Gutter Size Chart: Standard Sizes, Capacity and Downspouts by Roof Area

Byline: The Roofing Brief Team · Last reviewed: September 2026

A gutter size chart tells you the exact width, capacity, and matching downspout for every common residential profile. In North America the two standard residential gutter widths are 5 inch and 6 inch K-style, plus 5 inch and 6 inch half-round. The width you read on a chart is the face measurement, and each width pairs with a specific downspout and a maximum roof drainage area. Use the chart below to read the specs, then use the drainage-area method further down to size a real roof.

Gutter size chart: standard sizes at a glance

Standard residential gutters come in four common sizes: 5 inch K-style, 6 inch K-style, 5 inch half-round, and 6 inch half-round. K-style is the boxy profile on most U.S. homes; half-round is the rounded profile common on older and higher-end houses. The table lists nominal width, capacity per linear foot, the roof area each size drains in moderate rain, and the downspout it pairs with.

Profile	Nominal size	Capacity (approx.)	Roof area drained (moderate rain)	Paired downspout	Source
K-style	5 inch	about 1.2 gal per linear ft	up to about 5,500 sq ft	2×3 inch	SMACNA sizing method (Berger Building Products, Proper Gutter and Downspout Sizing); contractor norms
K-style	6 inch	about 2.0 gal per linear ft	about 5,500 to 7,960 sq ft	3×4 inch	SMACNA sizing method; contractor norms
K-style	7 inch	higher (custom)	large or commercial roofs	4×5 inch or round	Manufacturer custom profiles
Half-round	5 inch	lower than K-style at same width	up to about 2,500 sq ft	3 inch round	SMACNA sizing method; contractor norms
Half-round	6 inch	lower than K-style at same width	about 2,500 to 3,840 sq ft	4 inch round	SMACNA sizing method; contractor norms

Roof-area figures assume a low to moderate rainfall region and level gutter. In high-intensity rainfall areas the same gutter drains a smaller area, which is why the rainfall-intensity method further down matters. For the decision of which one fits your house, see what size gutters your house needs.

What is the standard gutter size?

The standard residential gutter size is 5 inch K-style. It is the default on most single-family homes in the United States and handles a roof area up to roughly 5,500 square feet in moderate rain. Builders step up to 6 inch K-style for larger roofs, long single runs, steep pitches that throw water fast, or regions with heavy downpours. Half-round is a style choice more than a capacity choice and drains less water at the same nominal width.

5 inch vs 6 inch K-style gutters

The jump from 5 inch to 6 inch K-style adds roughly 40 percent more drainage capacity and pairs with a larger 3×4 downspout instead of 2×3. A 5 inch trough holds about 1.2 gallons of water per linear foot; a 6 inch trough holds about 2.0 gallons per foot. The 6 inch size costs more per foot and needs larger hangers and outlets, but it clogs less and overflows less on steep or large roofs.

Spec	5 inch K-style	6 inch K-style	Source
Face width	5 inch	6 inch	Manufacturer profiles
Water held per foot	about 1.2 gal	about 2.0 gal	Contractor norms
Roof area drained (moderate rain)	up to about 5,500 sq ft	about 5,500 to 7,960 sq ft	SMACNA method
Standard downspout	2×3 inch (6.0 sq in)	3×4 inch (12.0 sq in)	SMACNA Table of downspout dimensions
Best for	most homes, dry to moderate rain	large or steep roofs, heavy rain	Contractor norms

Half-round gutter sizes

Half-round gutters come mainly in 5 inch and 6 inch, measured as the diameter across the open top. They carry less water than a K-style of the same nominal width because the semicircular shape holds less volume than the boxy K-style cross-section. A 5 inch half-round runs about 3.1 inches deep and a 6 inch about 3.75 inches deep. Half-round is common on historic homes and copper installations and usually pairs with round downspouts.

Half-round size	Approx. depth	Roof area drained (moderate rain)	Paired downspout	Source
5 inch	about 3.1 inch	up to about 2,500 sq ft	3 inch round (7.07 sq in)	SMACNA method; manufacturer specs
6 inch	about 3.75 inch	about 2,500 to 3,840 sq ft	4 inch round (12.57 sq in)	SMACNA method; manufacturer specs

Downspout sizing: 2×3 vs 3×4

Downspout size is half of gutter sizing, because a gutter can never drain faster than its outlet and downspout. The two standard rectangular downspouts are 2×3 inch and 3×4 inch. A 2×3 has a 6.0 square inch cross-section and drains roughly 600 square feet of roof per downspout; a 3×4 has a 12.0 square inch cross-section and drains roughly 1,200 square feet, about double. Round downspouts of 3 inch and 4 inch give 7.07 and 12.57 square inches.

