GutterPilot › Free tools › Gutter & downspout size

What gutter size and how many downspouts?

Enter the roof footprint, the pitch, and your local rainfall. You get the gutter size and number of downspouts the roof actually needs, using the standard sheet-metal sizing method, not a guess. Undersize the gutter and it overflows in the first hard rain.

SMACNA / IPC method No login, nothing stored Sources shown

The roof

Footprint is the area on the ground the roof covers, not the slope length.

Roof footprint length

Roof footprint width

Roof pitch

Steeper roofs catch more wind-driven rain, so the area is adjusted up (SMACNA Table 1-1).

Region (sets a typical rainfall)

Local rainfall intensity

in/hr

The 5-minute design storm for your area. Look up your exact number on NOAA Atlas 14.

Gutter profile

Half-round holds less water, so it sizes up one step.

Recommended size

gutter for this roof

Roof area it servesfootprint adjusted for pitch

0sq ft

This size handles at your rainfall

0sq ft

Downspouts, 3x4

Downspouts, 2x3if using the smaller profile

How the sizing is figured

The standard sheet-metal method. Every step is shown so you can check it.

Design area = footprint length × width × pitch factor. 40 × 30 × 1.10 = 1,320 sq ft (pitch factors from SMACNA Table 1-1).
Required capacity = design area × rainfall in/hr. Gutter tables are rated at 1 in/hr and scale by dividing by your rainfall. 1,320 × 4 = 5,280
Gutter size = the smallest whose 1 in/hr rating covers that. K-style ratings: 5″ = 5,520, 6″ = 7,960, 7″ = 11,040 sq ft. Half-round holds less: 5″ = 2,500, 6″ = 3,840, 7″ = 5,520.
Downspouts = required capacity ÷ downspout rating. A 3×4 drains 1,200 sq ft and a 2×3 drains 600 sq ft, both at 1 in/hr (one square inch of outlet per 100 sq ft of roof).
Ratings are for a roughly level gutter. Sloping it about 1/8 in per foot adds roughly 40% capacity. Space downspouts no more than about 40 ft apart, 50 ft max.

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