How much gutter do you need?

How do I calculate how much gutter I need?

Measure the length of every eave that will get a gutter and add them up. A house with a 40 ft run along the front and a 25 ft run on the garage needs 65 feet of gutter — seven 10 ft sections, or 65 feet of seamless gutter.

How many downspouts do I need?

A common rule is one downspout for every 30 to 40 feet of gutter, and at least one on every run. A run longer than 40 feet usually gets one at each end, with the gutter sloping both ways from the middle. Each downspout needs pipe from the gutter to the ground — with 10 ft eaves, one 10 ft piece.

How many gutter hangers do I need?

One every 2 feet, as Amerimax and Home Depot recommend, so a 40 ft run takes 20 hangers. In places with heavy snow, many installers space them closer, about every 18 inches — enter a longer run or add the extra by hand.

What parts do I need besides the gutter itself?

For each downspout: a drop outlet, three elbows (two at the top to bring the pipe back to the wall, one at the bottom to turn the water away) and the pipe. For each run: two end caps, unless it wraps a corner — then add an inside or outside corner piece instead. Plus hangers, and sealant and screws for the joints.

Seamless or sectional gutters?

Seamless gutters are formed on site in one piece per run, so they have no joints to leak, but need a contractor with a machine. Sectional gutters come in 10 ft pieces you can hang yourself, joined with connectors. Compare the prices in the table above.

How many downspout pipes do I need for a two-story house?

Pipe is counted by the height from the gutter to the ground, in 10 ft pieces, for each downspout. An 18 ft eave needs 2 pieces per downspout; a single 30 ft run at 20 ft height has 1 downspout and needs 2 pieces. Elbows are added separately: three per downspout.

Do I measure the roof edge or the eaves?

Measure along the eaves, the low edges of the roof where the water runs off, and enter each straight stretch as a separate run. Do not use the roofline length of a gable or the ridge. If a run turns a corner, enter the two sides separately and add the corner piece yourself.

How the parts are counted

Each run is counted on its own. The calculator gives every run one downspout for each 40 feet of gutter or part of 40 feet, and at least one per run. Pipe is counted per downspout, in 10 ft pieces, from the height of the gutter, and each downspout takes a drop outlet and three elbows. Each run gets two end caps, and hangers are placed every 2 feet, rounded up per run. These are rules of thumb from manufacturers such as Amerimax (a downspout for every 40 feet of gutter for best drainage capacity), and roof area and rainfall in your region may justify more downspouts than the minimum.

Worked example

A house has a 40 ft run along the front and a 25 ft run on the garage, with the gutter 10 ft above the ground. Each run is short enough for one downspout, so the tool counts 2 downspouts, 2 drop outlets, 6 elbows and 2 lengths of 10 ft pipe. There are 4 end caps, because each run is closed at both ends, and 33 hangers: 20 for the 40 ft run and 13 for the 25 ft run, which is 12.5 hangers rounded up.

A single 60 ft run with its gutter 18 ft up needs 2 downspouts, 4 lengths of pipe, 6 elbows and 30 hangers; a long run like this is usually sloped from the middle toward a downspout at each end.

Things to watch

Entering one total length is a common shortcut, but it undercounts parts: two 20 ft runs need four end caps and two downspouts, while one 40 ft run needs two caps and one downspout. List each straight run separately. The count does not include inside or outside corner pieces, splice connectors between sections, sealant or screws; add these from your layout. Finally, round up the gutter itself: it is sold in 10 ft sections, so 65 feet of run needs seven sections and leaves 5 feet to cut off.

Sources: one downspout per 40 feet of gutter from Amerimax product guidance; hanger spacing as cited by Amerimax and Home Depot; part counts follow from those spacings. Local codes and rainfall can call for more. Related tools: roof, roof pitch and rainwater harvesting calculators.