Downspout	Cross-section area	Roof area per downspout (moderate rain)	Pairs with	Source
2×3 inch rectangular	6.0 sq in (plain)	about 600 sq ft	5 inch K-style	SMACNA downspout dimensions; contractor norms
3×4 inch rectangular	12.0 sq in (plain)	about 1,200 sq ft	6 inch K-style	SMACNA downspout dimensions; contractor norms
3 inch round	7.07 sq in	about 700 sq ft	5 inch half-round	SMACNA downspout dimensions
4 inch round	12.57 sq in	about 1,250 sq ft	6 inch half-round	SMACNA downspout dimensions

SMACNA guidance notes that corrugated downspouts carry less than their plain cross-section suggests (a corrugated 2×3 works out to about 4.8 effective square inches), and that downspouts under 7.0 square inches should be used only for small areas such as porches. To connect a downspout into buried drainage, see how to fit a gutter to PVC adapter.

How to measure gutter size

Measure a K-style gutter from the back edge that meets the fascia to the front lip; that face measurement is the size. A 5 inch gutter measures 5 inches back to front. Do not measure the top opening, which is narrower than the face. For half-round, measure the diameter straight across the open top.

1. Set a tape measure flat across the top of the gutter.
2. For K-style, read the distance from the back wall (against the fascia) to the outer front lip. That number is the nominal size, usually 5 or 6 inches.
3. For half-round, read the diameter across the widest part of the open top.
4. Note the downspout too: measure its two sides, for example 2 by 3 inches or 3 by 4 inches.
5. Confirm the profile shape (boxy face equals K-style, rounded equals half-round) so a replacement section matches.

Gutter capacity by roof area (quick lookup)

For a fast estimate, match the roof area draining into one gutter run to a size using the chart below. This is the shortcut most installers use before refining with the rainfall method. Figures assume moderate rainfall and a level gutter; wetter regions and steep pitches push you up a size.

Roof area to one run	K-style size	Half-round size	Downspout	Source
Under 2,500 sq ft	5 inch	5 inch	2×3 or 3 inch round	SMACNA method; contractor norms
2,500 to 3,840 sq ft	5 inch	6 inch	2×3 or 4 inch round	SMACNA method; contractor norms
3,840 to 5,500 sq ft	5 to 6 inch	6 inch (near limit)	3×4	SMACNA method; contractor norms
5,500 to 7,960 sq ft	6 inch	step to K-style	3×4	SMACNA method; contractor norms
Over 7,960 sq ft	7 inch or extra downspouts	not typical	4×5 or multiple	Manufacturer custom profiles

How to size gutters with the drainage-area and rainfall method

The precise method used in the SMACNA Architectural Sheet Metal Manual sizes gutters from three inputs: the roof drainage area, the roof pitch, and the local rainfall intensity. You adjust the flat (plan) roof area upward for pitch, look up your local design rainfall rate, and convert to the flow the gutter and downspout must carry. It replaces guessing with numbers, which matters in high-rainfall regions where a 5 inch gutter can overflow on a roof a chart calls safe.

First, adjust the plan area for pitch using the SMACNA design-area factors. A steeper roof presents more area to driving rain, so you multiply the flat footprint by a factor.

Roof pitch	Design-area factor	Source
Level to 3 in 12	1.00	SMACNA Table 1-1 (Berger reprint)
4 to 5 in 12	1.05	SMACNA Table 1-1
6 to 8 in 12	1.10	SMACNA Table 1-1
9 to 11 in 12	1.20	SMACNA Table 1-1
12 in 12	1.30	SMACNA Table 1-1

Second, find your local rainfall intensity. Use the 100-year, 1-hour rate from NOAA Atlas 14 (the National Oceanic and Atmospheric Administration precipitation data server) for your address. Gulf and Southeast locations can exceed 7 inches per hour; northern and western regions are often 2 to 4 inches per hour.

