Gutter and Downspout Calculator

Last reviewed How we calculate

A gutter job is mostly a shopping list: lengths of gutter, downspouts, the elbows that bring the downspouts back to the wall, hangers, end caps and outlets. This builds that list from the length of your eaves and a few choices about spacing.

Gutter and downspout materials

Gutter sections, downspouts, elbows, hangers and end caps from the length of your eaves.

How to measure
A gutter along an eave, seen from the front. Measure the total gutter length, the distance between downspouts, and the height from the gutter to the ground. Hangers are spaced along the gutter. Hangers along the gutter Distance between downspouts Gutter to ground Gutter length

This calculator needs JavaScript to work. The measurements below are the ones you would need to do the sum by hand.

Every eave that needs a gutter, added together.

A run is one continuous length with two ends of its own.

Sectional gutter is commonly sold in 10 ft lengths. For seamless gutter, enter the longest run.

A common rule of thumb is one downspout per 30 to 40 ft of gutter, closer where heavy rain is normal.

Commonly 24 in, closer where heavy snow sits in the gutter.

Gutter sections —

Downspouts
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Downspout pipe
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Elbows Three per downspout: two to step back from the eave, one at the foot.
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Downspout outlets
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Hangers
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End caps
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Downspout spacing here is a rule of thumb, not a sizing calculation. How much water a gutter carries depends on the roof area feeding it and on local rainfall intensity; the rainwater runoff calculator gives that flow. Gutter work is ladder work, and if you cannot do it safely it is a job to hand over.

How the calculation works

Gutter sections are the total length divided by the section length, rounded up. Downspouts are the total length divided by the spacing you choose, with at least one on every run. Each downspout gets pipe to reach the ground, one outlet and three elbows. Hangers are spaced along the gutter with one extra per run, and every run needs two end caps.

A worked example

80 ft of gutter in two runs, with 10 ft sections and downspouts no more than 35 ft apart, needs 8 sections and 3 downspouts. At 10 ft to the ground that is 30 ft of downspout pipe, 9 elbows and 3 outlets. With hangers every 24 in the job takes 42 hangers and 4 end caps.

How many downspouts is enough

The spacing here is a rule of thumb. The real question is how much water each downspout has to carry, and that depends on the area of roof draining to it and on how hard it rains where you live. In heavy-rain areas, or where a large roof drains to a short gutter, more downspouts or a larger size are needed. The rainwater runoff calculator works out the peak flow in gallons per minute.

Working at height

Gutter length can be measured from the ground with a tape along the eaves. Fitting gutters means working from a ladder at the roof edge, and ladder falls are among the most common serious injuries in home repair. If that is not something you can do safely, it is a job to hand to a contractor.

Frequently asked questions

How many downspouts do my gutters need?

A common rule of thumb is one downspout for every 30 to 40 feet of gutter, with at least one on every run. Large roofs, heavy rainfall and long runs need them closer together.

What size gutters do I need?

It depends on the roof area draining to them and how hard it rains where you live. 5-inch K-style gutters are common on houses; larger or steeper roofs and heavy-rain areas often need 6-inch gutters.

How far apart should gutter hangers be?

Commonly every 24 inches, and closer where heavy snow sits in the gutters. The gutter manufacturer states the maximum spacing for its hangers.

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