Gutter Size Calculator
Pick 5 in or 6 in gutters from roof area, pitch and rainfall.
Enter your values
Enter the roof plan area in sq ft.
Choose one of: Flat to 3/12, 4/12 to 5/12, 6/12 to 8/12, 9/12 to 11/12, 12/12+.
Enter the rainfall intensity in in/hr.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- Recommended gutter
- 6 in K-style
- Adjusted roof area
- 7,560 sq ft-equivalent
- Downspouts (5 in)
- 2
- Downspouts (6 in)
- 1
Working
- 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.
adjusted area = roof area x pitch factor x rainfall intensity
At a glance
| What it does | Pick 5 in or 6 in gutters from roof area, pitch and rainfall. |
|---|---|
| Category | Home & DIY |
| Inputs needed | Roof plan area, Roof pitch, Rainfall intensity |
| Main output | Recommended gutter |
| Formula | adjusted area = roof area x pitch factor x rainfall intensity |
| Cost | Free — no sign-up, no download |
How to use the Gutter Size Calculator
- 1Measure the roof footprint served by that gutter run, not the sloped surface.
- 2Choose the pitch band - steeper roofs throw more water into the gutter.
- 3Use your local 5-minute rainfall intensity; 6 in/hr is a safe default in much of the US.
Inputs explained
- Roof plan area(sq ft)
- Enter the roof plan area in sq ft.
- Roof pitch
- Choose one of: Flat to 3/12, 4/12 to 5/12, 6/12 to 8/12, 9/12 to 11/12, 12/12+.
- Rainfall intensity(in/hr)
- Enter the rainfall intensity in in/hr.
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
Results
- Recommended gutter6 in K-style
- Adjusted roof area7,560 sq ft-equivalent
- Downspouts (5 in)2
- Downspouts (6 in)1
- 1,200 sq ft x pitch 1.05 x rain 6 = 7,560
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.
What do I need to enter into the Gutter Size Calculator?
Just 3 values: roof plan area, roof pitch, rainfall intensity. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Gutter Size Calculator work out the 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. It applies the formula adjusted area = roof area x pitch factor x rainfall intensity and shows the working so you can check each step by hand.
Is the Gutter Size Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.