Work out gutter size instantly with clear inputs, formula shown and shareable results.
One millimetre of rain on one square metre is one litre, so runoff in litres per hour is plan area times intensity, and dividing by 3600 gives litres per second. The gutter is then selected on the flow reaching each outlet, since a gutter with outlets at both ends carries only half the total at any point.
Design flow
L/s = roof plan area x rainfall intensity mm/h / 3600
Per outlet
Flow per outlet = total flow / number of outlets
L/s = roof plan area x rainfall intensity mm/h / 3600. One millimetre of rain on one square metre is one litre, so runoff in litres per hour is plan area times intensity, and dividing by 3600 gives litres per second.
No. Use the plan projected area, because rain falls vertically. A steeply pitched roof collects the same rainfall as its footprint, though wind-driven rain justifies a small increase.
This calculator takes 3 inputs: Roof plan area drained, Design rainfall intensity, Number of outlets. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.