K-style gutter & downspout capacity chart

Gutter and downspout capacities are stated as the roof drainage area they handle at 1 in/hr of rainfall. Multiply your plan area by the roof-pitch factor and your local rainfall intensity, then compare. Use the gutter size calculator for your own rainfall figure.

K-style gutter capacity (drainage area at 1 in/hr)

GutterArea at 1 in/hr
5" K-style5,520 sq ft
6" K-style7,960 sq ft

Downspout capacity (drainage area at 1 in/hr)

DownspoutArea at 1 in/hr
2×3 in600 sq ft
3×4 in1,200 sq ft

Roof-pitch drainage factor

Roof pitchFactor
Flat to 3/12×1.00
4/12–5/12×1.05
6/12–8/12×1.10
9/12–11/12×1.20
12/12+×1.30

Adjusted area = plan area × pitch factor × rainfall intensity (in/hr). Values are industry/SMACNA-derived.

Related calculators

Gutter SizeDecide between 5-inch and 6-inch K-style gutters from the roof drainage area, the roof pitch factor and your local rainfall intensity in inches per hour.Downspout CapacityFind how many 2x3-inch or 3x4-inch downspouts a roof needs from its adjusted drainage area, using industry outlet capacities at one inch per hour.Gutter LengthAdd up your eave runs to get the total gutter footage and the number of downspouts you need, at one outlet per 30 to 40 linear feet of gutter.