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Stormwater pipe size calculator

Choose the method, enter the flow or the roof, paved and garden areas at the design storm, then the pipe's grade and material. The calculator finds the smallest pipe that carries it within the Standard's limits, says what sets the size, and checks the cover and a kerb connection.

Under NCC 2025, site stormwater drainage is designed to AS/NZS 3500.3 (Volume One F1D3 for Class 2 to 9 buildings, Volume Two H2D2(1)(a) for houses and Class 10 buildings), and this calculator applies that Standard's pipe sizing method.

How it works

It reads from

  • AS/NZS 3500.3:2025 Clauses 5.4.3 to 5.4.8 (rational method; Table 5.4.3 design AEP), 5.4.11 and Figure 5.4.11.2
  • Clause 5.5 (nominal method); NCC 2025 Volume Two H2D2(1)(b) and Housing Provisions 3.3.5 (Part 3.3 route)
  • Table 5.4.11.2 (roughness), Table 6.3.4 (minimum grades), Clause 6.3.3 (minimum sizes), Table 6.2.5 and Clause 6.3.6 (cover)
  • Clause 7.5.1.2(c) and Figure 7.5.1.2 (kerb connection), Clause 7.9 (anchor blocks), Clauses 5.2.3 and 5.4.12.1 (box gutters)

Which method applies

The general method (Clause 5.4) sizes the pipe for the design flow. For a single dwelling in a non-urban area or on an urban lot under 1 000 m², the nominal method (Clause 5.5) may be used instead: pipes follow local practice without calculations, and must meet the minimum size, cover and grade. For a Class 1 or 10 building where the 5% AEP, 5-minute intensity is 255 mm/h or less, NCC 2025 Volume Two H2D2(1)(b) also allows Housing Provisions Part 3.3, provided the system otherwise complies with AS/NZS 3500.3; its 3.3.5(c) covers apply to 90 mm Class 6 uPVC drains.

Design flow

Q = ΣCA × I / 3600, with C of 1.0 for roofs, 0.9 for paving and a calculated value for gardens and lawns (Clause 5.4.6). Table 5.4.3 sets the design AEP by what a surcharge would do: 63% for a small impact in a low density area, 50% for normal impacts, 10% for ponding in flat topography, a car park flooded over 150 mm or impeded access to commercial and industrial buildings, and 5% for ponding against adjoining buildings or impeded access to important ones. Read the 5-minute intensity for that AEP from the Bureau of Meteorology's design rainfall (IFD) at bom.gov.au/water/designRainfalls/revised-ifd.

Capacity

Clause 5.4.11.2(c) takes the capacity from Figure 5.4.11.2, whose four panels chart roughness k of 0.015, 0.15, 0.6 and 3.0 mm, from DN 90 to DN 375 and from 1:5. Nothing is read beyond a line's printed ends, and the chart stops at DN 375: a larger flow needs a steeper grade or a second drain. Panel (d) prints both of its lines as "DN 150"; by their position they are DN 300 and DN 375, and they are read as such.

Limits

Full-pipe velocity v = 4000 Q / (π D²) with the nominal size, as the Standard's worked examples tabulate it: at most 2.0 m/s, and 1.5 m/s advised for a pipe leaving a pit. Minimum grades from Table 6.3.4 (1:100 for DN 90 to DN 150 in Australia); it does not list DN 200 or DN 250, and the steeper neighbouring grade is applied to them. Grades run from 1:5, where Figure 5.4.11.2 starts; a drain steeper than 1:5 also needs anchor blocks (Clause 7.9). The result says what sets the size and, where a smaller pipe would do on a steeper grade, which grade.

Minimum sizes

Clause 6.3.3: DN 90 for single dwellings in rural areas and residential buildings on urban lots under 1 000 m². On other properties a pipe leaving a pit is at least DN 150 and at least the largest pipe entering the pit. The Standard's own examples take DN 150 between pits on house lots too: the calculator sizes to DN 150 there, and marks a smaller pipe Check rather than failing it.

