Construction

Gutter & Downspout Calculator

Undersized gutters are why people end up with rot behind the fascia, washed-out flowerbeds, and water in the basement. Oversized gutters and downspouts cost more without performing better. This calculator takes your eave length and the roof area draining into that gutter run and returns the linear feet of gutter, number of downspouts, end caps, and hangers you need — sized to actually move the water in a hard summer thunderstorm without overflowing.

Gutter LF needed

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Downspouts: —
End caps: —
Hangers (32 in oc): —

How the math works

Gutter sizing comes down to two numbers: linear feet of gutter (matches your eave length, plus a bit for corners) and downspout count (one downspout per ~600 sq ft of roof area for 5" K-style with 2"x3" downspouts).

Gutter LF = eave length, rounded up to the next 10-ft section. Buy in 10-ft sticks (sectional) or have seamless run on-site.
Downspouts = roof area ÷ 600, minimum 2 per continuous gutter run. A 1,500 sq ft single-story ranch needs 3 downspouts; a 3,000 sq ft two-story needs 5-6 across multiple roof planes.
End caps = 2 per gutter run (one at each end), plus corner pieces (mitered or strip miters) at each inside or outside corner.
Hangers = one every 32" on-center, or every 24" in snow-load regions. A 40-ft gutter run needs about 15 hangers at 32" oc; 20 in heavy-snow country.

For high-flow areas (steep roofs, heavy rain regions, big roof areas dumping into one gutter) upsize to 6" K-style gutter with 3"x4" downspouts — it moves about 40% more water per linear foot. One 6" downspout handles roughly 1,200 sq ft of roof.

How to use this calculator

Measure eave length in feet — just the straight runs that need gutter. Skip gable ends (no gutter goes on a gable).
Estimate roof drainage area in sq ft. For a simple gable roof, half the footprint goes to each eave. For a hip roof, divide the total roof area by 4.
The calculator returns gutter linear feet, downspout count, end caps, and hangers (sized at 32" on-center).
For each separate gutter run (front, back, side), run the math separately and add the parts together.

Common scenarios

1,400 sq ft single-story ranch, gable roof, 50-ft front and back eaves. Each eave drains 700 sq ft. Front gutter: 50 LF, 2 downspouts (round up from 1.2 to meet the 2-minimum), 2 end caps, 19 hangers. Back gutter same. Total: 100 LF gutter, 4 downspouts, 4 end caps, 38 hangers.

2,400 sq ft two-story colonial, complex hip roof. Total eave perimeter 160 ft across multiple roof planes. Total roof area drained ~2,800 sq ft. 5 downspouts (preferably 3"x4" since the upper-floor roof dumps onto the lower-floor roof). Plan 160 LF of 6" K-style, 8-10 end caps for the various roof breaks, and 60 hangers.

Detached 24x24 garage with gable roof. 24 ft eaves each side, ~290 sq ft drained per side. 24 LF gutter each side, 2 downspouts per run (minimum), 2 end caps, 9 hangers. Total: 48 LF, 4 downspouts.

FAQ

5-inch or 6-inch gutter? +

5" K-style is the residential default and handles roughly 5,500 sq ft of roof when paired with 2"x3" downspouts. 6" K-style with 3"x4" downspouts handles around 7,800 sq ft — use it on big two-story houses, steep roofs, or wet climates. The price difference is 25-40%, mostly worth it on the larger house.

Seamless vs. sectional gutters? +

Seamless gutter is run on-site by a contractor with a portable forming machine and has zero joints along its length — just at corners and downspouts. It leaks less and lasts 25-30 years. Sectional gutter is 10-ft sticks you join with slip connectors; DIY-friendly but the joints fail in 8-15 years. Most homeowners hire seamless ($8-15/LF installed); sectional makes sense for outbuildings, sheds, and short detached-garage runs.

Aluminum, copper, or steel? +

Aluminum is the standard — cheap, won't rust, paints up nicely, lasts 20-30 years. Galvanized steel costs less but rusts at scratches and seam joints; not worth the savings. Copper looks gorgeous, lasts 50+ years, and runs 4-6x the price of aluminum. Vinyl is a non-starter for anything but the smallest sheds — it sags and cracks in cold.

How far should downspouts discharge from the foundation? +

At least 6 ft. Most water-in-the-basement problems are downspouts dumping right at the foundation. Use 6-ft or longer extensions, splash blocks angled away from the house, or buried drain tile leading to daylight (best option). The water has to land somewhere that drains downhill away from the foundation.

What about gutter guards — worth it? +

Mixed bag. Foam inserts are useless. Mesh screens ($1-3/LF) work well for pine needles and big leaves but pollen and seed pods still wash in. Reverse-curve guards ($8-15/LF installed) work well but cost more than the gutter. If you have heavy tree cover, gutter guards plus an annual cleaning beat cleaning 4x a year.

What pitch should the gutter run at? +

1/4 inch of fall per 10 ft of gutter — enough that water flows, gentle enough that the slope isn't visible from the ground. For runs over 35 ft, slope from both ends down toward a center downspout so the visible slope on each end is half. Long single-slope runs look crooked.

Where do hangers go? +

Hidden hangers (the modern standard) screw through the back of the gutter into the fascia board behind, every 32" on-center, every 24" in snow regions. Old-style spike-and-ferrule looks ugly and tends to back out; modern installs all use hidden hangers. Buy them stamped with the gutter style (K-style vs half-round).

Do I need to remove old gutters before installing new? +

Yes — you can't sister new gutter to old in any reliable way. Tear-off is straightforward (4-6 hours for a typical house) and lets you inspect the fascia for rot. Replace any rotted fascia sections before mounting new gutter; the screws can only hold in solid wood.