Rain falls straight down
This is the one thing to take away from the page. The catchment a gutter has to carry is the horizontal projection of the roof — the footprint, the shadow, the plan area. Not the sloped surface.
A 35° roof has more surface than a 5° roof over the same footprint, but it does not collect more rain. The water arriving on it is the water that fell through the footprint, and the footprint is identical. Steepening a roof makes the water arrive faster; it does not make more of it.
So do not multiply the catchment by the pitch multiplier. That's the classic hand-calculation error, it inflates every flow figure by 5–40%, and on a job sitting near a capacity boundary it doubles the downpipe count for nothing. The same multiplier that is essential on the roof sheet calculator is flatly wrong here.
The mistake in numbers
Take a 50 m² catchment in south-east Queensland at 250 mm/hr, running into a 100 × 50 rectangular downpipe rated about 3.6 L/s.
- Done properly: 50 × 250 ÷ 3600 = 3.47 L/s → 3.47 ÷ 3.6 → one downpipe
- Pitch multiplier wrongly applied at 22.5°: 50 × 1.0824 = 54.1 m² → 54.1 × 250 ÷ 3600 = 3.76 L/s → 3.76 ÷ 3.6 → two downpipes
Same roof, same storm. One extra downpipe, one extra trench, one extra thing to explain on the quote.
The number this tool won't pick for you
Rainfall intensity is site specific and it is the input that actually decides the answer. It comes from the Bureau of Meteorology's design rainfall data, or from your council, for the real address, at the storm duration and return period the job requires. Two suburbs an hour apart can differ by 50 mm/hr, and coastal versus escarpment can be worse than that.
| Rough regional band | Starting figure |
|---|---|
| Southern Victoria / Tasmania | 150 mm/hr |
| Sydney / Adelaide / Perth | 200 mm/hr |
| South-east Queensland | 250 mm/hr |
| Tropical north Queensland / NT | 300 mm/hr |
Treat those as a sanity check for a rough sizing conversation on site, never as the design figure. Look up the real one before anything gets ordered or installed.
A worked example
A brick veneer house, 15m long and 9m wide in plan, gutter down both long sides, in south-east Queensland.
- Plan catchment: 15 × 9 = 135 m²
- Each gutter takes half of it: 67.5 m²
- Flow at 250 mm/hr: 67.5 × 250 ÷ 3600 = 4.69 L/s
- A 100mm round downpipe carries about 6 L/s, so on flow alone: one downpipe
- But the gutter run is 15m, at one downpipe per 12m: 15 ÷ 12 = 1.25 → two downpipes
Two downpipes per side. The length rule governed, not the flow — which is completely normal on a long, narrow house. Put them at opposite ends so the water only ever travels half the run.
Downpipe capacities
| Downpipe | Typical capacity |
|---|---|
| 100mm round | about 6.0 L/s |
| 100 × 75 rectangular | about 5.4 L/s |
| 90mm round | about 4.5 L/s |
| 100 × 50 rectangular | about 3.6 L/s |
These assume a decent outlet. A downpipe fed through a small hole punched in a flat gutter sole carries a fraction of what the pipe itself could take — the hole is the restriction, not the pipe. A sump, a rainhead or a properly formed outlet is what lets a downpipe work at its rating.
Why length matters as well as flow
A gutter is a very shallow channel with very little fall. On a long run the far end fills and runs deep long before the outlet is working hard, and any sag in the fascia sends it over the front. Spacing outlets keeps the water shallow the whole way, which is why the length rule can beat the flow rule on a roof nowhere near capacity.
Overflow, and why blocked gutters cost so much
Gutters block. Leaves, a tennis ball, a possum, a bird's nest against the outlet. The design question is not whether it happens but where the water goes when it does.
Over the front of the gutter is a wet garden bed and an annoyed customer. Over the back is water into the eaves, down the wall cavity, into the ceiling and along the top plates — rotted framing, stained plasterboard, and an insurance argument about maintenance. High-front gutters and box gutters are the ones that fail backwards, and they need a deliberate overflow: slots, a spitter, or an overflow outlet sized to shift the same storm the downpipe was.
Extra catchment people forget
- Walls above a roof. A two-storey wall standing over a lower roof throws water onto it. Part of that wall area counts as catchment — AS/NZS 3500.3 sets how much.
- Upper roofs discharging onto lower ones, unless the water is carried through.
- Verandahs, carports and pergola roofs tied into the same gutter line.
What this calculator does not do
It sizes the number of downpipes against flow and gutter length. It does not size the gutter profile itself, and it does not touch box gutters, sumps, rainheads, overflow devices or siphonic systems. Box gutters in particular have far less tolerance than an eaves gutter and are a design job — get a hydraulic consultant or your building surveyor onto them.
Sized it up. Now get the quote away.
Yamate turns gutter metres, downpipes and your hours into a quote the customer can accept on their phone — then an invoice, then a payment, with GST sorted and the follow-ups chased.
Try Yamate free → Free to try · no card · sign in with a link, no password to rememberGutter & downpipe sizing FAQ
How do I calculate roof catchment area for guttering?
Use the horizontal plan area — length by width as measured flat, not up the slope. Then split it between the gutters that actually drain it, which on a simple gable is half each side.
Does roof pitch affect gutter size?
No. Rain falls vertically, so the water a gutter receives is set by the footprint, not the sloped surface. Multiplying catchment by the pitch factor is the most common hand-calculation error in roof drainage and it oversizes downpipes.
How do I calculate flow rate for a gutter?
Catchment in square metres times rainfall intensity in millimetres per hour, divided by 3600. A 67.5 m² catchment at 250 mm/hr gives 4.69 litres per second.
How many downpipes do I need?
Take the larger of two answers: flow divided by the downpipe capacity, and gutter run divided by about 12 metres. On long narrow houses the length rule usually wins, even when one downpipe could carry the flow.
How much water can a 100mm downpipe take?
Around 6 litres per second with a properly formed outlet. A 90mm round is about 4.5 and a 100 by 50 rectangular about 3.6. A downpipe fed through a small punched hole carries far less than its rating.
Where do I get rainfall intensity for my site?
The Bureau of Meteorology publishes design rainfall data by location, and most councils will state the figure they expect used. It is site specific — the regional bands on this page are a starting point for a rough sizing, not a design figure.
Other calculators you'll want
General information for Australian trades, and an estimate only — not tax, financial or legal advice. It is not engineering, building or design advice either: a calculator works out a quantity from the figures you enter, and anything that needs sign-off still needs your engineer, certifier or building surveyor. Roof drainage in Australia is governed by AS/NZS 3500.3; the relationships here are described in our own words from published Australian guidance on it. Downpipe capacities are typical values and depend heavily on the outlet detail. The design rainfall intensity must come from the Bureau of Meteorology or your council for the actual site — the regional bands here are a starting point only. This is a sizing aid for quoting and sanity-checking, not a hydraulic design.