Calculators/ Roofing/Gutter Calculator
Gutter Calculator
Gutter length, downspouts and roof drainage load.
Gutter Calculator
ft
ft²
in/hr
Undersized gutters overflow at the worst possible moment, usually into a foundation. Sizing depends on the roof area draining into each run and on how hard it rains locally, not just on the length of the eave.
How this is calculated
- Gutter length is simply the total eave run to be guttered.
- Downspout capacity scales with roof area and rainfall intensity. A 2×3 inch downspout handles roughly 600 square feet at one inch per hour; a 3×4 inch handles about 1,200.
- Divide effective drainage load by that capacity, with a minimum of two downspouts so a single blockage cannot flood the whole run.
Frequently asked questions
What size gutters do I need?
Five inch K-style handles most single-storey homes. Move to six inch for large roof areas, steep pitches that shed fast, or regions with intense downpours.
How many downspouts does a house need?
One per 30 to 40 feet of gutter run as a rule of thumb, but size by roof area and rainfall rather than length alone.
What slope should a gutter have?
About a quarter inch of fall per 10 feet toward the downspout. Too little and it pools; too much and it looks visibly crooked from the street.