Work out downpipe sizing instantly with clear inputs, formula shown and shareable results.
Rainwater downpipes run part full with an annular film, and a working design velocity of about 1.5 m/s gives the required bore: area in square millimetres is roughly 667 times the flow in litres per second. The result is rounded up to the next standard pipe size.
Flow per pipe
L/s = area x intensity / 3600 / number of downpipes
Bore
Area mm² = flow L/s x 667; diameter = sqrt(4A / pi)
L/s = area x intensity / 3600 / number of downpipes. Rainwater downpipes run part full with an annular film, and a working design velocity of about 1.5 m/s gives the required bore: area in square millimetres is roughly 667 times the flow in litres per second.
Two smaller pipes give redundancy if one blocks with leaves, and they halve the flow each gutter run has to carry, which often allows a smaller gutter.
This calculator takes 3 inputs: Roof plan area drained, Design rainfall intensity, Number of downpipes. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.