Gutter Size Calculator

Most houses get 5-inch gutters, and most of the time that's right. But a gutter doesn't overflow because the house is long. It overflows because too much roof pours into one stretch of it during a cloudburst, and that depends on the size of the roof, which way it slopes and how hard it rains where you live.

This sizes them from three things you already know: the ground-floor footprint, whether the roof is a gable or a hip, and your region. You get the gutter size, how many downspouts and where they go, the downspout size to buy, and how full the gutters run in your area's hardest rain.

More downspouts or bigger gutters. On a big roof or in a stormy region, a 5-inch gutter can still work if you add downspouts, so each one drains a shorter stretch. The calculator only moves you up to 6-inch when it saves downspouts, and tells you what 5-inch would take instead.

Split the roof between its gutters. The calculator treats the house as a rectangle about twice as long as it is deep. A gable roof sheds water to its two long sides, so each long gutter takes half the roof. A hip roof slopes down on all four sides, so one gutter runs all the way around and takes all of it. Pitched roofs catch more wind-driven rain than their footprint, so the area is raised by 10 percent for an average-pitch roof.

Size for the hardest few minutes of rain. Gutters are sized for the most intense 5-minute burst your area sees about once in 10 years, measured in inches per hour. Along the Gulf Coast and in Florida that's about 10 inches an hour; across the East, South, Midwest and Plains about 8; in the West about 5. A level 5-inch K-style gutter can carry the runoff from 5,520 sq ft of roof in a 1-inch-an-hour rain, and a 6-inch gutter 7,960 sq ft, about 45 percent more. Divide by your area's rate and you have how much roof one stretch of gutter can take.

Count the downspouts. Each run gets enough downspouts that no stretch carries more than that, with no downspout serving more than 50 feet of gutter. Each square inch of downspout drains about 100 sq ft of roof, so a 2-by-3-inch downspout handles 600 sq ft and a 3-by-4-inch one 1,200. The smaller size is used when it doesn't add downspouts. Then 6-inch gutters with 3-by-4 downspouts are tried, and win only if they need fewer.

Worked example. A 2,000 sq ft ranch with a gable roof in the East is about 63 by 32 feet. Raised 10 percent, that's 2,200 sq ft of roof, 1,100 on each long side. Each 63-foot side needs 2 downspouts, so each drains 550 sq ft, under the 600 a 2-by-3 handles. A 5-inch gutter can take 690 sq ft a stretch at 8 inches an hour, so 550 fills it to about 80 percent. That's 5-inch gutters and 4 downspouts.

House length = 2 × √(footprint ÷ 2); roof area = footprint × 1.1. Gable: two runs, each the house length and half the area. Hip: one loop of 2 × (length + depth) with all of it.

Downspouts per run = the largest of: 1; run length ÷ 50 ft; run area × rain rate ÷ gutter capacity; run area ÷ downspout capacity, each rounded up. Gutter capacity at 1 in/hr: 5-inch 5,520 sq ft, 6-inch 7,960. Downspout capacity: 2 by 3 inch 600 sq ft, 3 by 4 inch 1,200. Rain rate: Gulf Coast or Florida 10 in/hr, East, South, Midwest or Plains 8, the West 5.

6-inch gutters only when their total downspouts are fewer than 5-inch's. How full = roof per downspout × rain rate ÷ gutter capacity.

2,000 sq ft gable, East: 1,100 sq ft a side, 2 downspouts a side of the 2 by 3 inch size, 5-inch gutters about 80% full. 3,200 sq ft gable, East: 1,760 sq ft a side; 5-inch would need 3 downspouts a side, 6-inch needs 2, so 6-inch gutters with 4 downspouts, about 90% full.

📎Sources:NOAA — Precipitation Frequency Data Server (Atlas 14 rainfall intensities),This Old House — How To Size Gutters and Downspouts (gutter and downspout capacities),Berger Building Products — Proper Gutter and Downspout Sizing (pitch factors, downspout spacing)