Gutter and Downspout Size Calculator

Size gutters and downspouts from roof area and rainfall, then price the run. Built for gutter installers and handy homeowners.

How the sizing works

Flow in gallons per minute equals rainfall in inches per hour times drainage area times 0.0104, the factor used in the International Plumbing Code. Drainage area is the flat footprint of roof that sheds into this gutter, scaled up for steep pitches.

A 5 inch K style gutter at a 1/8 inch per foot slope is treated as carrying about 74 gallons per minute and a 6 inch about 120. Those are planning figures; check your manufacturer's table. Downspout count is the larger of flow divided by downspout capacity and run length divided by 50 feet.

Common mistakes

Undersized downspouts cause more overflow than undersized gutters. Valleys dump concentrated water at one spot, so place a downspout near them. In snow country, hangers every 24 inches or less keep gutters from tearing off under ice.

Questions

How many downspouts do I need?

Divide the roof area draining to the gutter by the area one downspout can handle at your design rainfall. A 2x3 downspout carries about 30 gallons per minute and a 3x4 about 92. Also keep each downspout to about 50 feet of gutter.

5 inch or 6 inch gutters?

5 inch K style handles most homes. Large or steep roofs and areas with heavy rain bursts often need 6 inch. The calculator checks capacity against your rainfall.

What rainfall number should I use?

Use the 5 minute rainfall intensity for a 10 year storm from NOAA Atlas 14 or your local code. Many inland mountain towns fall between 2 and 4 inches per hour.

Does roof pitch matter?

Codes size from the horizontal roof area. Steep roofs catch more wind driven rain, so many installers add 10 to 30 percent for steep pitches, which this tool does.