Size gutters and downspouts from roof area and rainfall intensity.
Rainfall lands on the horizontal projection, so roof pitch does not increase the volume collected. Downspout count usually governs the design because a single downspout carries far less than a long gutter run delivers. Undersized drainage overflows at the gutter rather than backing up visibly, which is how water ends up against the foundation.
Rain Gutter
Flow in L/s = roof plan area × rainfall intensity ÷ 3,600
Flow in L/s = roof plan area × rainfall intensity ÷ 3,600 Rainfall lands on the horizontal projection, so roof pitch does not increase the volume collected. Downspout count usually governs the design because a single downspout carries far less than a long gutter run delivers.
Undersized drainage overflows at the gutter rather than backing up visibly, which is how water ends up against the foundation.
This calculator takes 4 inputs: Roof plan area drained, Design rainfall intensity, Gutter capacity per run, Downspout capacity. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.