Gutter calculator

How to work out gutters and downspouts

Gutters are bought by the foot, but sized by the water they must carry. Here is the math the calculator runs, with the values it opens with: a 40 x 30 ft house with a gable roof, 1 ft overhangs, 10 ft eaves, a 6/12 pitch and Chicago's rainfall.

1. 1

   Measure the edges that shed water

   On a gable roof, water runs off the two long sides only. Each gutter runs the length of the house plus the overhang at both ends: 40 + 1 + 1 = 42 ft. Two runs make 84 ft.

2. 2

   Find the roof area behind each run

   With the overhang, the roof covers 42 x 32 = 1,344 sq ft seen from above. Each long side drains half, 672 sq ft.

3. 3

   Adjust for pitch and rain

   A steeper roof catches more wind-driven rain, so a 6/12 roof gets a pitch factor of 1.1. Multiply by the rainfall intensity, 7.4 inches per hour: 672 x 1.1 x 7.4 = 5,470.

4. 4

   Pick the gutter size

   A 5 inch K-style gutter is rated for about 5,520 on this scale and a 6 inch for about 7,960. 5,470 fits a 5 inch gutter, at 99% of its rating. That is close to the limit, so 6 inch is a sensible upgrade if the price gap is small.

5. 5

   Count downspouts

   Two rules, and the larger count wins. By length, one downspout per 40 ft: a 42 ft run needs 2. By area, a 2x3 downspout drains about 600 sq ft: 672 x 1.1 = 739 sq ft also needs 2. That is 4 downspouts, one at each corner, and 40 ft of pipe for 10 ft eaves.

Priced out, the DIY parts list comes to $496 and a seamless install about $1,116, at $9 a foot for 84 ft of gutter and 40 ft of downspout. Switch to "Hip roof" and the gutter wraps all four sides: 148 ft, with four corners and no end caps.

5-inch or 6-inch gutters?

The 5 inch K-style gutter is the standard on American houses. The 6 inch holds about 40% more water and pairs with a bigger 3x4 downspout. The ratings below are for the sizing method above (roof area x pitch factor x inches of rain per hour).

Half-round gutters look right on older homes but carry far less, so size them with care. The pitch factor runs from 1.0 for flat to 3/12 roofs, 1.05 for 4/12 to 5/12, 1.1 for 6/12 to 8/12 and 1.2 for 9/12 to 11/12, up to 1.3 for 12/12 and steeper. Not sure of your pitch? Hold a level out 12 inches from the roof and measure the rise in inches, or use the roof pitch calculator. If you are replacing the roof at the same time, the roofing calculator prices shingles for the same house.

How hard does it rain where you live?

Gutters overflow in short, violent bursts, not in a long drizzle. The calculator uses NOAA's 5-minute rainfall intensity for a storm that comes about once every 10 years. Here are values for some cities, from NOAA Atlas 14 at each city center.

That is why 6 inch gutters are common along the Gulf Coast and in Florida, while 5 inch gutters handle most houses in the dry West. Rain varies a lot within a metro area, so look up your own address if the result is close to the line.

How many downspouts?

A gutter is only as good as its outlets. A common rule is one downspout every 30 to 40 ft of gutter, and any run of 60 ft or more needs at least two. Area matters too: plan about 1 square inch of downspout opening per 100 sq ft of roof.

The calculator applies both rules to each run and keeps the higher count, with at least one per run. Where you can, put downspouts at the corners and let a long run slope both ways from the middle. Extra downspouts are cheap. They also cut the load on each stretch of gutter, which helps in heavy-rain areas.

What goes on the parts list

• Gutter sections come in 10 ft lengths at home centers. Each run is rounded up to whole sections.
• Connectors join sections. Every joint is a possible leak, which is the main argument for seamless gutter.
• End caps close the open ends of each run, and corners (miters) turn the hip-roof corners.
• Outlets (drop outlets) are the short spouts the downspout slips over, one per downspout.
• Elbows: three per downspout. Two make the jog from the gutter back to the wall, and one kicks the water out at the bottom.
• Hidden hangers every 2 ft, plus one at each end of a run. They screw through the gutter into the fascia.
• Downspout brackets, two per 10 ft of pipe, and a splash block at each outlet.

