Downpipe Calculator
Size rainwater downpipes from roof catchment, pitch and rainfall using AS/NZS 3500.3 — round and square Colorbond, Zincalume, copper and uPVC.
Downpipe Calculator
Downpipe shape:
Effective catchment area
154 m²
Cross-section
80 cm²
Drains per downpipe
80 m²
Min downpipes
2
Peak flow
3.21 L/s
AS/NZS 3500.3 simplified: 1 cm² downpipe cross-section per 1 m² catchment as a starting point.
What this calculator does
This calculator sizes the vertical downpipes that carry rainwater from the gutter to ground level, a stormwater pit, or a rainwater tank. It takes the roof catchment area, pitch, rainfall intensity, and your chosen downpipe profile and dimensions, then tells you how many downpipes you need and what the peak flow into the system will be.
Downpipe sizing is a separate calculation from gutter sizing: a 150 mm half-round gutter sized for a large detached’s catchment still needs the right downpipes under it or you’ll get overflow at the gutter’s high end during a 1-in-20 ARI 5-minute storm.
How to use it
- Enter the projected roof area in m². Plan-view footprint, not sloped surface. Sum the projected area of each slope for hipped and complex roofs.
- Set the pitch factor. Default is 1.10 (around 22.5° pitch, the Australian project-home default). Use 1.00 for skillion / low-pitch (10–15°), 1.05 for 15–20°, 1.20 for 30–35° pitches, 1.30 for steep architect-designed 45°+ pitches.
- Enter the rainfall intensity in mm/hr. Default is 90 mm/hr — the AS/NZS 3500.3 ARI 1-in-20-year 5-minute baseline for the eastern seaboard (Sydney, Melbourne, Brisbane, Adelaide). Use 120–150 mm/hr for the Tropical Wet (Cairns, Darwin, Townsville), 100 mm/hr for the NSW Northern Rivers and SE Queensland subtropics, 75 mm/hr for SW Western Australia (Perth) and southern Tasmania (Hobart). The Bureau of Meteorology IFD (Intensity-Frequency-Duration) tool gives the per-postcode value.
- Pick the downpipe shape and dimensions. Rectangular for the Colorbond / Zincalume volume default, round for heritage and modernist architect builds, copper round for high-end residential.
- Read the result. The big number is the minimum number of downpipes needed to handle the effective catchment area.
The AS/NZS 3500.3 simplified rule
The rule of thumb is: 1 cm² of downpipe cross-section drains 1 m² of effective catchment at the ARI 1-in-20 5-minute design intensity for the eastern seaboard (90 mm/hr). For the Tropical Wet zone, halve the rule — 1 cm² drains 0.5 m² because the design intensity at Cairns / Darwin is 150–200 mm/hr.
For cyclone regions C1/C2/C3, the AS/NZS 3500.3 rational method is mandatory; the rule of thumb is for cycloneless residential only.
Common Australian downpipe sizes and capacities
| Size | Cross-section | Drains up to (90 mm/hr) | Drains up to (150 mm/hr Tropical) |
|---|---|---|---|
| 75 × 75 mm rectangular | 56 cm² | 56 m² | 28 m² |
| 100 × 50 mm rectangular | 50 cm² | 50 m² | 25 m² |
| 100 × 75 mm rectangular | 75 cm² | 75 m² | 37 m² |
| 100 × 100 mm rectangular | 100 cm² | 100 m² | 50 m² |
| 80 mm round | 50 cm² | 50 m² | 25 m² |
| 90 mm round | 64 cm² | 64 m² | 32 m² |
| 100 mm round | 78 cm² | 78 m² | 39 m² |
| 110 mm round | 95 cm² | 95 m² | 47 m² |
| 150 × 100 mm rectangular | 150 cm² | 150 m² | 75 m² |
Pairing downpipes to gutter sizes
Standard Colorbond / Zincalume / Lysaght system pairings:
- 115 mm quad gutter (project-home default) with 100 × 75 mm rectangular downpipe — drains up to 75 m² per downpipe (standard) or 37 m² (tropical).
