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Gutter Calculator
Use this free gutter calculator to size gutters and downspouts for your roof before you get quotes. Enter the roof area or the roof length and width, pick the roof pitch and your local rainfall intensity, and the calculator recommends a 5 inch or 6 inch K-style gutter, works out how many downspouts you need, and estimates the installed cost. The sizing follows standard published K-style gutter capacity charts, so the recommendation matches what a gutter installer would use as a starting point.
| Item | Amount |
|---|---|
| Effective roof area | 1,590 sq ft |
| Recommended gutter | 5-in K-style |
| Gutter runs | 60 ft total |
| Downspouts needed | 2 (max 30 ft apart) |
| Estimated installed cost | $390 – $840 |
Capacity basis: standard K-style gutter charts — a 5-in gutter handles about 5,500 sq ft and a 6-in gutter about 7,900 sq ft of roof at 5 in/hr rainfall; capacity scales inversely with rainfall intensity. Figures are planning estimates — confirm with local code and a gutter installer.
How it works
The calculator starts with the effective roof area, which is the area the gutters must actually drain. If you enter the roof area directly, it is multiplied by the pitch factor for your roof: a flat or low-slope roof uses 1.00, a 4 in 12 pitch uses 1.06, a 6 in 12 pitch uses 1.12, an 8 in 12 pitch uses 1.20, and a steep 12 in 12 pitch uses 1.41. A steeper roof has more surface and sheds water faster, so it counts as a bigger roof for gutter sizing. A 1,500 square foot roof at a 4 in 12 pitch has an effective area of 1,500 × 1.06 = 1,590 square feet. If you do not know your pitch, measure the rise over a 12 inch run, or use 1.06 for a typical suburban roof.
Gutter size is chosen from published K-style capacity charts. At a rainfall intensity of 5 inches per hour, a 5 inch K-style gutter handles about 5,500 square feet of roof, and a 6 inch K-style gutter handles about 7,900 square feet. Capacity scales inversely with rainfall intensity: at 7 inches per hour, the 5 inch gutter handles about 5,500 × 5 ÷ 7 = 3,929 square feet. The calculator compares your effective roof area with the capacity at your rainfall intensity and picks the smallest gutter that can handle it. If even the 6 inch gutter is too small, it says so and recommends extra downspouts and a professional review, because no standard residential gutter will do the job alone.
Rainfall intensity is the design rain rate for your area, usually the 5-minute or 10-minute storm intensity from local plumbing or building code tables. Five inches per hour is a common design value for much of the country, while Gulf Coast and Florida locations often design for 7 to 8 inches per hour, and dry western areas for 3 to 4. If you are unsure, check your local code or use 5 inches per hour as a middle value, and go higher if your gutters have overflowed in past storms.
Downspouts are sized two ways, and the calculator takes the larger count. The drainage rule is that a standard 2 by 3 inch downspout drains about 800 square feet of roof at 5 inches per hour, and a 3 by 4 inch downspout drains about 1,200 square feet, with both numbers scaling inversely with rainfall intensity. The spacing rule is one downspout per 40 feet of gutter at most, and at least two downspouts per house, so a single clog cannot flood the whole system. A 60 foot gutter needs at least 2 downspouts by spacing, and a 1,590 square foot roof needs 2 by drainage, so the answer is 2. On a long run, put a downspout at each end rather than both at one end, and slope the gutter about 1/4 inch per 10 feet toward each downspout.
Cost is the installed price per linear foot of gutter plus a per-downspout installed price of about $75–$150 each. Seamless aluminum is the most common choice at about $4–$9 per foot for 5 inch and $6–$12 for 6 inch. Steel runs about $8–$15 and is stronger but can rust, while copper runs about $12–$25 and lasts decades but costs several times more. Gutter guards, which keep leaves out, typically add $7–$12 per foot and are not included here. Prices vary by region and by how high and awkward the roof is, so get two or three local quotes before committing.
Capacity at your rain = chart capacity × 5 ÷ rainfall (in/hr)
Downspouts = larger of ⌈area ÷ per-downspout⌉ and ⌈gutter length ÷ 40⌉, minimum 2
Worked example
You have a 1,500 square foot roof at a 4 in 12 pitch, a design rainfall of 5 inches per hour, and 60 feet of gutter in 5 inch aluminum with 2 by 3 inch downspouts.
Step 1: Effective area — 1,500 × 1.06 = 1,590 square feet.
Step 2: Peak flow — 1,590 × 5 × 0.0104 = 83 gallons per minute at the peak of the storm.
Step 3: Gutter size — a 5 inch K-style gutter handles about 5,500 square feet at 5 in/hr, which is far above 1,590, so the 5 inch gutter is adequate and the 6 inch is not needed.
Step 4: Downspouts by drainage — 1,590 ÷ 800 = 1.99, rounded up to 2.
Step 5: Downspouts by spacing — 60 ÷ 40 = 1.5, rounded up to 2, and the minimum is 2.
Step 6: Cost — 60 feet × $4–$9 = $240–$540 for the gutter, plus 2 downspouts × $75–$150 = $150–$300, for a total of $390–$840 installed.
The answer is a 5 inch K-style gutter with 2 downspouts, at an estimated installed cost of $390–$840. If the same roof were 6,000 square feet, the 5 inch gutter would be too small and the calculator would recommend the 6 inch gutter instead.
FAQ
What size gutter do I need, 5 inch or 6 inch?
A 5 inch K-style gutter is enough for most homes: at a rainfall intensity of 5 inches per hour it handles about 5,500 square feet of roof area. A 6 inch K-style gutter handles about 7,900 square feet at the same intensity. Choose the 6 inch gutter if your roof area is above about 5,500 square feet, if your area sees rainfall over 5 inches per hour, or if you have long gutter runs with few downspouts.
How many downspouts do I need?
Use at least one downspout per 40 feet of gutter, and at least two downspouts per house so one clog does not flood the whole system. A standard 2 by 3 inch downspout drains about 800 square feet of roof at 5 inches per hour of rain, and a 3 by 4 inch downspout drains about 1,200 square feet. This calculator takes the larger of the spacing count and the drainage count, so both rules are satisfied.
How is roof area calculated for gutters?
Start with the roof footprint, length times width, then multiply by a pitch factor, because a steep roof sheds water faster and has more surface. The factors used here are 1.00 for a flat or low-slope roof, 1.06 for a 4 in 12 pitch, 1.12 for 6 in 12, 1.20 for 8 in 12, and 1.41 for a steep 12 in 12 pitch. A 1,500 square foot roof at a 4 in 12 pitch has an effective area of about 1,590 square feet.
How much do gutters cost?
Seamless aluminum gutters typically cost about 4 to 9 dollars per linear foot installed for a 5 inch K-style gutter, and about 6 to 12 dollars per linear foot for a 6 inch gutter. Downspouts typically add about 75 to 150 dollars each installed. A 60 foot run with two downspouts in 5 inch aluminum therefore runs about 390 to 840 dollars installed, before gutter guards.
How far apart should downspouts be?
Downspouts should be no more than 40 feet apart along a gutter run, and closer is better on steep or large roofs. A gutter run longer than 40 feet needs a downspout at each end or one in the middle, and the gutter should slope about 1/4 inch per 10 feet toward each downspout so water does not pool.