Third, convert to flow. SMACNA notes that 96.15 square feet of roof at 1 inch per hour of rain equals 1 gallon per minute. So required flow in gallons per minute equals adjusted roof area times rainfall intensity divided by 96.15, which is the same as adjusted area times intensity times 0.0104. Size the downspout so its cross-section carries that flow: the plumbing-code allowance is roughly 1,100 square feet of roof per 4 inch downspout at 1 inch per hour, scaled down by your actual intensity.

Worked sizing example

Take a two-story gable home with a 2,400 square foot roof footprint split into two equal gutter runs of 1,200 square feet each, a 6 in 12 pitch, in a Southeast region with a design rainfall intensity near 7 inches per hour.

1. Adjust for pitch: 1,200 sq ft times the 1.10 factor (6 in 12) equals a 1,320 square foot design area per run.
2. Pick the gutter: 1,320 sq ft in a heavy-rain region is past what a 5 inch K-style handles comfortably, so choose 6 inch K-style.
3. Convert to flow: 1,320 sq ft times 7 in per hour times 0.0104 equals about 96 gallons per minute per run.
4. Size and count downspouts: a 3×4 downspout drains roughly 1,200 sq ft in moderate rain, but at 7 inches per hour that capacity falls sharply, so split the run with two 3×4 downspouts, one per roughly 660 sq ft.
5. Check spacing: two downspouts on a run near 50 feet also satisfies the common rule of one downspout per 20 to 35 linear feet and the SMACNA 50 foot maximum per downspout.

Result: 6 inch K-style gutter with two 3×4 downspouts per run. In a dry region with a 3 inch per hour intensity, the same roof could run 5 inch K-style with one 2×3 downspout per run. The rainfall input, not the roof area alone, drives the answer. Before install, confirm hanger spacing so a loaded 6 inch trough does not sag; see how many gutter hangers per 10 feet.

How many downspouts do you need?

Plan on one downspout for every 20 to 35 linear feet of gutter, and never let a single run exceed about 50 feet without an outlet, per SMACNA guidance. Match downspout capacity to the roof area draining to it: a 2×3 clears roughly 600 square feet and a 3×4 roughly 1,200 square feet in moderate rain. Add downspouts (not just a bigger gutter) when runs are long, valleys dump into one trough, or rainfall intensity is high.

Frequently asked questions

What is the standard gutter size?

The standard residential gutter size is 5 inch K-style, the default on most U.S. homes. It drains up to roughly 5,500 square feet of roof in moderate rain and pairs with a 2×3 inch downspout. Builders move to 6 inch K-style for larger or steeper roofs, long runs, or heavy-rain regions, which adds about 40 percent more capacity and uses a 3×4 downspout.

How do I measure my gutter size?

Measure a K-style gutter across the top from the back edge against the fascia to the front lip; that face number is the size, usually 5 or 6 inches. Do not measure the narrower top opening. For half-round, measure the diameter straight across the open top. Note the downspout separately, for example 2 by 3 inches, so a replacement matches.

What is the difference between 5 inch and 6 inch gutters?

A 6 inch K-style gutter carries about 40 percent more water than a 5 inch, holding roughly 2.0 gallons per linear foot versus 1.2, and uses a larger 3×4 downspout instead of 2×3. The 6 inch size suits large roofs, steep pitches, and heavy rainfall, while 5 inch fits most homes in dry to moderate climates.

Should I use a 2×3 or 3×4 downspout?

Use a 2×3 downspout with 5 inch gutters on smaller roofs and a 3×4 with 6 inch gutters on larger or heavy-rain roofs. A 2×3 has a 6.0 square inch cross-section and clears about 600 square feet per downspout; a 3×4 has 12.0 square inches and clears about 1,200 square feet, roughly double the flow and far less prone to clogging.

What size gutters do I need for a 2,000 square foot roof?

A 2,000 square foot roof in a dry to moderate region is usually fine on 5 inch K-style gutters with 2×3 downspouts placed every 20 to 35 feet. In a heavy-rain region, on a steep pitch, or where several roof planes drain into one run, step up to 6 inch K-style with 3×4 downspouts. Confirm with the rainfall-intensity method for your local rate.

Do half-round gutters hold less water than K-style?

Yes. A half-round gutter carries less water than a K-style of the same nominal width because its semicircular cross-section holds less volume than the boxy K-style shape. A 5 inch half-round drains up to about 2,500 square feet versus roughly 5,500 for a 5 inch K-style, so half-round is chosen mainly for its look on historic and copper homes rather than for capacity.