Measuring gutter runs and sizing downspouts
Gutter is priced by the linear foot, so the whole job comes down to measuring the eaves accurately. Walk the perimeter and measure only the edges that actually need a gutter: the low horizontal eaves. The sloping edges of a gable end (the rakes) carry no gutter, and a hip roof by contrast needs gutter on all four sides. Add the runs together, divide by your stock section length, and round up — this calculator does that for you and counts the downspouts separately, since a downspout is the same material sold by the foot but runs vertically from the gutter to the ground or to a drain.
Downspout count matters more than most people expect. A common rule of thumb is one downspout for every 30 to 40 feet of gutter run, with an extra one wherever two roof planes dump into a single valley. Too few and the gutter overflows at the far end in heavy rain no matter how big the trough is. Keep a slope of roughly a quarter inch of fall for every 10 feet toward each outlet, hang brackets at a consistent spacing so long runs cannot sag, and plan where the water goes once it leaves the bottom elbow: extensions or a buried line that carries it several feet away from the foundation are cheap compared to fixing a wet basement. Budget separately for end caps, inside and outside corners, elbows, hangers and any leaf guard, as these are usually priced per piece rather than per foot.
Per-foot prices vary widely between sectional and seamless gutter, material and whether the figure includes installation. Rain intensity in your area may call for larger gutters or more downspouts than the rule of thumb.