What Size Gutters Do I Need? 5 Vs. 6-Inch Gutters Explained

If you're asking "what size gutters do I need?" you're already ahead of most homeowners. Gutters aren't one-size-fits-all, and choosing the wrong size can lead to overflowing water, foundation damage, and costly repairs down the road, especially here in the Pacific Northwest, where rain is practically a year-round resident.

The two most common residential options are 5-inch and 6-inch gutters, and the right choice depends on a few specific factors: your roof's square footage, its pitch, and how much rainfall your area actually gets. Pick too small and your gutters can't keep up during heavy storms. Go oversized without reason and you're spending money you didn't need to.

At Legacy Exteriors LLC, we install gutter systems across Kirkland, Bellevue, Redmond, and the surrounding areas every week. We see firsthand what happens when gutters are undersized for a home, and it's never pretty. This guide breaks down exactly how to determine the right gutter size for your house, walks you through the 5-inch vs. 6-inch comparison, and helps you make a confident decision before your next project.

What controls gutter size on your home

Three variables determine what size gutters you need for your home: the drainage area of your roof, its pitch, and the rainfall intensity in your area. None of these works in isolation. A steep roof over a large footprint in a high-rainfall region pushes far more water through your gutters than a low-slope roof on a small home in a drier climate. Getting a handle on all three before you order materials saves you from undersized gutters and unnecessary overflows.

Roof drainage area

Your roof's drainage area is the horizontal square footage each gutter run must handle. This differs from your home's floor plan because roof overhangs extend past the foundation walls. Every square foot of roof surface that sheds water toward one side adds to the load that specific gutter section carries. Larger drainage areas call for wider gutters and more downspouts spaced at shorter intervals.

Roof valleys concentrate the problem further. A valley channels water from two roof planes into a single point, effectively doubling the volume hitting that section of gutter during a heavy storm. That's a detail most homeowners miss until water is already pouring over the side.

A roof valley can deliver twice the water flow of a standard flat run, which is one of the most common reasons gutters overflow even when they appear properly sized.

Roof pitch

Roof pitch controls how fast water reaches your gutters. A steep roof accelerates runoff significantly, so water arrives with more velocity and volume than the same storm produces on a low-slope roof. Gutter sizing charts apply a correction factor based on pitch, and that adjustment alone can push a borderline 5-inch installation into 6-inch territory.

Roof Pitch	Correction Factor
Up to 6:12	1.0x (no adjustment needed)
7:12 to 11:12	1.1x
12:12 and above	1.2x

Local rainfall intensity

Here in the Pacific Northwest, rainfall intensity pushes many homes toward 6-inch gutters. What matters isn't just annual totals but the peak inches-per-hour rate during storm events. A region with intense short bursts overwhelms gutters far faster than one with steady, lighter rain spread across many hours.

For homeowners in Kirkland, Bellevue, Redmond, and nearby cities, local storm data consistently supports sizing up. A 5-inch gutter may handle average days without issue, but undersized gutters fail hardest on the worst storm days, which is exactly when your system needs to perform.

Step 1. Estimate your roof drainage area

To answer what size gutters do I need, you first need to know how much roof surface each gutter run actually drains. You don't need to climb on your roof to get this number. A solid estimate from the ground using your home's exterior measurements is accurate enough for selecting the right gutter size before you order anything.

Your drainage area is the horizontal projection of your roof, not the actual sloped surface, so ground-level measurements give you an accurate and usable starting point.

Measure each roof section

Start by measuring the length and width of each exterior wall, then add 1 to 2 feet per side to account for roof overhangs extending past the foundation. Multiply the adjusted length by the adjusted width to get the square footage for each drainage zone. A rectangular home with a basic gable roof will have two drainage zones, one per side. More complex roof shapes will have additional sections you'll need to calculate separately.

Here's a simple calculation template you can fill in for your home:

Roof Section	Length (ft)	Width (ft)	Drainage Area (sq ft)
Left side	45	22	990
Right side	45	22	990
Front addition	20	18	360

Add up the load per gutter run

Once you have each section's square footage, assign those numbers to the specific gutter runs that collect water from them. A single gutter run along the left side of your home only handles its own drainage zone, not the full roof total. This per-run figure is the number you carry into the next step when you factor in pitch and rainfall intensity.

Step 2. Factor in pitch, valleys, and rainfall

Take your per-run drainage area from Step 1 and run it through two adjustments before you land on a final number. Roof pitch and valley placement both increase the effective load your gutters carry, and local rainfall intensity determines whether your adjusted number pushes you into 6-inch territory or keeps you at 5-inch.

