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Stormwater network calculator

List the pipes, what each drains and where it flows, and which leave a pit. The calculator adds each branch's equivalent impervious area down the network and sizes or checks every pipe; with surface levels it also sets the inverts, checks the cover, and sizes the pits and grates.

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 checks every pipe in a network by that Standard's Appendix J method.

How it works

It reads from

  • AS/NZS 3500.3:2025 Appendix J.3 and J.4 (the general method applied to a network; Table J.1 levels and covers)
  • Clauses 5.4.6, 5.4.8, 5.4.11; Table 6.3.4; Clause 6.3.3; Table 6.2.5 (cover); Clause 7.5.3 (falls across pits); Table 7.5.2.1 (pit sizes)
  • Equation 5.4.10.1 (grates); Clauses 5.2.3 and 5.4.12 (surcharge outlets); Clause 7.5.1.2(c) (kerb connection); Clause 5.5 (nominal method)

One intensity, cumulative areas

The Standard's own network example applies one 5-minute intensity to every pipe and adds the equivalent impervious area of everything upstream, and any known flow entering (a pump, a device, another system). No time of travel is added, which suits a single property.

Each pipe

For every pipe: the flow, the capacity read from Figure 5.4.11.2 at its grade, the full-pipe velocity with the nominal size (at most 2.0 m/s, and 1.5 m/s advised for a pipe leaving a pit), the grade against Table 6.3.4 and the Clause 6.3.3 minimum size, including the largest pipe entering a pit. A pipe set to Auto takes the smallest size that works, never smaller than a pipe entering it.

Levels, pits and grates

With surface levels, a blank invert is set as the Standard's Appendix J example sets it: at the minimum cover of Table 6.2.5 at both ends, level with the pipes arriving at a junction, and at least 20 mm below them across a pit (Clause 7.5.3). Each pit is sized by its depth (Table 7.5.2.1), each grate is checked by Equation 5.4.10.1, the outlet is checked against the point of connection, and the longest run is drawn as a long section.

Box gutters and surcharge outlets

Where box gutters connect, the pipes above the surcharge outlet carry the 1% AEP flow and those from it down the Table 5.4.3 AEP (Clause 5.2.3). Mark the surcharge pit, and its grate is sized for the whole flow at that point (Equation 5.4.12.2).

A worked example

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

Flow at the outlet

20.60 L/s

Every pipe carries its flow. The busiest is A, at 84% of its capacity.

  1. Runoff coefficientsCr = 1.0, Ci = 0.9, Cp as entered = Cp 0.00AS/NZS 3500.3:2025 Clause 5.4.6
  2. One 5-minute intensity for every pipe = 201 mm/hAS/NZS 3500.3:2025 Clause 5.4.4; Appendix J.3
  3. Pipe A → CQ = ΣCA × I / 3600 + known inflow; capacity from Figure 5.4.11.2; v = 4000 Q / (π DN²) = ΣCA 120.0 m² at 201 mm/h, DN 100 (sized) at 1:100, fall 0.12 m = 6.70 L/s of 8.00 L/s · 0.85 m/s
  4. Pipe B → CQ = ΣCA × I / 3600 + known inflow; capacity from Figure 5.4.11.2; v = 4000 Q / (π DN²) = ΣCA 117.0 m² at 201 mm/h, DN 100 (sized) at 1:100, fall 0.08 m = 6.53 L/s of 8.00 L/s · 0.83 m/s
  5. Pipe C → DQ = ΣCA × I / 3600 + known inflow; capacity from Figure 5.4.11.2; v = 4000 Q / (π DN²) = ΣCA 309.0 m² at 201 mm/h, DN 150 (sized) at 1:100, fall 0.15 m, leaves a pit = 17.3 L/s of 24.7 L/s · 0.98 m/s
  6. Pipe D → outletQ = ΣCA × I / 3600 + known inflow; capacity from Figure 5.4.11.2; v = 4000 Q / (π DN²) = ΣCA 369.0 m² at 201 mm/h, DN 150 (sized) at 1:100, fall 0.06 m = 20.6 L/s of 24.7 L/s · 1.17 m/s

Questions

How do I size a stormwater system with several downpipes?

Split it into pipes between pits and junctions, add up the equivalent impervious area draining to each (roof × 1.0, paving × 0.9, garden × Cp), and size each pipe for ΣCA × I / 3600 at the 5-minute intensity — the method of AS/NZS 3500.3:2025 Appendix J.

Does the network calculator set invert levels?

Yes, when you enter the surface levels. A blank invert is set at the minimum cover of AS/NZS 3500.3:2025 Table 6.2.5 at both ends of the pipe, level with the pipes arriving at a junction and at least 20 mm below them across a pit (Clause 7.5.3), as the Standard's Appendix J example does. Enter your own inverts and it checks them instead.

Can a house lot use a DN 100 pipe out of a pit?

AS/NZS 3500.3:2025 Clause 6.3.3(a) reads DN 90 for single dwellings and residential lots under 1 000 m². The Standard's own worked examples take DN 150 between pits on those lots, so the calculator sizes to DN 150 there and marks a smaller pipe Check for you to confirm with the certifier.

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The method behind every calculator, in one guide:roof and stormwater drainage to AS/NZS 3500.3:2025.

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