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Calculation Machine

Gutter Size Calculator

Pick 5 in or 6 in gutters from roof area, pitch and rainfall.

Reviewed by James Whitfield · How we check our maths

Your values

ⓘ Live results

Results
Recommended gutter
6 in K-style
Adjusted roof area
7,560 sq ft-equivalent
Downspouts (5 in)
2
Downspouts (6 in)
1

Working

  1. 11,200 sq ft x pitch 1.05 x rain 6 = 7,560

Quick answer

Effective roof area = plan area x pitch factor. A 5 inch K-style gutter handles about 5,500 sq ft of adjusted area per inch of rainfall per hour; 6 inch handles about 7,900.

The formula

adjusted area = roof area x pitch factor x rainfall intensity

Worked example

Worked example

Using the values the calculator loads with.

Inputs

  • Roof plan area1200 sq ft
  • Roof pitch4/12 to 5/12
  • Rainfall intensity6 in/hr

Result

  • Recommended gutter6 in K-style
  • Adjusted roof area7,560 sq ft-equivalent
  • Downspouts (5 in)2
  • Downspouts (6 in)1

How to use the Gutter Size Calculator

  1. 1Measure the roof footprint served by that gutter run, not the sloped surface.
  2. 2Choose the pitch band - steeper roofs throw more water into the gutter.
  3. 3Use your local 5-minute rainfall intensity; 6 in/hr is a safe default in much of the US.

Frequently asked questions

When is a 6 inch gutter worth it?

Large roof planes, steep pitches and heavy-rain regions. Six inch gutters carry roughly 40% more water and clog far less often.

How far apart should downspouts be?

No more than about 35 to 40 feet apart, and at least one per gutter run regardless of length.

About this calculator