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Roofing

Gutter Size Calculator

Work out whether 5-inch or 6-inch gutters are needed for a roof's drainage load.

Your numbers

sq ft
ft

Result

Recommended gutter size 6-inch K-style with extra downspouts
Effective drainage area 1,540.0 Square feet, pitch-adjusted
Peak flow 64.0 Gallons per minute
Downspouts recommended 3
An effective drainage area of 1,540.0 sq ft at 4 in/hr produces about 64.0 gallons per minute. Use 6-inch K-style with extra downspouts with 3 downspouts.

A simplified sizing guide based on drainage area, pitch and rainfall intensity. Local rainfall data and plumbing code drainage tables should govern the final specification on anything unusual.

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Questions

About this calculator

Are 6-inch gutters worth it?

On steep or large roofs, yes. They carry roughly 40% more water and pair with larger downspouts that clog less. On a small single-storey roof they are usually unnecessary.

Why does roof pitch affect gutter size?

Steeper roofs deliver water to the gutter faster and with more momentum, so the effective load is higher than the flat area suggests.

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