- 150 mm half-round gutter (architect-designed and rural) with 100 × 100 mm rectangular or 100 mm round — drains up to 100 m² (standard) or 50 m² (tropical).
- 125 mm fascia gutter / box gutter (commercial and modernist residential) with 150 × 100 mm rectangular — drains up to 150 m² per downpipe.
- 115 mm half-round heritage gutter with 90 mm round copper or galvanised — drains up to 64 m² per downpipe.
When to step up a size
Add cross-section or add downpipes if any of these apply:
- Concentrated valleys. A roof valley dumps the flow from two slopes into a short stretch of gutter — place a downpipe directly under the valley termination.
- Long single-fall runs. Beyond 12 m of single-fall gutter, the high-end gutter sees standing water during heavy rain.
- Steep pitches above 30°. Wind-driven rain factor increases nonlinearly above 30° — apply 1.20 to 1.30 pitch factor.
- Tropical Wet and Cyclonic zones. Cairns, Darwin, Townsville, Northern Rivers, Wet Tropics — double the rule-of-thumb downpipe count, or use the full AS/NZS 3500.3 calculation with 150–200 mm/hr design intensity.
- Standing-seam metal and glazed clay tile. These shed rain faster than corrugated Colorbond and concrete tile, concentrating flow at the eave with less detention. Add 10–15% to the calculated peak flow.
- Box gutters and parapet gutters. Lined parapet gutters carry concentrated flow over a long horizontal distance; sizing is governed by AS/NZS 3500.3 Section 5, not the rule of thumb. Always specify with overflow weirs.
Discharge to council stormwater, soakwells and rainwater tanks
- Council stormwater pit / kerb discharge. The default for metropolitan Sydney, Melbourne, Brisbane, Adelaide. Council connection point is typically at the front kerb or at a stormwater easement; the plumber connects via 90 mm or 100 mm uPVC underground.
- Soakwell. Mandatory in WA outside reticulated stormwater zones (Perth metropolitan), commonly used in SA and rural Vic. Sized at 1 m³ per 60 m² of impermeable area, located at least 1.8 m from any building under WAPC SPP 2.9.
- Rainwater tank. Mandatory in NSW under BASIX, Victoria under 7-Star NCC 2022 dwelling targets, Queensland under MP 4.2 outside reticulated zones. Tank sizing is typically 3,000–5,000 L for a 3-bed metropolitan family home, 10,000+ L for rural or off-grid.
- Detention tank. Required by some councils (Sydney City, Inner West, North Sydney, Willoughby) on lots over a certain hardstand percentage. Holds the peak storm flow and releases at a regulated rate to the council main. Typically 1.5–3.0 m³ for a 4-bed home.
Common edge cases
Two-storey house, no upper-storey gutter. Combine the upper and lower roof catchments for the lower-storey downpipe count. Better fix: add a small upper-storey gutter routed via a kickout into the lower-storey gutter.
Cyclone Region C1 / C2 / C3. AS 4055 wind loading applies — 14 g × 65 mm Type 17 self-drillers at every clip, not the standard 12 g × 50 mm. Downpipe brackets at 1.0 m max centres in C2 (Cairns / Townsville) vs 1.2 m in N3 (Sydney / Melbourne).
Heritage Federation / Queenslander home. Cast-iron round or galvanised round downpipes are the typical original specification. Replace like-for-like with copper round (Wakool / Argosy / heritage merchants) or galvanised steel; uPVC is usually refused under heritage overlay (HCA / NSW Heritage Council).
Bushfire region BAL-FZ / BAL-40 / BAL-29. Steel downpipes only (Colorbond, Zincalume, copper, galvanised). uPVC is non-compliant under AS 3959:2018 — it ignites at radiant heat exposure 29 kW/m² and would propagate flame to the gutter line.
