Gutter Size Calculator

Background.

One inch of rain on 1,000 square feet of roof is 623 gallons of water, and in a heavy storm a gutter has to move it in minutes. Gutters rarely fail on an average day; they overflow in the short, intense bursts of a summer thunderstorm, when the rate of rainfall, not the annual total, decides whether water stays in the channel or pours over the edge onto the foundation, the siding and the flower beds.

Sizing a gutter system therefore starts with a flow rate. Take the roof area that drains to one gutter, multiply by a design rainfall intensity in inches per hour, convert to gallons per minute, and compare that flow with what each downspout can carry. For a 44-by-28-foot roof section at a 6:12 pitch in a 5.5-inch-per-hour storm, the drainage area works out to 1,377 square feet and the runoff to about 78.7 gallons per minute. With downspouts rated at 40 gallons per minute, one is not enough; two are.

The two numbers you supply carry most of the uncertainty. Design rainfall intensity varies enormously by region and by how rare a storm you design for, so it is an input rather than a built-in figure; NOAA's Precipitation Frequency Data Server gives intensities for any US location, and local codes or design guides say which storm duration and return period to use. Downspout capacity depends on the downspout's size and shape and on the outlet feeding it, so it comes from a manufacturer's table or a sizing method such as SMACNA's.

What comes out is the flow your gutters and downspouts must handle, the number of downspouts and the length of gutter each one serves. Choosing the gutter profile itself, a 5-inch or 6-inch K-style for instance, means checking that flow against the manufacturer's capacity for that profile.

What is gutter size calculator?

Gutter sizing is the job of matching a roof's peak runoff to the capacity of the gutters and downspouts that carry it away. The runoff is a flow: roof drainage area times rainfall intensity, expressed here in US gallons per minute. The drainage area is the part of the roof, and any wall above it, that sheds water into one gutter run. The rainfall intensity is the rate of a design storm, in inches per hour, chosen for the location. Downspout capacity is the flow one downspout and its outlet can take, and the number of downspouts is the runoff divided by that capacity, rounded up to whole downspouts. The gutter itself also has to carry water along its length to each outlet, which is why gutter capacity tables depend on the distance to the outlet as well as the profile: more downspouts mean shorter runs and less water in any one stretch.

How to use this calculator.

1. 01Enter the length and width, in plan, of the roof section that drains into this gutter run. On a gable roof each eave gutter takes only one slope, so the width is the horizontal distance from eave to ridge, not the full depth of the building.
2. 02Enter the pitch as rise over run, for example 6 and 12 for a 6:12 roof. A flat roof is a rise of 0.
3. 03Add vertical wall area only where a wall above this roof sheds water onto it, and enter the share your code or design method counts.
4. 04Enter the design rainfall intensity in inches per hour, from NOAA's precipitation-frequency data for your location or from your local building department, for the storm duration and return period your code or design method specifies.
5. 05Enter the capacity of one downspout in gallons per minute from the manufacturer or a SMACNA table, along with the gutter run length, and keep the runoff coefficient at 1 for ordinary hard roofing.
6. 06Read the runoff flow and downspout count, then check the flow against the capacity of the gutter profile you plan to buy.

The formula.

The plan area is A = length × width, in square feet. The pitch ratio is p = rise ÷ run, and the pitch factor √(1 + p²) is the length of a roof slope per foot of horizontal run, so A × √(1 + p²) is the roof's true sloped surface area. Any vertical wall area entered is added in full to give the design area.

The runoff is design area × rainfall intensity × 0.6234 × runoff coefficient, giving gallons per hour, and dividing by 60 gives gallons per minute. The 0.6234 is exact: an inch of rain on a square foot is 144 cubic inches, a US gallon is 231 cubic inches, and 144 ÷ 231 = 0.623377 gallons. The downspout count is the runoff divided by one downspout's capacity, rounded up. Flow per downspout and gutter length per downspout simply divide the runoff and the run length by that count, assuming the flow splits evenly between downspouts.

The pitch factor needs a word of caution. Rain falling straight down lands on a roof's footprint whatever its slope, and plumbing codes size roof drainage on the horizontally projected area. Some gutter-sizing methods enlarge the area of pitched roofs to allow for wind driving rain against the slope. Using the full sloped area is the generous version of that allowance: it adds 5.4% at 4:12, 11.8% at 6:12, 20.2% at 8:12 and 41.4% at 12:12. If you are following a code method based on plan area, enter a pitch rise of 0.

Rainfall intensity and area scale the flow in direct proportion, while downspout capacity sets the count. Because the count rounds up, small changes in either can tip the answer from one downspout to two.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Sheet Metal and Air Conditioning Contractors' National Association (n.d.). "Downspout and Gutter Sizing Calculator." SMACNA.
2. [2]National Oceanic and Atmospheric Administration (n.d.). "Precipitation Frequency Data Server." NOAA National Weather Service.
3. [3]National Weather Service (2024). "NOAA Atlas 14 is the authoritative source for precipitation frequency information." NOAA.
4. [4]International Code Council (2024). "2024 International Residential Code, Chapter 33: Storm Drainage." ICC.
5. [5]National Institute of Standards and Technology (2023). "NIST Handbook 133, Appendix E: General Tables of Units of Measurement." U.S. Department of Commerce.