About the Gutter Calculator
This gutter calculator sizes a gutter run and its downspouts from three things: the roof area that drains into that gutter, the roof pitch, and the rainfall intensity you design for. It recommends a 5-inch or 6-inch K-style gutter (or flags when you need something bigger), counts the downspouts needed and estimates peak runoff in gallons per minute.
Use it when replacing gutters, adding an addition, or when existing gutters overflow in heavy storms. Work one gutter run at a time: measure the footprint (plan) area of the roof plane that drains to it, not the whole house.
Steeper roofs catch more wind-driven rain, so the plan area is increased by a pitch factor. Downspouts are sized with the common rule of thumb of 1 square inch of downspout opening per 100 sq ft of adjusted roof area, with at least one downspout every 40 feet of gutter. The gutter capacity ratings (about 5,520 for a 5-inch and 7,960 for a 6-inch K-style gutter, in sq ft × in/hr), the pitch factors and the downspout rule are common industry rules of thumb, not a building-code table — treat them as planning figures; local codes, gutter manufacturers and installers may require more. For design rainfall, look up your location’s short-duration intensity on NOAA’s Precipitation Frequency Data Server.
How to use the gutter calculator
- 1Measure the plan area of the roof plane draining into one gutter run.
- 2Select the roof pitch range.
- 3Enter a design rainfall intensity for your area (heaviest short storm).
- 4Enter the gutter run length and preferred downspout size.
- 5Read the recommended gutter size and downspout count; repeat for each run.
Formula and method
Gutter sizing starts with the plan (bird’s-eye) area of the roof plane that drains into the gutter. Because steep roofs intercept more wind-driven rain, that area is multiplied by a pitch factor from 1.0 (flat to 3/12) up to 1.3 (12/12 and steeper). Multiplying the adjusted area by the design rainfall intensity gives the drainage demand, which is compared with widely used capacity ratings of about 5,520 for a 5-inch K-style gutter and 7,960 for a 6-inch K-style gutter.
Downspouts follow the rule of thumb of one square inch of downspout opening per 100 sq ft of adjusted roof area, and at least one downspout per 40 feet of gutter. Peak runoff in gallons per minute is area × intensity ÷ 96.25, since one inch of rain per hour on one square foot is 144 cubic inches ÷ 231 per hour, or about 0.0104 gpm.
- Pitch factor
- 1.0–1.3 multiplier for roof steepness
- Rainfall
- Design rainfall intensity in inches per hour
- 96.25
- sq ft·in/hr per gallon per minute
Worked examples
1,200 sq ft, 6/12 pitch, 5 in/hr, 40 ft run
The pitch factor of 1.1 raises 1,200 sq ft to 1,320 sq ft. At 5 in/hr the demand is 6,600, above a 5-inch gutter’s 5,520 rating, so a 6-inch gutter is needed. With 2 × 3 downspouts (600 sq ft each) you need three.
Low-slope garage, 800 sq ft, 4 in/hr, 30 ft run
A flat-to-3/12 roof keeps the area at 800 sq ft. Demand is 3,200, well within a 5-inch gutter. Two 2 × 3 downspouts handle the 800 sq ft (600 each).
Steep 2,000 sq ft roof in a heavy-rain area
A 9/12 roof adjusts 2,000 sq ft to 2,400 sq ft, and 6 in/hr gives a demand of 14,400 — beyond a 6-inch gutter. Either upsize or split the roof; 3 × 4 downspouts (1,200 sq ft each) need at least two.
Frequently asked questions
Do I need 5-inch or 6-inch gutters?+
Multiply the pitch-adjusted roof area by your design rainfall in inches per hour. If the result is under about 5,520, a 5-inch K-style gutter works; up to about 7,960 calls for 6-inch; above that, upsize or split the roof into more runs.
How many downspouts do I need?+
A common rule is one square inch of downspout opening per 100 sq ft of roof, so a 2 × 3 downspout drains about 600 sq ft and a 3 × 4 about 1,200 sq ft, with at least one downspout every 30–40 feet of gutter.
Why does roof pitch matter for gutters?+
Rain rarely falls straight down. A steep roof presents more surface to wind-driven rain and sheds water faster, so its effective drainage area is larger than its plan area — hence the 1.05–1.3 pitch factors.
What rainfall intensity should I use?+
Use the heaviest short-duration (about 5-minute) rainfall expected in your area. NOAA publishes precipitation frequency data; many US locations fall between 4 and 8 inches per hour for design storms.
How much slope should a gutter have?+
Gutters are commonly pitched about 1/4 inch per 10 feet toward the downspout. Long runs are often sloped both ways from the middle to downspouts at each end.