Size downpipes and check gutter overflow provisions per AS/NZS 3500.3:2021. Select your location, enter roof details, and get instant results.
Enter roof catchment area and select downpipe size to check capacity. Uses 5% AEP (20-year) rainfall intensity.
Check gutter overflow capacity for extreme rainfall events. Uses 1% AEP (100-year) rainfall intensity.
Roof area and rainfall intensity determine the design flow rate. Each downpipe must handle its share of the total flow.
Standard downpipe sizes have defined maximum flow capacities under charged conditions per AS/NZS 3500.3.
Overflow provisions handle extreme rainfall events (1% AEP). The weir formula determines how much water can safely overflow the gutter.
CCC Engineering delivers complete hydraulic design packages, including roof drainage, downpipe sizing, overflow provisions, and on-site detention systems.
This downpipe and eaves gutter calculator helps hydraulic engineers, plumbers, roofers, and builders determine the number and size of downpipes required for a roof area, and check whether the gutter system can handle the design rainfall. Select your project location to load the local rainfall intensity, enter the roof catchment area and downpipe details, and the calculator will determine whether your drainage system has adequate capacity. The overflow provisions tab checks gutter performance under the 1% AEP (100-year) rainfall event.
Calculations follow AS/NZS 3500.3:2021 (Stormwater drainage systems). Downpipe capacities are taken from Table 5.2 for charged conditions. The primary sizing uses the 5% AEP (20-year average recurrence interval) rainfall intensity at a 5-minute storm duration, sourced from the Bureau of Meteorology IFD data. Overflow provisions are checked against the 1% AEP (100-year) intensity to ensure water can safely escape the gutter system during extreme events without causing building damage.
This tool provides sizing results suitable for residential and small commercial projects. For complex roof geometries, multi-level drainage systems, or projects with specific council requirements for on-site detention, consult a qualified hydraulic engineer. Final designs must comply with local council stormwater policies and any site-specific conditions of development consent.