Gutter Calculator: Size, Downspouts and Cost

Gutter size depends on roof area, pitch and rainfall

Each roof section needs three numbers: how much roof (measured flat, from above), how steep it is, and how hard it rains. Steep roofs catch more wind-driven rain, so the area gets increased by a pitch factor. The tool multiplies that by rainfall in inches per hour and compares the result with what a gutter can carry to one outlet.

• Adjusted area = plan area x pitch factor (1.00 flat to 3/12, 1.05 at 4 to 5/12, 1.10 at 6 to 8/12, 1.20 at 9 to 11/12, 1.30 at 12/12 and up)
• Design load = adjusted area x rainfall (in/hr)
• One 5-inch K-style outlet handles a load of 5,520. A 6-inch handles 7,960.
• Downspouts per section = the larger of load / gutter capacity and adjusted area / downspout rating (600 sq ft for 2x3, 1,200 for 3x4), rounded up
• Total downspouts = the larger of that sum and eave length / 40, rounded up
• Hangers = eave length in inches / 24 (or / 18 in snow country). End caps = 2 per run. Elbows = 3 per downspout.

The tool starts at 5-inch gutters with 2x3 downspouts. If rain alone would force extra outlets on any section, beyond the downspouts it already needs, it moves up to 6-inch with 3x4s.

The default inputs make a worked example: a ranch with two roof planes, each 50 feet long and 20 deep, at 6/12 pitch, in Chicago. Each plane is 1,000 sq ft flat, times 1.10 = 1,100. Chicago's 5-minute, 10-year rain is 7.43 in/hr, so the load is 1,100 x 7.43 = 8,173.

On 5-inch gutter that's 8,173 / 5,520 = 1.48, so two outlets. The 2x3 downspouts need 1,100 / 600 = 1.83, also two. Rain isn't asking for anything extra, so the result is 5-inch gutter with 2 downspouts per side, 4 in all. Spacing alone would have called for 3, since 100 / 40 = 2.5.

Parts come to 100 feet of gutter, 4 x 10 = 40 feet of downspout, 1,200 / 24 = 50 hangers, 4 end caps and 12 elbows. At $6 to $14 a foot for seamless aluminum on 140 feet, the price is $840 to $1,960.

The sizing rules come from This Old House and SMACNA

The pitch factors, the 5,520 and 7,960 capacities and the downspout ratings are the residential shortcut This Old House publishes. The downspout part is the old rule of 1 square inch of downspout per 100 sq ft of roof. It's a simplified version of the method in SMACNA's sheet metal manual, which SMACNA's downspout and gutter sizing calculator runs. SMACNA calls 50 feet the practical maximum run per downspout. The tool uses 40, closer to what installers quote.

Rainfall is the heaviest five minutes of a storm

The rainfall presets come straight from NOAA's precipitation frequency server (Atlas 14), one city per region: the heaviest 5 minutes of a 10-year storm, in inches per hour. If the presets don't fit your area, look up your own address there and read the 5-min row, 10-year column.

Five minutes sounds short, but gutters overflow in the first burst, not over an hour of drizzle. Plumbing codes generally size roof drains on a 1-hour, 100-year storm instead, which comes out less than half as intense. Don't mix the two up if you're comparing against a drain plan.

5-inch gutters fit most houses

Most houses do fine with 5-inch K-style gutters and 2x3 downspouts. The exceptions usually have one long, steep plane draining into a single run, or a valley that pours right at a corner. Six-inch holds about 40 percent more and adds only $1 to $2 a foot. In Florida or along the Gulf it's close to automatic.

Adding a downspout often fixes overflow as well as upsizing the gutter, and it costs less. That's why the tool lets you force 5-inch and simply adds outlets. It sizes K-style only. Half-round holds less at the same width, and the guide to gutter types explains why.

Hangers, corners and small parts

Hidden hangers every 24 inches is standard, or every 18 in snow country, because packed ice is heavy and the screws strip out of the fascia first. Corners are miters, inside or outside, and each adds about $20 to $40 of labor.

Every downspout gets an outlet and usually three elbows: two to jog back to the wall and one at the bottom to send water away from the foundation. End caps are counted by run. A gable ranch with gutters on the two long sides has two runs and four caps, while a hip roof wrapped all the way around might have none.

How much do gutters cost per foot?

Seamless aluminum costs about $6 to $14 a foot installed, vinyl about $3 to $6, galvanized steel $9 to $18 and copper $25 to $50. Those prices match our gutter cost guide. Sectional aluminum shows up here at $4 to $10, since seamless typically runs $2 to $4 more a foot. Downspouts are priced by the foot like the gutter.

Guards range from DIY plastic at fifty cents to $2.50 a foot up to $15 to $35 for a pro micro-mesh system. The gutter guards guide sorts out which kind is worth it.

The estimate leaves out hauling away old gutters (a couple hundred dollars), rotten fascia at $6 to $12 a foot, and a $1 to $3 a foot premium on tall or steep houses. Local quotes vary, sometimes by a lot. If a new roof is going on too, the roof cost calculator prices that side. In cold climates, read up on ice dam prevention before you pick guards, because some of them make icicles worse.