What size gutters do I need?

The short version

This Old House states it plainly on their guide to gutter types: gutters with 5-inch or 6-inch widths handle the rainfall on most houses in most parts of the US, which makes them the optimal gutter size. Their K-style guide says most residential homes use 5-inch K-style gutters, and homes with larger roofs or heavy rainfall might need 6-inch.

Why the size question is not trivial

One inch of rain falling on an average-size roof is a 1,900 gallon torrent off the eaves, a figure This Old House attributes to its own experts. A gutter that cannot carry it does not slightly underperform, it overflows, and the water lands where gutters exist to stop it landing.

That is the whole argument for going up a size rather than down.

The downspout has to match

This is the part most size advice leaves out, and it is where undersized systems actually fail. From This Old House's K-style guide:

A 6-inch gutter feeding a 2-by-3 downspout is a bottleneck with extra steps. If you are being quoted an upgrade to 6-inch gutters, ask whether the downspouts are being upgraded too, and what our downspout page costs out per foot.

Shape changes capacity as much as size does

Two gutters of the same width do not carry the same water. According to This Old House Magazine, as quoted on both their K-style and their gutter types guides, K-style gutters handle more than twice the water volume of half-round gutters of the same width.

So a 6-inch half-round is not an upgrade on a 5-inch K-style in capacity terms, even though it is wider. Our half-round and K-style page works through that trade and what each style costs.

Half-round sizes run from 4 to 8 inches in diameter, with 5 and 6 inches the most popular for homes, according to This Old House's half-round guide. The larger 7 and 8 inch sizes exist for heavy rainfall or steep and large roof areas.

What the size decision costs you

Size is not usually a big line on a gutter quote, but it has one expensive consequence.

This Old House notes on its LeafFilter guide that the average gutter measures 4 to 5 inches, with larger gutters up to 7 inches and narrower ones as small as 3, and that most gutter guards fit standard 5-inch systems, so larger or smaller systems may require a custom installation. They add that the wider the gutters, the more you pay to install guards.

Our reading, marked as ours: that makes 5-inch the cheapest size to own, not just to buy, because it is the one every guard manufacturer builds for off the shelf. If you are choosing between 5 and 6 inches and guards are anywhere in your plans, ask the guard price for both before you decide. Neither source frames the decision that way.

Getting the slope right matters more than one inch of width

A correctly sized gutter hung flat still backs up. This Old House Magazine, quoted on their K-style guide, gives the proper slope as about half an inch for every 10 feet of run.

Their method: mark the high point of the run on the fascia a quarter inch below the metal drip-edge flashing, mark the low end at the downspout, and snap a chalk line between them. And before any of that, check the fascia for rot and repair it first.

How to work out your own footage

This Old House's LeafFilter guide gives the rule as about 100 linear feet per 1,000 square feet of living space, and its own table maps a 2,000 square foot house to 200 linear feet. That 200 feet is the benchmark every per-foot price on this site multiplies against, and our cost per linear foot page sets the published per-foot figures side by side.

If you are thinking of hanging them yourself, the slope is the part that decides whether it works: our do-it-yourself page covers what that takes and what it saves.

What we would ask an installer, and this is ours rather than either source's: what size are you quoting, what size are the downspouts, how many downspouts, and what slope. A quote that answers all four is a quote from someone who has looked at your roof.