Gutter Calculator
Work out a gutter system from the eave length: 10 and 20 ft sections, downspouts by the 40 ft spacing rule, elbows, hangers, end caps and corner miters.
A gutter system counted piece by piece
A gutter job is a small kit of standard parts, and the count is mostly arithmetic. Gutters come in 10 and 20 ft sections. Downspouts follow the spacing rule every installer uses: one downspout for roughly every 40 ft of gutter, so a long eave gets the flow split before the gutter can overflow in the middle. Each downspout drops the height of the house in 10 ft sections, turns out at the bottom with two elbows, and the gutter itself hangs on a hanger every 2 ft.
Sections = gutter length (with allowance) ÷ section length, rounded up · Downspouts = length ÷ spacing (at least 2) · Hangers = length ÷ 2 ft
Example: 120 ft of eaves on a two-story house, 4 runs and 4 corners: 14 sections of 10 ft (7 in 20 ft lengths), 3 downspouts taking 6 sections of downspout pipe, 6 elbows, 66 hangers and 8 end caps.
Two quiet details in the numbers: the downspout count works from the true eave length (before the waste allowance pads it), because water does not care about your spare material; and the allowance is there for the miter cuts at corners and the occasional section a ladder bends. A single story drops about 10 ft; count a section per story per downspout. Where a valley dumps both roof slopes into one short eave, add a downspout there regardless of the spacing — concentrated flow beats the rule of thumb every time.
Frequently asked questions
How far apart should downspouts be?
About every 40 ft of gutter is the standard planning rule — closer (30 ft) for big roof areas, steep pitches or heavy-rain regions, since the limit is really how much water one downspout can swallow. Very short eaves still get two, one toward each end where the layout allows.
5 inch or 6 inch gutters — which do I need?
5 in K-style is the residential default and handles most roofs. Step to 6 in when the roof is large or steep (more water arriving faster), in heavy-rain climates, or where long runs feed few downspouts. 6 in gutters take matching 3 × 4 in downspouts, which move about twice the water of 2 × 3 in.
How many hangers does a gutter need?
One about every 2 ft, and within a foot of every corner and end. Hidden hangers with screws are the modern standard; spikes and ferrules are the older pattern the hangers replaced. Snow and ice country tightens the spacing to 18 in or less.
Do gutters need to slope?
Yes — a gentle fall toward the downspout, about 1/4 in per 10 ft (roughly 1 in per 40 ft run). Too flat and water stands and breeds mosquitoes; too steep and the gutter visibly dives below the roof edge at the low end. Long runs split the difference: high in the middle, sloping to downspouts at both ends.
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