Gutter Size Calculator
Calculate the peak rainwater flow rate from a roof catchment area and recommend the minimum gutter width and number of downspouts required.
Results
What is it?
A gutter sizing calculator determines what gutter width can handle the peak flow of rainwater from your roof during a design storm. It converts your catchment area and local rainfall intensity into a flow rate (litres per second), then works back to the minimum gutter cross-section needed.
How to use
Measure the horizontal projected area of the roof section draining into the gutter. Find your local design storm intensity (often called "10-year storm" intensity in mm/hr � available from your local council or weather authority). Enter the gutter slope (3 mm/m is typical). The result guides gutter profile width and downspout count.
Example scenario
A 150 m2 roof in a region with 75 mm/hr design rainfall generates 0.003125 L/s of flow. The formula suggests a gutter of approximately 281 mm � so rounding up to the nearest standard size, use a 300 mm half-round gutter with at least 3 downspouts spaced evenly.
Pro tip
Always upsize gutters slightly � undersized gutters overflow in heavy rain, potentially damaging soffits and foundations. A 125 mm K-style gutter is the minimum for most suburban homes; go to 150 mm or 300 mm half-round for larger roofs. Clean gutters twice a year to maintain full flow capacity.