Reference standards (Australia)
- AS/NZS 3500.3:2021 — Plumbing and drainage Part 3: Stormwater drainage.
- AS/NZS 3500.0:2021 — Plumbing and drainage Part 0: Glossary.
- AS 4055:2021 — Wind loads for housing (downpipe bracket fixings in cyclonic regions).
- AS 3959:2018 — Construction of buildings in bushfire-prone areas (BAL ratings for downpipe materials).
- NCC 2022 Volume 2 Part 13.6 — Roof drainage (deemed-to-satisfy reference to AS/NZS 3500.3).
- NSW BASIX SEPP — Mandatory rainwater capture for new dwellings.
- Queensland MP 4.2 — Mandatory rainwater plumbing for newbuild outside reticulated zones.
- Victoria Plumbing Regulations 2018 — AS/NZS 3500 compliance.
- WAPC SPP 2.9 — Western Australian stormwater management policy (soakwell sizing).
- BoM IFD tool — Bureau of Meteorology Intensity-Frequency-Duration design rainfall data per postcode.
- BlueScope / Stramit / Lysaght / Fielders / Metalcorp installation manuals — Manufacturer-specified sizing for warranty compliance.
Related calculators
- Gutter size calculator — choose 115 mm quad, 125 mm fascia, or 150 mm half-round by catchment.
- Gutter slope calculator — set the right fall on the gutter run.
- Gutter installation cost calculator — full first-time install pricing per linear metre.
Sources: AS/NZS 3500.3:2021 Stormwater drainage; AS 4055:2021 Wind loads for housing; AS 3959:2018 Bushfire construction; NCC 2022 Volume 2 Part 13.6; BASIX SEPP NSW; Queensland Plumbing and Drainage Regulation 2003; WAPC SPP 2.9; Bureau of Meteorology IFD design rainfall data; BlueScope, Stramit, Lysaght, Fielders, Metalcorp installation manuals; ARC technical bulletin 22.
Frequently asked questions
How many downpipes do I need for a 200 m² roof?
Apply the AS/NZS 3500.3 simplified rule of thumb — 1 cm² of downpipe cross-section per 1 m² of effective catchment. A 22.5° pitched roof has a pitch factor of about 1.10, so 200 m² of plan-view area becomes 220 m² of effective catchment. A standard 100 mm × 75 mm Colorbond rectangular downpipe (75 cm² cross-section) drains 75 m² per pipe, so you need ceiling(220 / 75) = 3 downpipes. The standard Australian project home is built that way: one downpipe at each rear corner plus a third at the end of the front gutter run, draining to a stormwater PVC trunk that discharges to the street kerb or a rainwater tank.
What size downpipe pairs with a 115 mm Colorbond gutter?
A 100 × 75 mm rectangular Colorbond downpipe pairs with the standard 115 mm quad gutter — that's the BlueScope / Stramit / Lysaght / Fielders system default. Cross-section is 75 cm², drains 75 m² per pipe. For larger homes with the 150 mm half-round gutter, step up to 100 × 100 mm rectangular (100 cm², drains 100 m²) or 100 mm round (78 cm², drains 78 m²). For tropical North Queensland and high-rainfall regions of NSW Northern Rivers and the Wet Tropics of FNQ, double the calculation — that's where the AS/NZS 3500.3 rule of thumb breaks down and the full ARI 1-in-20 5-minute rainfall calculation governs.
Round vs rectangular downpipes — which is better?
Hydraulically identical for the same cross-section. The choice is structural and aesthetic: rectangular is the volume default in Colorbond and Zincalume because it sits flat against the wall and the BlueScope / Stramit / Lysaght / Fielders system is engineered around it. Round is the heritage default for pre-war Federation, Queenslander, and California Bungalow homes where copper and galvanised steel round downpipes were original. Round in copper is the high-end-residential default for modernist architecture (Burley Katon Halliday, Dunn DeSantis Wood). Round in uPVC is the cheap default for sheds, granny flats and low-spec rental builds — never specify uPVC on the main residence in cyclone regions C1/C2/C3 (it perishes in UV plus fails the AS 4055 wind loading test).
