Gutter Calculator
Size your gutters and downspouts from eave length and roof area.
What you'll need to order
- K-style gutter
- downspouts
- hidden hangers
- end caps
- outlet drops
Prices vary by region and haul distance. Get quotes from at least two local suppliers.
How this calculator works
downspouts = roof area ÷ 600 · hangers = eave length ÷ 2
One standard 2 by 3 inch downspout drains roughly 600 sq ft of roof in typical rainfall. In high-intensity rainfall regions, size down to 400 sq ft per downspout or step up to 3 by 4 inch.
Worked example
With total eave length 100 ft, roof area draining to it 1800 sq ft, this calculator returns 3 .
Frequently asked questions
How many downspouts do I need?
One 2 by 3 inch downspout per 600 sq ft of roof area is the usual rule. A 1,800 sq ft roof needs three. In heavy-rainfall regions, work to 400 sq ft per downspout instead.
What size gutter should I use?
Five inch K-style handles most homes. Go to 6 inch where the roof is large, steep, or the rainfall intensity is high — a 6 inch gutter carries nearly twice the water of a 5 inch.
How far apart should gutter hangers be?
Twenty-four inches in most climates, and 18 inches where snow and ice load the gutter. Hidden hangers screwed into the fascia hold far better than the old spike-and-ferrule method.
What slope should a gutter have?
About 1/4 inch of fall per 10 ft toward the downspout. Too little and water stands; too much and the gutter looks visibly crooked against the fascia line.