Updated · Built and checked by the PlinzoTools team, formulas tested against published sources
Calculate linear feet of gutter and downspout count from your roof's eave length.
Gutter length simply matches your roof's eave length, but downspout count is a separate question -- water needs an outlet at reasonably spaced intervals along the run so gutters don't overflow during heavy rain. This calculator handles both.
Enter your total eave length (the horizontal roof edge gutters attach to) and a downspout spacing interval (roughly every 35-40ft is common general guidance, though heavier rainfall regions or larger roof drainage areas often need closer spacing).
Gutter Needed = Eave Length
Downspouts = ceil(Eave Length ÷ Spacing Interval), minimum 1
This is a general planning estimate -- actual downspout placement should also consider corners, valleys concentrating extra water flow, and your specific gutter size's drainage capacity, which a contractor or local code may size more precisely.
Once you know your linear footage and downspout count, the Gutter Cost Calculator turns that into a material and labor cost estimate.
Consider a house with 90 feet of total eave length and a downspout spacing of 40 feet. The gutter needed equals the eave length, 90 feet. Downspouts are 90 divided by 40, or 2.25, rounded up to 3. If instead you want a closer 30-foot spacing for a wetter climate, the count becomes 3 exactly. It is worth trying a few spacings to see how the count changes.
The calculator gives a count, but not where the downspouts should go. Good locations are at the low end of a sloped gutter, near corners, and where water can discharge away from the foundation. A long run should slope slightly toward each outlet. Valleys can concentrate flow from two roof planes into one spot and may deserve a dedicated downspout. Also consider where the discharge lands: onto a splash block, into an underground drain, or onto a lawn slope that leads away from the house.
Gutter size and downspout size affect how much water can be handled. A larger gutter or a larger downspout can move more, so a house with a big roof area feeding one edge may need bigger components or more outlets. Local rainfall intensity is the other factor. This calculator does not compute roof drainage area, so treat its downspout count as a starting point and ask a gutter installer or your local code office if you live in an area with heavy storms.
Sectional gutters come in standard lengths, and seamless gutters are formed on site. Either way, plan for a bit of extra material, end caps, corner pieces, and hangers. Those accessories are not part of the linear footage number.
Walk the perimeter of the house before ordering and note obstacles such as chimneys, roof valleys, and porch roofs that add extra gutter runs.
Roughly every 35-40 feet is common general guidance, but heavy rainfall areas or large roof catchment areas often need closer spacing -- check local guidance or a gutter contractor's recommendation for your climate.
Often yes -- a roof valley concentrates significantly more water flow than a straight eave run, and may warrant a dedicated downspout nearby regardless of the general spacing interval.
Only if gutters go on every edge. Add up only the edges where gutters will actually be installed.
Not necessarily. One outlet can serve a run if the gutter slopes toward it, though long runs often use a center or two end outlets.
Because a gutter with no outlet cannot drain. Even a very short run needs at least one.