Drain safely.

Gutters are sized to your roof, not your house

Here is the thing most people get backward. The size of gutter you need has almost nothing to do with how big your house is. It comes down to how much roof drains into each run of gutter, how steep that roof is, and how hard it rains where you live. A small house with a steep roof in Florida can need bigger gutters than a sprawling ranch with a low roof in Oregon. Water does not care about your square footage. It cares about how fast it arrives.

A gutter system is simple once you see it laid out. Rain runs off the roof, hits the drip edge, and drops into the gutter hung off the fascia board. The gutter slopes gently toward an outlet, the outlet feeds a downspout, and the downspout carries the water down the wall and away from the foundation. The calculator up top sizes every piece of that from your roof numbers. The diagram below shows how it all fits together.

How to measure your gutters

Start with a tape measure and the eaves. Gutters run along the eaves, the low horizontal edges of the roof where the water sheets off. They do not run up the gable ends, the sloped edges. So the linear feet of gutter you need is the total length of all the eaves you want to protect, not the perimeter of the whole house.

For a plain rectangular house, that is usually the two long sides. A 30 by 40 foot ranch with gutters on both long eaves needs about 80 feet of gutter. Put gutters on all four sides and it is closer to 140. Measure each eave at ground level along the wall below it, since the gutter length matches the wall length underneath. Add it all up and that is your gutter run. The calculator turns that number into hangers, end caps, and outlets automatically.

The other number you need is roof area, and specifically the area that drains into your longest single run of gutter. That is what sizes the gutter itself. A gutter is only as good as its ability to carry away the water from the worst section, so the calculator sizes to the busiest run, not the average.

Five inch or six inch: what size gutter you need

Almost every house in America runs 5-inch K-style gutters. They are the default at every home center, they handle the great majority of residential roofs, and they are what a contractor hangs unless something about your roof says otherwise. The question is really just: does your roof push enough water to need the bigger 6-inch size?

Why the two multipliers matter: a steep roof throws water into the gutter faster than a shallow one, so the gutter has to move it faster, which is the same as needing more capacity. And rain intensity is regional. The Gulf Coast and Florida see cloudbursts that drop 9 inches an hour in the worst five minutes of a storm, while the Pacific Northwest rains constantly but gently and rarely tops 5 inches an hour. Same roof, different gutter, purely because of the sky above it. When you are on the line between sizes, go bigger. Six-inch gutters cost only a little more per foot and the extra capacity is cheap insurance against the one storm a year that overwhelms an undersized run.

Slope: the part everyone forgets

A gutter is not level. If you hang it dead flat it will hold standing water, breed mosquitoes, sag under the weight, and eventually pull off the fascia. Gutters need a slope, sometimes called the fall, that tips them toward the downspout so water actually moves. The standard is about a quarter inch of drop for every 10 feet of run.

That is subtle enough that you cannot eyeball it, but it adds up. On a 40-foot run, a quarter inch per 10 feet means the downspout end sits a full inch lower than the far end. Chalk a line from your high point down to the outlet before you hang a single bracket, because fixing the slope after the gutter is up means taking the whole run back down. On a very long eave, contractors often slope from a high point in the middle down to a downspout at each end, so no single stretch has to fall too far and drop the gutter awkwardly below the roofline.

Downspouts: how many, and where they go

Downspouts are the limiting part of the whole system. You can hang perfect gutters at a perfect slope and still get water pouring over the front edge in a hard rain if you did not give it enough places to drain. The baseline rule is one downspout for every 40 feet of gutter run, and more if the roof area feeding that run is large. A 120-foot run of gutter needs three downspouts minimum, and often four.

Placement matters as much as count. Downspouts go at the low ends of each sloped run, which is where the water is heading anyway. Then the discharge has to go somewhere sensible. Never aim a downspout at the foundation, at a walkway that sends the water back toward the house, or onto a neighbor's property. Get it 4 to 6 feet away from the foundation with an extension or a buried drain line. A downspout dumping against the foundation is one of the most common causes of a wet basement, and it is entirely avoidable.

Hangers, and why spacing is not optional

Hangers are the brackets that hold the gutter to the fascia. Space them 24 to 32 inches apart in most of the country. The calculator uses 32-inch spacing, which is standard for climates that do not get heavy snow. If you live where snow piles up on the roof and slides into the gutter, tighten that to 16 to 20 inches. A gutter full of wet snow and ice is astonishingly heavy, and widely-spaced hangers are exactly how a gutter ends up hanging off the house by one bracket in February. Hangers cost a few dollars each, so tighter spacing adds almost nothing to the bill and saves you a spring repair.

Materials and what they actually cost

The material you pick sets both the price and how long the gutters last. Here is the honest comparison, installed, in 2026 dollars.

Seamless aluminum earns its place at the top. It is formed on site from a continuous coil, so a 40-foot run is one unbroken piece with no seams to leak. That single fact matters more than anything else, because the seams are where roughly all gutter leaks start. Sectional gutters, the 10-foot snap-together pieces from the home center, are the DIY-friendly option and they work fine, but every joint is a future leak waiting for its moment.

Where gutter jobs go wrong

The failures repeat, and they are all preventable. Hanging the gutter flat with no slope tops the list, because it looks fine on install day and only reveals itself as standing water and sagging months later. Too few downspouts is next, and it shows up as water sheeting over the front edge in the first hard storm. Then there is the downspout that dumps straight at the foundation, quietly filling a basement one rain at a time.

The rest are material and spacing mistakes. Vinyl in a freezing climate, which cracks. Hangers spaced too far apart, which pull loose under snow. And undersizing the gutter to save a few dollars a foot, then watching it overflow every time the sky really opens up. None of these are hard problems. They are planning problems, which means getting the numbers right before you buy solves almost all of them. If you are replacing gutters as part of a roof replacement, do both at once and let the roof replacement planner chain the quantities together. If you are steep-roof or snow country, the snow load calculator tells you what that roof is really carrying, and for capturing the runoff instead of just shedding it, the rainwater calculator sizes a tank from the same roof area.