Gutter Size Calculator
Enter the roof size drained by a gutter run, the roof pitch and the heavy-rain rate for your area to estimate the flow and the number of downspouts. Capacity figures are editable.
How to use this gutter size calculator
- Enter the length of the gutter run in feet.
- Enter the roof run, the horizontal distance from the eave to the ridge. For a gable roof this is half the building width.
- Enter the roof pitch as inches of rise per 12 inches of run.
- Enter your design rainfall in inches per hour. Look up local figures, because the number varies a lot by place.
- Enter the roof area one downspout can handle, from the maker data or local rules, then press Calculate.
How to size gutters and downspouts
The calculator finds the roof area that drains to the gutter and the flow that heavy rain sends into it.
- Slope factor = the square root of (1 + (pitch / 12) squared)
- Roof area drained = gutter length x roof run x slope factor
- Flow (gallons per minute) = roof area x rainfall (in per hour) / 96.23
- Downspouts = roof area / roof area per downspout, rounded up
- Spacing = gutter length / downspouts
Worked example
A gutter run is 40 ft long and drains a roof that has a run of 15 ft from the eave to the ridge at a 6 in 12 pitch. The slope factor is 1.118, so the roof area is 40 x 15 x 1.118 = 670.8 sq ft. At a design rainfall of 6 inches per hour the flow is 670.8 x 6 / 96.23 = 41.8 gallons per minute. If one downspout is rated for 600 sq ft, you need 2 downspouts, each handling about 335 sq ft and spaced about 20 ft apart.
Assumptions & limits
- The roof drains to one side only, and the gutter and downspouts are clear of leaves and debris.
- One inch of rain per hour on one square foot of roof is about 0.0104 gallons per minute. This is the conversion behind the 96.23 in the formula.
- The design rainfall is your own input. Use a local heavy-rain figure, for example from NOAA Atlas 14 or local code, not a yearly average.
- The roof area per downspout is an editable rule of thumb. It depends on the gutter and downspout size.
- Gutter slope, shape and the maker data decide how much a gutter can carry, so confirm the gutter size with the supplier or a roofer.
Frequently asked questions
How do I calculate what gutter size I need?
Find the roof area that drains into the gutter, multiply by the heavy-rain rate and convert to flow. Then compare the flow and area with the maker capacity data for the gutter and downspout size.
How many downspouts do I need?
A common rule of thumb is one downspout for a set roof area. The calculator uses the figure you enter, with 600 sq ft as an editable default.
Why does roof pitch matter?
A steeper roof has more surface under the same footprint and sheds water faster, so the slope factor increases the roof area.
What rainfall should I use?
Use a heavy short-burst figure for your location, from NOAA Atlas 14 or local code. Yearly rainfall is not the right number.
Is a bigger gutter better?
For heavy rain or a large roof, yes. A larger gutter and downspout carry more water and clog less often.
Does this replace a roofer?
No. It gives an estimate. A roofer or the gutter maker can confirm the sizes for your roof.
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Last updated: November 2026