Gutter Calculator

Follow these steps to size the gutters and downspouts for one side of your roof.

1. Enter the roof length in feet. This is the length of the eave where the gutter will run.
2. Enter the roof width in feet. Measure the horizontal distance from the ridge to the eave on that one side, not the sloped rafter length.
3. Select the roof pitch from 3/12 to 12/12. Steeper roofs shed water faster, so the calculator adds a small allowance.
4. Enter the rainfall intensity in inches per hour. Use your local 10-year, 5-minute rate if you know it. About 2 in/hr is a common planning figure for the United States.
5. Choose the gutter style, K-style or half-round. The recommended size, drainage area, flow rate and downspout count update right away.

Treat the output as a planning figure. Your installer or local code office makes the final call on size and downspout placement.

Gutter sizing is the process of matching the width of a rain gutter and the number of downspouts to the amount of water your roof sheds in a heavy storm. A gutter that is too small overflows, which can soak siding, rot fascia boards and erode soil beside the foundation. A gutter that is far larger than needed costs more and may look out of scale on the house.

Three things drive the answer. The first is the roof area that drains to the gutter. The second is how hard it rains, measured as an intensity in inches per hour. The third is the gutter itself, because its shape and width set how much water it can carry before it spills over the front edge.

Two gutter styles dominate home building. K-style gutters have a flat back and a front profile that resembles crown molding. Half-round gutters have a smooth, curved trough, like a pipe cut in half lengthwise. For the same width, a K-style gutter holds more water than a half-round gutter, so the style changes the size you need.

The calculator finds the roof area that drains to the gutter, adjusts it for pitch, and then checks that area against the capacity of each gutter size at your rainfall rate.

Step by step:

1. The pitch factor is 1.00 for pitches below 5/12, 1.05 for 5/12 through 8/12, and 1.10 for steeper roofs.
2. The roof length is multiplied by the roof width and the pitch factor to get the adjusted drainage area.
3. The calculator holds a table of the largest area each gutter can serve at a rainfall of 1 in/hr. K-style is 5,520 sq ft for a 5-inch gutter and 7,960 sq ft for a 6-inch gutter. Half-round is 2,500 sq ft and 3,840 sq ft.
4. Each capacity is divided by your rainfall intensity. Doubling the rainfall halves the area a gutter can serve.
5. The smallest gutter whose adjusted capacity covers the drainage area is recommended. If neither size is enough, the result says that more runs or downspouts are needed.
6. The flow rate comes from the standard rule that 1 inch of rain per hour on 96 square feet is about 1 gallon per minute.
7. The downspout count divides the drainage area by the area one downspout can serve at your rainfall, then rounds up.

A house has a 50 ft eave, a 20 ft run from ridge to eave, a 6/12 pitch and K-style gutters. The local rainfall rate is 2 in/hr.

1. Pitch factor: 6/12 falls in the 5/12 to 8/12 band, so the factor is 1.05
2. Adjusted area: 50 x 20 x 1.05 = 1,050 sq ft
3. 5-inch K-style capacity: 5,520 / 2 = 2,760 sq ft
4. Check: 1,050 is below 2,760, so a 5-inch K-style gutter is enough
5. Flow rate: 1,050 x 2 / 96 = 21.9 GPM
6. Downspout capacity: a 2×3 inch downspout serves 600 / 2 = 300 sq ft at this rainfall
7. Downspouts: 1,050 / 300 = 3.5, which rounds up to 4

The result is a 5-inch K-style gutter with 4 downspouts of 2×3 inch size. If the same roof sat in a region with 4 in/hr of rain, the 5-inch capacity would drop to 1,380 sq ft, which still covers 1,050 sq ft, but the downspout count would rise to 7. That shows why downspouts, not gutter width, often become the limit in heavy-rain areas.

Roof Area

The larger the roof area draining to a gutter, the more water it must carry. A long eave on a wide roof sends far more water to the gutter than a short eave on a narrow one. Splitting a long run into two sections that drain to separate downspouts lowers the load on each.

Rainfall Intensity

The rainfall rate is the biggest swing factor. A gutter sized for 2 in/hr has only half the margin at 4 in/hr. Use a local design rate rather than annual rainfall, because gutters fail in short, intense bursts, not in steady drizzle.

Roof Pitch

Steeper roofs shed water faster and with more speed, which can make water shoot over the front lip of a gutter. The pitch factor in this tool adds a modest allowance for that effect.

Gutter Style and Width

A 6-inch gutter carries more than a 5-inch gutter, and a K-style gutter carries more than a half-round gutter of the same width. Moving up one width is often the simplest way to add capacity.