Cover and the kerb

Table 6.2.5 sets the minimum cover by where the pipe runs and what it is made of (Clause 6.2.5); under a building's slab, Clause 6.3.6. Where the drain connects to a street kerb and gutter and is larger than DN 100, an inlet pit is needed at the boundary (Clause 7.5.1.2(c)), and several DN 100 or smaller pipes, or rectangular conduits, cross the footpath from it (Clause 6.3.3, Figure 7.5.1.2). The calculator works out how many.

Box gutters

Where box gutters connect upstream of a surcharge outlet, every drain upstream of that outlet carries the 1% AEP flow from all the surfaces above it (Clauses 5.2.3 and 5.4.12.1).

A worked example

The calculator's example inputs, worked through step by step.

Pipe size

DN 150

DN 150 at 1:100 carries 24.7 L/s on Figure 5.4.11.2(a), enough for the 20.0 L/s design flow, at 1.13 m/s.

  1. Equivalent impervious areaΣCA = 1.0 Ar + 0.9 Ai + Cp Ap = 250.0 + 0.9 × 120.0 + 0.00 × 0.0 = 358.0 m²AS/NZS 3500.3:2025 Clause 5.4.6
  2. Design flow, 5% AEPQ = ΣCA × I / 3600 = 358.0 × 201 / 3600 = 20.0 L/sAS/NZS 3500.3:2025 Eq 5.4.8
  3. Hydraulic capacity, full boreread from Figure 5.4.11.2(a) = DN 150, 1:100, k 0.015 mm = 24.7 L/sAS/NZS 3500.3:2025 Clause 5.4.11.2(c); Figure 5.4.11.2(a); Table 5.4.11.2
  4. Full-pipe velocity at the design flowv = 4000 Q / (π D²), D the nominal size = 4000 × 19.9883 / (π × 150²) = 1.13 m/sAS/NZS 3500.3:2025 Eq J.3.3.2(3)
  5. What sets the size = DN 100 carries 8.00 L/s at 1:100, less than 20.0 L/s. = capacityAS/NZS 3500.3:2025 Clause 5.4.11.2(d); Figure 5.4.11.2

Questions

What size stormwater pipe do I need?

Work out the flow by the rational method (Q = ΣCA × I / 3600), then find the smallest pipe whose capacity on Figure 5.4.11.2 at your grade exceeds it, with a full-pipe velocity of no more than 2.0 m/s. In smooth plastic at 1:100 a DN 90 carries about 6 L/s, a DN 100 8 L/s and a DN 150 about 25 L/s.

What is the minimum fall for a stormwater pipe?

1:100 for DN 90, DN 100 and DN 150 in Australia, 1:200 for DN 225, 1:250 for DN 300 and 1:300 for DN 375 (AS/NZS 3500.3:2025 Table 6.3.4). The table does not list DN 200 or DN 250.

What is the minimum cover for a stormwater pipe?

AS/NZS 3500.3:2025 Table 6.2.5, finished surface to the top of the pipe. For plastics: 100 mm under a garden at a single dwelling and 300 mm elsewhere without pavement, 100 mm under brick or plain concrete paving, 450 mm where vehicles cross unpaved ground, 100 mm below a reinforced concrete pavement for vehicles, and 600 mm under a sealed road. Under a building's slab, 25 mm (Clause 6.3.6). For 90 mm Class 6 uPVC on the Housing Provisions route, 3.3.5(c) sets 100 mm under soil and 50 mm under paving.

When does a stormwater drain need a pit at the kerb?

When it connects to a street kerb and gutter and is larger than DN 100 (AS/NZS 3500.3:2025 Clause 7.5.1.2(c)). Place an inlet pit with a sump beside the property boundary; from it, several DN 100 or smaller pipes, or rectangular conduits, cross the footpath to the kerb (Figure 7.5.1.2). The council approves the kerb outlet.

Can a single dwelling skip the calculation?

Yes. The nominal method of AS/NZS 3500.3:2025 Clause 5.5 covers single dwellings in non-urban areas and on urban lots under 1 000 m²: pipes follow local practice without design calculations, but still meet the minimum size (Clause 6.3.3), cover (Clause 6.2.5) and grade (Table 6.3.4). The calculator runs it, and shows the general-method flow for comparison when you give the areas.

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