-style, 3 × 4 in rectangular | 1,120 | 750 | 560 | 450 |
| 6 in K-style, 3 × 4 in rectangular | 1,840 | 1,230 | 920 | 740 |
| 5 in half-round, 3 in round | 710 | 470 | 350 | 280 |
| 6 in half-round, 4 in round | 1,150 | 770 | 580 | 460 |
Tips and common mistakes
Count the downspouts, not just the gutter size. Most overflowing gutters on houses have too few outlets, not too small a trough. Adding a downspout is usually cheaper than upsizing a whole run.
Watch inside corners and valleys. Water from two roof planes meets at a valley and hits the gutter in a concentrated stream that can shoot over the front edge. A valley splash guard, or an outlet near the corner, helps.
Keep them clean. A gutter half full of leaves has half the capacity, and a clogged outlet has none. Gutter guards help under trees, but even guarded gutters need checking.
Move the water away from the house. Discharge downspouts onto splash blocks or into extensions that carry water several feet from the foundation, or into a drain system where local rules allow. Check whether your city restricts tying downspouts into sewers.
Check upper roofs that drain onto lower ones. A second-story downspout emptying onto a first-floor roof adds all of its flow to the lower gutter. Include that upper roof area in the lower run’s numbers.
Work from a stable ladder. Gutter work is ladder work. Set the ladder on firm, level ground, keep three points of contact, and don’t lean out past the rails.
Frequently asked questions
Should I get 5 inch or 6 inch gutters?
Five-inch K-style gutters handle most single-story roofs with typical rainfall, as long as you have enough downspouts. Six-inch gutters carry about 65% more water, so they suit long or deep roof sections, steep roofs and places with heavy downpours, like the Gulf Coast and the Southeast. They also clog less easily.
How many downspouts do I need?
Enough that no outlet takes more water than it can pass. A 2 × 3 inch downspout on a 5-inch gutter serves about 720 sq ft of roof at 6 in/h of rain, and a 3 × 4 inch outlet roughly doubles that until the gutter itself becomes the limit. We also keep downspouts no more than about 40 feet apart.
What rainfall intensity should I use for gutters?
Residential gutters are usually sized for the most intense 5 minutes of a 10-year storm. That intensity comes from NOAA Atlas 14, and our state values come from its data server for a major city in each state. Use the 100-year value where an overflow would soak a doorway, a basement window or a walkway.
Why does roof pitch change gutter size?
Rain rarely falls straight down in a heavy storm. Wind pushes it against the sloped face of a steep roof, so the roof catches more than its footprint. The calculator adds half of the roof's vertical projection to the plan area, the same allowance the plumbing code uses for walls that drain onto a roof.
Do gutters need to slope?
A slight slope toward the outlets, often around 1/16 inch per foot, helps the gutter drain completely and avoids standing water. Our capacity figures assume a level gutter, which is the conservative case, so a sloped gutter has a little extra margin.