Downpipe Size Calculator
Get an indicative downpipe size for your roof. Enter the catchment area draining to the downpipe, the right size depends on your local rainfall intensity, so treat this as a planning guide to confirm with a plumber.
How we estimate this
Downpipe sizing follows AS/NZS 3500.3 and depends on the roof area draining to each pipe and your local rainfall intensity, not just the roof size. As a rough guide, one standard 90 to 100mm downpipe handles a modest suburban catchment, and large roofs need several.
Pricing reviewed: November 2026.
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Pricing reviewed November 2026. Indicative Australian costs, not a quote.
Understanding downpipe sizes in Australia
Downpipe sizing follows AS/NZS 3500.3 and depends on the roof area draining to each pipe and your local rainfall intensity, not just the roof size. As a rough guide, one standard 90 to 100mm downpipe handles a modest suburban catchment, and large roofs need several.
The common mistake is too few downpipes, which makes gutters overflow in heavy rain even when the gutter itself is big enough. A plumber sizes both the gutter and the number of downpipes to your area's rainfall data.
Frequently asked questions
What size downpipe do I need?
It depends on the roof area draining to it and your local rainfall intensity. As a guide, a standard 90 to 100mm downpipe suits a modest catchment, large roofs need several. Confirm against AS/NZS 3500.3 with a plumber.
Why do my gutters overflow even though they are big?
Usually because there are too few downpipes, so water cannot drain away fast enough in heavy rain. Adding downpipes often fixes overflow better than bigger gutters.
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