Skip to content

Water pipe size calculator

Choose a supply pipe to dwellings or a branch within one, then the pipe, the index length, the height of the highest outlet and the supply pressure. The calculator gives the size Table C.1 gives by the Standard's steps and checks it against the velocity and pressure limits.

NCC 2025 Volume Three B1D3 requires a cold water service in any class of building to comply with AS/NZS 3500.1, and this calculator applies that Standard's sizing method and checks the 3 m/s and 50 kPa / 500 kPa limits that Performance Requirements B1P2 and B1P6 also set.

How it works

It reads from

  • AS/NZS 3500.1:2025 Tables 3.2.1, 3.2.3 and 3.2.4; Clauses 3.3, 3.4 and 3.5
  • Appendix A (internal diameters); Appendices C and D (Table C.1, Equation C.1 and the sizing steps)

Demand

The supply pipe to one or more dwellings takes the Table 3.2.3 minimum (0.48 L/s for one dwelling); the equation in its Note 2, Q = 0.03n + 0.4554√n, extends it beyond 100 dwellings. Branch piping inside a dwelling takes a single fixture's Table 3.2.1 flow, or for several fixtures the Table 3.2.4 flow for their loading units.

Size

The pressure drop available is Hm − Hs − 5 m, with Hs the height of the highest outlet (Equation C.2), less the losses through meters, backflow devices and valves (Clause 3.3.3). Table C.1 is read for the next lower pressure drop and the index-length column at or above the run, and the first size whose flow is not less than the demand is taken (Appendix C Steps 7 to 9). Table C.1 is for copper Type B; other pipes take the same steps with Equation C.1 at their Appendix A internal diameters.

Checks

Velocity at or below 3.0 m/s (Clause 3.4). At least 50 kPa at the most disadvantaged outlet after friction over the index length (Equation C.1 plus 50% for fittings), the lift to the highest outlet and the other losses (Clauses 3.3.2 and 3.3.3). Static pressure at the lowest outlet at or below 500 kPa (Clause 3.3.4). A single dwelling's water service at least 15.0 mm bore (Clause 3.5.1).

A worked example

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

Pipe size

DN 25

DN 25 carries 0.530 L/s at 1.28 m/s and leaves 171 kPa at the most disadvantaged outlet.

  1. Loading unitsΣ outlets × loading units = 2 × 2 + 1 × 8 + 2 × 1 + 2 × 2 + 1 × 3 + 1 × 3 + 2 × 3 + 2 × 4 = 38.0AS/NZS 3500.1:2025 Table 3.2.1
  2. Probable simultaneous flow rateTable 3.2.4 at the loading units, the next whole unit up = 38 LU = 0.530 L/sAS/NZS 3500.1:2025 Clause 3.2.4; Table 3.2.4
  3. Pressure drop available for frictionHm − Hs − Hx − other losses = 25.50 − 4.00 − 5 (m head; 1 kPa = 0.102 m) = 16.50 mAS/NZS 3500.1:2025 Appendix C, Equation C.2; Clause 3.3.3
  4. Pipe size by Table C.1the table at or below the pressure drop, the column at or above the index length, then the first flow not less than the demand = 15 m table, 30 m column: 1.08 L/s in DN 25 = DN 25AS/NZS 3500.1:2025 Appendix C, Steps 7–9; Appendix D Step 6; Table C.1
  5. Pipe = DN 25, internal diameter 23.0 mmAS/NZS 3500.1:2025 Appendix A
  6. Velocityv = Q / A = 0.53 L/s in 23.0 mm = 1.28 m/sAS/NZS 3500.1:2025 Clause 3.4
  7. Head-loss gradientQ = 3.637×10⁻⁵ H^0.555 D^2.667, solved for H = 9.08 m per 100 mAS/NZS 3500.1:2025 Appendix C, Equation C.1
  8. Friction loss with fittingsh = H/100 × L × 1.5 = 9.0825/100 × 30.0 × 1.5 = 4.09 mAS/NZS 3500.1:2025 Appendix C; Clause 3.3.3 Note
  9. Pressure at the most disadvantaged outletP − (h + Hs) × 9.8 − other losses = 250 − (4.0871 + 4.0) × 9.8 = 171 kPaAS/NZS 3500.1:2025 Clause 3.3.2

Questions

What size water pipe does a house need?

The supply pipe to a house carries at least 0.48 L/s (AS/NZS 3500.1:2025 Table 3.2.3) and has at least 15.0 mm bore (Clause 3.5.1). Branches inside take their fixtures' loading units (a WC 2, basin 1, shower 2, bath 8, kitchen sink 3) through Table 3.2.4. Each size then comes from Table C.1 for the supply pressure, the height of the highest outlet and the index length, keeping the velocity at or below 3.0 m/s and at least 50 kPa at the most disadvantaged outlet.

What is the maximum water pressure in a house?

500 kPa static at any outlet other than a fire service outlet within a building (AS/NZS 3500.1:2025 Clause 3.3.4). Where the static pressure at an outlet would be higher, it has to be reduced, typically with a pressure-limiting valve.

More calculators

All calculators

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

Ask about your own roof, drain or detention tank

The free plan answers from the National Construction Code, state building and planning legislation, ABCB handbooks, regulator practice notes and codes of practice, with the clause cited. Plus adds the Standards, including AS/NZS 3500.1:2025, from A$79 a month plus GST.

Try it freeSee plans and pricing