Gutter Calculator
Calculate gutter length and downspouts needed from roof perimeter.
Assumptions used in this calculation
- Even downspout distribution: Spaces downspouts evenly along the perimeter, it doesn't account for specific low points, valleys, or corners where a downspout is structurally required regardless of spacing.
- No extra length for seams or end caps: Gutter length is taken as exactly the roof perimeter, with no added allowance for overlaps, end caps, or corner miter pieces.
About this calculator
Undersized gutter drainage leads to overflow during heavy rain, and the number of downspouts a roof needs isn't obvious from perimeter alone, it depends on how far water can travel along a gutter run before it needs somewhere to drain. This calculator takes your roof's total eave perimeter and a standard downspout spacing interval, divides perimeter by spacing and rounds up to find how many downspouts are needed, enforcing a minimum of one even for a short run since every gutter needs at least one drain point. Enter roof perimeter and your target downspout spacing, and it returns both the gutter length (matching the perimeter) and downspout count together instead of estimating drainage points by eye. Because it distributes downspouts evenly rather than accounting for specific low points or valleys where water naturally concentrates, review the actual roof layout and add a downspout at any low point regardless of what even spacing alone suggests.
Worked example
120ft roof perimeter, 35ft downspout spacing
Result: 120ft of gutter, 4 downspouts
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