Downspout Size and Count

A gutter can only drain as fast as its downspouts allow. Larger downspouts and more of them raise the real capacity of the system, and they keep water from backing up and spilling over.

Valleys, Slope and Debris

Roof valleys concentrate water at one point, and a gutter that is not pitched toward the downspout will hold water. Leaves and shingle grit reduce the working capacity of any gutter.

No prices are stated here because gutter material and labor costs change by region and by season. Ask for written quotes on both styles and compare the installed price per foot.

Long, Wide Roof Sections

If the calculator recommends a 6-inch gutter, use a 6-inch gutter. Large roof planes put a heavy load on a single run, and a wider trough gives you margin.

Heavy-Rain Regions

Areas with intense thunderstorms or tropical rain need bigger gutters and more downspouts. A rainfall rate of 4 in/hr or more can overwhelm a standard 5-inch gutter on a roof that works fine at 2 in/hr.

Steep Roofs and Valleys

Steep roofs and valleys speed up and concentrate runoff. A 6-inch gutter is a common upgrade below a valley or under a steep roof plane.

Large Trees Nearby

Debris cuts the working capacity of a gutter. A wider trough, paired with leaf guards, gives extra room before clogs cause overflow.

Protecting a Foundation or Entry

Where overflow would hit a doorway, a walkway or a basement window well, choose a larger gutter and add a downspout. The extra cost is small next to the cost of water damage.

1. Using the Sloped Rafter Length as the Width

Enter the horizontal distance from the ridge to the eave. The pitch factor already accounts for the slope, so using the sloped length counts the slope twice.

2. Guessing the Rainfall Rate

Annual rainfall is not the same as storm intensity. Look up a local design rate for a 5-minute storm with a 10-year return period, because that is the figure that decides whether a gutter overflows.

3. Sizing Only the Gutter and Ignoring the Downspouts

A wide gutter with too few downspouts still overflows. Check the downspout count as carefully as the gutter width.

4. Mixing Roof Sections

Run the calculation for each section that drains to its own gutter. Adding two separate sections together overstates the load on any single run.

5. Forgetting Slope and Placement

A gutter needs a slight pitch toward each downspout, and downspouts should sit near the ends or at low points. A correctly sized gutter that is hung level will still hold water.

The arithmetic is exact for the numbers you enter. The limits come from the simplified capacity table and the rule-of-thumb pitch factor, not from the math.

Most homes use 5-inch K-style gutters, and larger roofs, steep roofs and heavy-rain areas often need 6-inch gutters. Enter your roof size and local rainfall rate in the calculator for a specific recommendation.

Multiply the eave length by the horizontal distance from the ridge to the eave, then by a pitch factor. For a 50 ft by 20 ft section at 6/12 pitch, the factor is 1.05 and the area is 1,050 sq ft.

Use your local 10-year, 5-minute rainfall rate, which is the rate a gutter must handle in a short heavy storm. About 2 in/hr is a common planning figure for the United States, but many regions are higher.

It depends on the drainage area and the rainfall. This calculator divides the area by what one downspout can serve and rounds up. The typical 50 ft by 20 ft example at 2 in/hr needs 4 downspouts of 2×3 inch size.

A 6-inch gutter carries more water, so it is better for large roofs, steep pitches and heavy rain. A 5-inch gutter is enough for many ordinary roofs and costs less.

K-style gutters have a flat back and a decorative front, and they hold more water for the same width. Half-round gutters have a curved trough, suit traditional homes and carry less.

Steeper roofs shed water faster and can throw it over the lip of the gutter. This calculator adds a pitch factor of 1.05 for 5/12 through 8/12 and 1.10 for steeper roofs.

GPM is gallons per minute. It is the peak runoff from your roof at the rainfall rate you entered, using the rule that 1 inch of rain per hour on 96 square feet is about 1 gallon per minute.

Use it for each simple roof section on its own. Valleys, dormers and roofs that drain onto other roofs concentrate water, so have an installer or designer review those layouts.

No. It is a planning tool based on published gutter capacity methods. The building code and your local rainfall data decide the final design, so confirm with your code office or installer.

No. The calculator runs in your browser and nothing you enter is stored or sent anywhere.

• SMACNA Gutter Sizing Calculator (Official tool based on the Architectural Sheet Metal Manual 7th edition.)
• This Old House – Gutter Sizes (Gutter capacity tables by style and size.)
• Fine Homebuilding – Sizing Gutters (Step-by-step gutter sizing methodology.)
• IPC Table 1106.2 Gutter Sizing (Code-based gutter and leader sizing tables.)