Apply the pitch correction factor

Multiply your drainage area by the correction factor that matches your roof's pitch. A steep 12:12 pitch moves water fast, which increases the effective drainage load by 20 percent compared to the same roof at a shallow slope. Use the table below to apply the right multiplier to each gutter run.

Roof Pitch	Correction Factor	Example: 1,000 sq ft becomes
Up to 6:12	1.0x	1,000 sq ft
7:12 to 11:12	1.1x	1,100 sq ft
12:12 and above	1.2x	1,200 sq ft

Once you apply the pitch correction, treat the adjusted number as your true drainage area for every sizing decision that follows.

Add valley loads to the affected run

Roof valleys funnel water from two planes into one point, so you need to count the drainage area from both planes and add them together for the gutter run that sits below that valley. If a 1,000 sq ft left plane and a 600 sq ft rear plane both drain into the same corner, that gutter run carries a combined adjusted load of 1,600 sq ft, not 1,000.

Match your numbers to local rainfall

Figuring out what size gutters you need also depends on your peak rainfall rate, measured in inches per hour. For Kirkland and the broader Seattle area, the design rainfall rate typically runs around 0.5 to 1.0 inches per hour. Cross-reference your adjusted drainage area against that rate to confirm your gutter size before you order anything.

Step 3. Choose 5-inch or 6-inch gutters

Now that you have your adjusted drainage area for each run, the decision between 5-inch and 6-inch gutters comes down to a straightforward capacity comparison. Standard K-style gutters at 5 inches handle roughly 5,520 square feet of adjusted drainage area, while 6-inch gutters push that capacity to approximately 7,960 square feet. Match your per-run adjusted numbers against those thresholds to land on the right size for each section of your home.

When 5-inch gutters are enough

Five-inch gutters work well on smaller homes with simple rooflines and modest roof pitches. If your adjusted drainage area per run stays well below 5,000 square feet and you're not dealing with valleys concentrating water into one spot, a standard 5-inch K-style profile handles the load without issue. Most single-story homes with straightforward gable roofs fall into this category, even in wetter climates.

• Adjusted drainage area under 5,000 sq ft per run

• Roof pitch at or below 6:12

• No major valleys feeding the run

• Home under 1,500 sq ft of floor space

When to step up to 6-inch gutters

If your adjusted drainage area lands above 5,000 square feet per run, or if you're working with steep pitches and valley loads, 6-inch gutters are the right call. Homes in Kirkland, Redmond, and Bellevue frequently hit these thresholds because of larger footprints, complex rooflines, and high peak rainfall rates. That's the core of figuring out what size gutters do I need in this region.

Undersizing gutters to save money upfront almost always costs more in water damage repairs within a few years.

Step 4. Size downspouts and plan the layout

Gutters collect the water, but downspouts move it away from your home. Getting the downspout count and placement right is just as important as answering what size gutters do I need. An oversized gutter run with too few downspouts will still overflow at the drain point during a heavy storm, so plan both together before you finalize your order.

Calculate how many downspouts each run needs

One downspout handles roughly 600 to 800 square feet of adjusted drainage area when paired with a 5-inch gutter, and up to 1,200 square feet with a 6-inch gutter. Divide your adjusted drainage area per run by the appropriate figure to find the minimum number of downspouts that run requires.

Never rely on a single downspout for any gutter run longer than 40 feet, regardless of drainage area calculations.

Use this quick reference to confirm your counts before ordering:

Gutter Size	Max Drainage Area per Downspout	Run Length Guideline
5-inch	600 to 800 sq ft	1 downspout per 30-40 ft
6-inch	1,000 to 1,200 sq ft	1 downspout per 40-50 ft

Place downspouts where water can exit safely

Standard 3x4-inch rectangular downspouts pair with 6-inch gutters and move significantly more volume than the older 2x3-inch size. Position each downspout at the low end of a sloped gutter run so water naturally flows toward the drain rather than pooling mid-run. Extend downspout outlets at least 6 feet from your foundation using splash blocks or underground drainage to protect your home's base.

Final checklist before you order gutters

You now have everything you need to answer what size gutters do I need for your specific home. Before you place your order, run through this checklist to confirm you haven't missed anything.

• Drainage area calculated per gutter run, not the full roof total

• Pitch correction factor applied to each run's square footage

• Valley loads added to the affected runs

• 5-inch vs. 6-inch decision made based on adjusted area thresholds

• Downspout count confirmed at the right spacing for your gutter size

• Downspout outlets planned to discharge at least 6 feet from your foundation

Getting these details right before installation protects your home from water damage for years. If you want a professional set of eyes on your roof before committing to a gutter system, schedule a free inspection with Legacy Exteriors and we'll walk through the sizing with you.