Gutter & Downpipe Size
Choose a gutter width and the number of downpipes for a roof from its catchment area and rainfall, free online.
About this calculator
The roof area that drains to the gutter, the roof pitch and local heavy-rain intensity set the flow. Flow (L/min) = area × rainfall (mm/h) ÷ 60. As a rule of thumb, a 100 mm gutter serves up to about 50 m² of roof, 125 mm up to 100 m², 150 mm up to 150 m², and one downpipe is added per 50 m². Always confirm with local building codes.