What's the AS/NZS 3500.3 method for downpipe sizing?
AS/NZS 3500.3 specifies the rational method for stormwater drainage from roofs. The full calculation uses Q = (C × i × A) / 360 in litres per second where C is the runoff coefficient (1.0 for non-permeable roofs), i is the rainfall intensity in mm/hr at ARI 1-in-20-year 5-minute, and A is the catchment in m². The simplified rule of thumb for typical Australian rainfall (90 mm/hr design intensity for most of the eastern seaboard, 150 mm/hr for tropical FNQ) is 1 cm² of downpipe cross-section per 1 m² of catchment. Use the full calculation for any project on a contour map, in a known overland-flow path, or in cyclone regions C1/C2/C3 where the design ARI escalates.
Where should downpipes be placed on the gutter run?
Two principles: never longer than 12 m of single-fall gutter without a downpipe, and place at the ends not the middle on hipped or split-fall front gutters. For a typical 12 m project home front gutter run with a single fall to one side, one downpipe at the low corner is sufficient. For a 15 m+ front gutter on an architect-designed home, two downpipes — one at each end — works better than one at the high midpoint. Place downpipes adjacent to the soil stack on the rear elevation when possible; that simplifies discharge to the council stormwater pit or rainwater tank.
Can downpipes discharge to a rainwater tank in Australia?
Yes, and most newbuild contracts now include rainwater capture under BASIX in NSW (mandatory for new dwellings since 2004), 6-Star Energy Rating or 7-Star NCC 2022 in Victoria, Queensland MP 4.2 mandatory rainwater plumbing for newbuild outside the metropolitan reticulated zone, SA 50/50 plumbing rule (Water Conservation Code), and WA WAPC SPP 2.9 stormwater management. Tank sizing is typically 3,000–5,000 L for a 3-bed family home and 10,000+ L for a rural property on tank-only water. Plumbing is required for at least one tank-fed fixture (toilet, washing machine cold tap, or external tap depending on jurisdiction). Always check the local council requirements — BASIX, BCA, NCC, and state-level overlays vary.
How do I handle a two-storey house with no upper-storey gutter?
The upper roof discharges directly onto the lower roof, so the lower-storey downpipes must be sized for the combined catchment. For a 90 m² upper roof discharging to a 110 m² lower roof at 22.5° pitch, the combined effective catchment is (90 + 110) × 1.10 = 220 m². Three 100 × 75 mm Colorbond downpipes (75 cm² each, drains 75 m² each) handles it (3 × 75 = 225 m² > 220 m²). The better fix on architect-designed homes is to add a small upper-storey gutter that catches the upper roof and routes via a kickout into the lower-storey gutter — that distributes the flow rather than concentrating it where the upper roof terminates.
Are downpipe sizes required by Australian building code?
Yes — AS/NZS 3500.3 (Stormwater Drainage) is referenced as deemed-to-satisfy by the National Construction Code 2022 Volume 2 Part 13.6 for residential. Council stormwater connection requires the downpipe count and discharge location to be shown on the plumbing application drawings. State-level overlays vary: NSW BASIX requires rainwater capture sizing for the dwelling, Queensland Plumbing and Drainage Regulation 2003 requires AS/NZS 3500.3 compliance, Victoria Plumbing Regulations 2018 mandate AS/NZS 3500.3, and the BCA references AS/NZS 3500 across all states. For cyclone regions C1/C2/C3, the cyclonic wind loading on downpipes (AS 4055 Tbl 2.6 for C1) requires extra fixings — typically 14 g × 65 mm Type 17 self-drillers at every clip into the wall framing, not the standard 12 g × 50 mm of N1/N2/N3 regions.