Free Calculator
Gutter Size Calculator
Find out whether you need 5-inch or 6-inch K-style gutters — and how many downspouts — using the watershed method based on roof area, pitch, and local rainfall intensity.
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Size Your Gutters and Downspouts
Enter the roof area draining to your gutter run, the roof pitch, and your local rainfall intensity to get a recommended gutter size and downspout count.
Use the flat footprint (length × width), not the sloped surface area.
US ranges ~4 (Pacific NW) to ~9+ (Gulf Coast); check a rainfall intensity map.
📋 Important: All calculator results are ballpark estimates
The figures shown are approximate estimates based on typical averages and should be used for general planning purposes only. They are not a substitute for a professional assessment or written contractor quote. Actual costs, savings, and results will vary significantly based on your specific circumstances, local market conditions, equipment choices, and contractor pricing. Always confirm any estimate with a licensed contractor, financial advisor, or qualified professional before making purchasing or financial decisions.
How Gutter Size Is Determined
The watershed (adjusted-area) method
Professional gutter sizing isn't a guess — it's based on the watershed method. You start with the roof plan area (the flat footprint) draining to a single gutter run, multiply it by a pitch factor to get an effective drainage area, then compare that against each gutter size's carrying capacity at your local rainfall intensity. A gutter that can't move water fast enough overflows, no matter how big the roof looks.
| Gutter size (K-style) | Watershed capacity at 1 in/hr |
|---|---|
| 5-inch K-style | ~5,520 sq ft of roof plan area |
| 6-inch K-style | ~7,960 sq ft of roof plan area |
Divide those capacities by your rainfall intensity to get the real limit. At 7 in/hr, a 5-inch gutter handles roughly 790 sq ft and a 6-inch gutter roughly 1,140 sq ft of effective area per run.
Why roof pitch matters
A steep roof sheds water faster and lets wind push rain further up the slope, so more water reaches the gutter in a short burst than the flat footprint suggests. That's why sizing multiplies the plan area by a pitch factor — from 1.0 for a flat or low roof up to about 1.30 for a very steep 12:12+ roof. Skip this step and you'll undersize the gutter on exactly the roofs most prone to overflow.
Why rainfall intensity matters
Gutters are sized for the peak short-duration downpour your region sees, not the annual average. Rainfall intensity ranges from about 4 in/hr in the Pacific Northwest to 9+ in/hr along the Gulf Coast. Because capacity scales inversely with intensity, the same 5-inch gutter that easily drains a house in Seattle may overflow on an identical house in Houston. Always size to your local intensity, which building departments and rainfall maps publish.
Downspouts and spacing
A gutter is only as good as its outlets. As a guide, one 2×3 downspout drains about 600 sq ft of roof plan area and one 3×4 downspout about 1,200 sq ft. Beyond capacity, plan for roughly one downspout per 20–40 feet of gutter run so water doesn't have to travel too far to an outlet. Adding a downspout or splitting a long run is usually cheaper and more effective than upsizing the entire gutter profile.
5-inch vs 6-inch: the tradeoffs
- 5-inch K-style — the residential default; lower material cost, pairs with 2×3 downspouts, fine for typical roofs and moderate rainfall.
- 6-inch K-style — carries ~40% more water, pairs with larger 3×4 downspouts that clog far less, and is the better call for large watershed areas, steep pitches, or high-rainfall regions.
- 6-inch costs more per foot and needs correspondingly larger hangers and outlets, but often lets you use fewer downspouts.
- If your effective area exceeds even a 6-inch gutter's capacity, split the run or add outlets rather than assuming one long gutter will keep up.
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Book a Free DemoFrequently Asked Questions
Should I use 5-inch or 6-inch gutters?
For most single-family homes with a typical roof, 5-inch K-style gutters handle the runoff. Step up to 6-inch K-style when the roof plan area draining to one run is large, the pitch is steep, or you're in a high-rainfall region — 6-inch gutters carry roughly 40% more water and pair with larger 3×4 downspouts that clog less. The calculator above recommends a size based on your watershed area and local rainfall intensity.
How is gutter size actually determined?
Gutter sizing uses the watershed method: take the roof plan area (footprint, not the sloped surface) draining to a gutter run, adjust it upward for roof pitch, then compare that effective area against each gutter size's drainage capacity at your local rainfall intensity. Steeper roofs and higher rainfall both reduce how much roof area a given gutter can safely handle.
How many downspouts do I need?
A common rule of thumb is one downspout per 20–40 feet of gutter run, but capacity matters too. As a guide, one 2×3 downspout drains about 600 sq ft of roof plan area and one 3×4 downspout about 1,200 sq ft. Divide your effective drainage area by those figures and round up. Adding downspouts is often cheaper and more effective than upsizing the gutter itself.
Why does roof pitch affect gutter size?
A steeper roof sheds water faster and lets wind drive rain further, so more water reaches the gutter in a shorter burst than the flat footprint alone would suggest. Sizing tables account for this with a pitch factor that increases the effective drainage area — a 12:12 roof can add 20–30% to the load compared to a flat roof of the same footprint.
What rainfall intensity should I use?
Use the 5-minute peak rainfall intensity for your area, expressed in inches per hour. It ranges from about 4 in/hr in the Pacific Northwest to 9+ in/hr along the Gulf Coast and Southeast. Local building departments and rainfall intensity maps publish these figures. Sizing to a higher intensity gives you more overflow margin during heavy storms.
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Get a Free DemoAll calculations are ballpark estimates based on standard gutter watershed sizing rules and should be verified with licensed roofing or gutter contractors in your area before making any purchasing decisions.