TradesHow this works
Enter the roof area that drains to the gutter run and your rainfall level. The tool lays 5 inch and 6 inch K-style gutters side by side, shows the roof area each is rated for and the downspouts it needs, and recommends the size for your roof. Five inch suits most homes; a big roof or heavy rain moves you to 6 inch.
Downspouts help a gutter keep up, but they do not raise its carrying capacity, so past a point you need the bigger trough. And rainfall intensity beats the annual total: short violent storms can overrun a gutter the yearly number says is fine.
Common questions
Should I use 5 inch or 6 inch gutters?
Five inch K-style is the standard and handles most homes, up to roughly 5,500 square feet of roof drainage area in moderate rain. Step up to 6 inch for a larger roof, a steep pitch, or heavy rainfall. Six inch carries nearly double the water, about 2 gallons per foot versus 1.2.
How many downspouts do I need?
On a 5 inch gutter with 2 by 3 inch downspouts, plan one per about 600 square feet of roof. On a 6 inch gutter with 3 by 4 inch downspouts, one per about 1,200 square feet. Spread them out, ideally one near each end of a long run, rather than clustering them.
Does adding downspouts fix an undersized gutter?
Only up to a point. More downspouts help a gutter drain and reduce overflow at the outlets, but they do not raise how much water the trough itself can carry. Past a certain roof size or rain intensity you need the bigger 6 inch gutter, not just more downspouts.
How does roof pitch affect gutter size?
A steeper roof throws water at the gutter faster, so it effectively raises the area the gutter must handle. Sizing charts apply a pitch factor for this. If your roof is steep or has large planes feeding one run, size up rather than sitting right at the limit of the smaller gutter.
What about heavy rain areas?
Rainfall intensity matters more than the annual total. A region with short, violent downpours can overwhelm a 5 inch gutter that the yearly number says is fine. If you get intense storms or more than about 40 inches a year, lean to 6 inch gutters and generous downspouts.
Sources: 5 in K-style: to ~5,500 sq ft, 1.2 gal/ft, 2x3 downspout per ~600 sq ft; 6 in: to ~7,960 sq ft, 2.0 gal/ft, 3x4 per ~1,200 sq ft (This Old House) · Go 6 in for roofs over ~5,500 sq ft or heavy rainfall (over ~40 in/yr or intense storms). Engine version 1.0.0. Risk tier 1.