Gutter Calculator
Gutter length, downspouts and sizing.
| Gutter length | 80 linear ft(plus a little for corners and joints) |
| Downspouts | 4 |
| Roof area per downspout | 350 sq ft(keep this modest — a large area overwhelms one outlet) |
| Average downspout spacing | 20 ft(closer spacing drains faster) |
| Downspout material | 64 linear ft of pipe |
| Suggested gutter size | 4 in (small roofs) |
Results update instantly and are computed entirely in your browser. For general information and planning only — not engineering, structural or code advice.
About the gutter calculator
A gutter system has one job: move water off the roof fast enough that it never reaches the foundation. Two quantities decide whether it succeeds — how much water arrives, and how quickly it can leave. The first comes from the roof area draining into the gutter and the local rainfall intensity; the second from the gutter profile and the number and size of downspouts. Length is the easy part: the gutter runs along the eaves, so its length is the eave measurement, plus a small allowance for corners and joint overlaps. Downspouts are where estimates often go wrong. A single outlet can only carry so much water, and if the area it serves is too large the gutter overflows in a heavy storm, sending water down the wall and into the soil beside the foundation. Spacing outlets closer together, or making them larger, is the standard fix. Sizing follows from the same logic. A small roof with a short run can be handled by a modest gutter profile; a large, steep or wide roof feeding a long uninterrupted run needs a deeper gutter and more outlets. Sloped roofs shed more water because their real area is larger than the footprint, so a roof pitch calculation belongs in the input if the roof is steep. Local rainfall intensity, which varies enormously by region, is the other half of the sizing decision. This calculator returns gutter length, downspout material, average spacing and a rough gutter size suggestion from the area each outlet must serve. It is a planning tool, not a drainage design: for a large or complex roof, use the local code's rainfall tables. Check the slope of the gutter run too, because a flat run holds water and a badly pitched one misses the outlet.
This calculator returns estimates for planning and general information only. It is not engineering, architectural, structural, electrical or code advice, and it cannot replace a qualified professional who has seen the site. Material quantities depend on the actual dimensions, the product you buy and local building codes, so confirm the figures before ordering materials or starting work, and always follow local regulations.