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HeizerDesk Gutter Calculator
Undersized gutters overflow in a hard rain no matter how clean they are. This sizes the trough and the number of downspouts from the roof area actually draining into them. Use it as a free Gutter Calculator on site — the working is shown underneath, not hidden.
Your numbers
Result
Downspouts needed
3
downspouts
- Gutter size
- 5in K-style
- Space them about every
- 30ft
- Effective roof areaPitch-adjusted
- 1,540sq ft
- Area one downspout drains
- 600sq ft
- Gutter to buy
- 95linear ft
- HangersEvery 24 in
- 45hangers
How this is calculated
- Effective roof area = actual area × a pitch factor, because a steep roof throws more water at the gutter than its footprint suggests.
- A 3×4 in downspout drains roughly 600 sq ft of effective roof at 6 in/hr rainfall, and less as intensity rises.
- Above about 5,500 sq ft of adjusted area, 5 in K-style will overflow and you want 6 in.
Before you order
- Most residential overflow problems are downspout capacity, not gutter size. Adding a downspout is usually cheaper and more effective than upsizing the trough.
- 6 in gutter needs 3×4 in downspouts to be worth having. Pairing 6 in gutter with 2×3 in downspouts just moves the bottleneck.
- Slope the run about 1/4 in per 10 ft toward the outlet, and get the water away from the foundation once it lands.