What size gutter do I need for a 40 by 30 foot roof?

That's a 1,200 sq ft footprint. At a 5:12 pitch and a 4 in/hr storm the adjusted area is about 1,260 sq ft, which calls for a 5-inch K-style gutter and two 3×4-inch downspouts.

Is a 5-inch or 6-inch gutter better?

5-inch K-style handles most homes under about 5,520 sq ft of adjusted roof area. Step up to 6-inch for larger areas, steep roofs, valleys that concentrate flow, or regions with hard downpours.

How many downspouts do I need?

About one 3×4-inch downspout per 1,200 sq ft of adjusted roof area, and no more than 40 feet apart. Adding a downspout is almost always cheaper than upsizing the whole gutter.

Do half-round gutters hold as much as K-style?

No — a half-round carries roughly 20–30% less than a K-style of the same nominal width. If you want half-round for the look, size up one step.

What rainfall intensity should I use?

The 5- to 15-minute peak for a roughly 10-year storm in your area. It ranges from about 2 in/hr in the arid West to 8+ in/hr along the Gulf Coast. Local building departments and NOAA Atlas 14 publish the figures.

Does gutter slope matter?

Yes. Going from 1/16 to 1/8 inch of fall per foot moves noticeably more water through the same trough. If your size lands right on a threshold, adding slope is cheaper than a bigger profile.

Gutter Size Calculator

Gutters fail for one boring reason: they were sized for the roof’s footprint instead of the water the roof actually delivers during a downpour. Undersized gutters overshoot at the eaves, dump water against the foundation, and pull fascia apart under the weight of an ice dam. Sizing them takes four numbers: how much roof drains into the run, how steep it is, how hard it rains where you live, and how many downspouts you’re willing to hang.

The gutter sizing formulas

and are the roof plane’s horizontal footprint, is the pitch factor (1.00 flat–3:12, up to 1.30 at 12:12+), and is the local peak rainfall intensity in in/hr. normalizes to the 4 in/hr basis of published capacity tables; is the peak runoff in gallons per minute, since 1 in/hr on 1 sq ft is about 0.0104 gpm.

Worked example

The calculator’s defaults are a 40 ft × 30 ft roof plane at a 5:12 pitch (factor 1.05), in a 4 in/hr storm.

Downspouts: .

Step Five: Match the size and count the downspouts

4-inch K-style: up to 1,000 sq ft adjusted
5-inch K-style: up to 5,520 sq ft adjusted
6-inch K-style: up to 7,960 sq ft adjusted
Above that: 7-inch or commercial box gutter

Downspouts = adjusted area ÷ 1,200 sq ft, rounded up

1,260 sq ft adjusted → 5-inch K-style gutter
Downspouts = 1,260 ÷ 1,200 = 1.05 → 2 downspouts

Gutter Size Calculator worked example diagram — A visual read of the worked example above — the same numbers the calculator returns for its default inputs.

What your results mean

The recommended size is the smallest standard gutter profile that can carry your peak flow without overtopping, assuming a normal 1/16-inch-per-foot slope and clean troughs. The adjusted area is the number to compare against any manufacturer capacity table, since it already accounts for pitch and rainfall. The flow rate tells you what the downspouts have to swallow: a 2x3-inch downspout handles roughly 600 sq ft of adjusted area, a 3x4-inch handles about 1,200.

When to size up

Bump one size when the roof drains through valleys that concentrate water, when the gutter run exceeds 40 feet between outlets, when tree cover means chronic partial blockage, or when a second roof drains onto the one you measured. In snow country, wider gutters with hidden hangers also survive ice loads far better than narrow ones.

Slope is free capacity

Increasing pitch from 1/16 to 1/8 inch per foot moves noticeably more water through the same trough. If your calculated size lands right at a threshold, adding slope is cheaper than rebuilding in a larger profile.

Which roof area drains to which gutter

Use only the roof plane that sheds water into the gutter you’re sizing. On a simple gable roof with a 40-foot ridge and 30 feet of total depth, each side drains a 40 × 15 ft plane into its own gutter, so each gutter carries half the roof. Hip roofs and roofs with valleys need each plane measured separately, and a valley that dumps two planes into one corner is the usual reason a gutter overflows at one spot.

Setting the slope in practice

The usual minimum slope is about 1/16 inch per foot. Over a 40-foot run that’s inches of total drop, and 5 inches at 1/8 inch per foot. On long runs, many installers slope both ways from the middle to a downspout at each end, which halves the drop needed and the distance the water travels.

Common mistakes

Sizing from the house footprint. Measure the roof plane draining to that gutter, and apply the pitch factor.
Using average rainfall. Gutters must handle short, intense bursts. Use the local peak intensity, not the yearly total.
Too few downspouts. A gutter sized correctly can still overflow if all the water has to travel to a single outlet.
Forgetting the upper roof. Water running off a higher roof onto a lower one adds to the lower gutter’s load.

About the formula: Effective roof area is the horizontal roof footprint multiplied by a pitch factor, since steeper roofs catch more wind-driven rain. That area is normalized to a 4 in/hr rainfall basis so it can be compared against standard K-style gutter capacity limits.