What gutters cost: DIY vs. hiring out

Most new gutters are seamless aluminum, formed on site from a coil by a machine on the installer's truck. That is not a DIY option. DIY means sectional gutter from a home center, with a joint every 10 ft.

The calculator's installed price, $9 a foot, falls in the middle of the aluminum range. It is applied to both gutter and downspout footage. Expect 6 inch gutter to cost about $0.50 to $1 more per foot. Quotes rise for second- and third-story work, steep roofs, removing old gutters and rotten fascia that must be replaced first. On a one-story ranch, DIY saves real money. On a two-story house, the ladder work is the hard and risky part, and a crew with a seamless machine is often done in a day.

Tips from the trade

• Slope it right. About 1/4 inch of fall per 10 ft toward the outlet. Snap a chalk line on the fascia before hanging anything. Too little slope leaves standing water, and too much looks crooked from the street.
• Tuck under the drip edge. The back of the gutter goes behind the roof's drip edge, so water can't run down the fascia behind it.
• Space hangers closer in snow country. 24 inches is common. Go to about 18 inches where ice and snow load the gutters, and follow the hanger maker's spacing.
• Move the water away. Gutters only help if the water lands well clear of the foundation. Use extensions or splash blocks, and keep the ground sloping away from the house. The model residential code calls for 6 inches of fall in the first 10 ft.
• Seal joints from the inside. Use gutter sealant on clean, dry metal, and rivet or screw each connector.
• Clean them twice a year, more under trees. Guards reduce cleaning but don't end it, and some make overflow worse in heavy rain.
• Respect the ladder. Set it 1 ft out for every 4 ft of height, extend it 3 ft above the edge you step onto, and never lean on the gutter. Leave roof-edge work above one story to a pro if you are not comfortable at height.

Common questions

Sources and assumptions

Lengths and areas are plain geometry for a rectangular house. Sizing rules come from published trade guidance, and prices are typical US prices checked in October 2026. Gutter and installed prices can be edited.

• Gutter and downspout capacities, pitch factors, slope and the 60 ft rule come from This Old House, how to size gutters and downspouts (April 2026). The calculator compares roof area x pitch factor x rainfall intensity with those capacities, as many municipal and installer guides do.
• Rainfall intensity: 5-minute, 10-year values from NOAA Atlas 14, Precipitation Frequency Data Server, read at each city center in October 2026. Atlas 14 does not yet cover the Pacific Northwest; look up your own value there.
• Downspout spacing: one per 40 ft of gutter per run, the upper end of the common 30 to 40 ft rule, and at least one per run.
• Installed prices: $6.29 to $10.45 per ft for seamless gutter in HomeWyse's seamless gutter estimate (May 2026), and material ranges (vinyl, aluminum, steel, copper) from Inch Calculator's gutter installation cost guide. The default $9 per ft ($10 for 6 inch) is applied to downspout footage too; that part is our estimate.
• DIY prices: $13 per 10 ft of 5 inch white aluminum K-style gutter, from a Home Depot listing. Fittings are our estimates of typical shelf prices: for 5 inch, $5 connector, $5 end cap, $14 corner, $8 outlet and $2.50 hanger; for 6 inch, $22 section, $8, $7, $20, $12 and $3.50; 2x3 downspout $12 per 10 ft, $4 elbow, $1.50 bracket; 3x4 downspout $20, $7 and $2; $10 splash block; $8 tube of sealant per 8 joints.
• Grading: 6 inches of fall within the first 10 ft from the foundation is the requirement in section R401.3 of the International Residential Code. Ladder angle follows OSHA's ladder rule (1 out for every 4 up, 3 ft above the landing).