Gutters, box gutters, downpipes, and stormwater pipes — sized to your catchment and rainfall, not a textbook guess. Based on AS/NZS 3500.3.
Stormwater drainage is sized using the rational method: design flow rate equals the runoff coefficient multiplied by rainfall intensity multiplied by catchment area, divided by 3,600. The runoff coefficient depends on the roof surface — colorbond metal runs off faster than concrete or gravel. Rainfall intensity comes from AS/NZS 3500.3 for the location and design storm frequency.
For standard residential work, the 20-year ARI is the typical design standard. For critical drainage where overflow would cause property damage, 100-year ARI is used. This tool includes rainfall intensity data for 12 Australian cities and regions and checks the design flow against gutter profiles, box gutter sections, downpipe capacities, and stormwater pipe sizes at the installed grade.
Use this when sizing new roof drainage on a new build or extension, checking whether existing gutters and downpipes are adequate, or designing a stormwater drainage system. The tool shows the formula calculation and the utilisation percentage so you can see how much headroom the sizing provides.
Plan area — the area of roof draining to this gutter/downpipe run. Not the roof surface area.
Auto-filled from location. Override with BOM IFD data for the exact site.
Box gutters must be designed to 1-in-100 year rainfall per AS/NZS 3500.3. Freeboard (top 1/3 of depth) is not counted as usable capacity.
Flatter grades need larger pipes. If site constraints limit the grade, you may need to upsize.