How the downspout calculator works
Capacity per outlet
A 2×3 downspout on a 5-inch K-style gutter handles about half the gutter's flow; 3×4 outlets about 70%. The calculator divides your run's flow by outlet capacity and enforces the spacing rule — no more than 30–40 feet of gutter per downspout — because long runs also fight the slope budget (see the slope calculator).
Placement is free capacity
Corners are natural downspout homes: they end runs, hide pipes, and shortening the effective run raises the slope each half can carry. Two downspouts sloped from a center high point outperform one terminal outlet on any run over 40 feet.
Below the outlet
Downspouts end 1–2 feet above grade with splash blocks, or extend 4–10 feet with buried pipe — the distance matters more than the hardware: foundation problems follow water dumped at the wall. Burying needs pitch too (1/8 inch per foot minimum) and an outlet that will not ice in winter.