AS/NZS 3500.1:2021 Standard Reference — Last Updated: 8 August 2026
Getting your pipe sizing wrong is a surefire way to cop callbacks for low pressure or noisy pipes. Under AS/NZS 3500.1, plumbers need to ensure proper flow rates to all fixtures without exceeding maximum velocity limits. Work out your probable demand first using our Fixture Unit Calculator and calculate flow requirements with the Water Flow Rate Calculator.
Why Velocity Matters Just As Much As Pressure
When most plumbers think about sizing a water service, they focus on delivering enough pressure. But AS/NZS 3500.1 puts a hard cap on water velocity—generally 3.0 m/s for standard domestic supply piping, though aiming for under 2.4 m/s delivers quieter, longer-lasting installations.
Pushing water too fast causes severe water hammer, erodes copper pipe fittings internally through impingement attack, and generates loud scouring noise whenever a valve opens. High mains pressure does not eliminate velocity issues—smaller pipe sizes might pass pressure drop criteria but fail velocity limits. Always verify velocity when stepping down pipe sizes.
Friction Loss and Pipe Materials
Not all pipe materials behave identically under flow. A DN20 Copper Type B pipe has a different internal bore (17.01mm) compared to a DN20 PEX pipe (14.40mm), meaning they carry different fluid volumes and create different head loss per metre. Friction loss is calculated using the Hazen-Williams equation:
| Hazen-Williams Friction Loss Equation |
|---|
| hf = (10.67 × L × Q1.852) / (C1.852 × d4.87) Where hf = head loss in metres, L = pipe length (m), Q = flow rate (m³/s), C = roughness coefficient (Copper 140, PEX 150, PVC 150, Galv 100), and d = internal bore (m). |
Plastic pipes such as PEX and PVC are smooth with a high C-factor (150), reducing internal hydraulic drag. Older galvanised iron pipes have a poor C-factor (100) due to internal rust scaling. Check detailed pressure losses across specific runs with our Water Pressure Drop Calculator.
Worked Example: Sizing a PEX Run to a Granny Flat
Consider sizing a new cold water service to a detached granny flat 45 metres from the meter. Required peak flow rate is 0.45 L/s (shower, toilet, and sink running concurrently), and mains pressure at the meter reads 300 kPa:
- 1. Test DN20 PEX (14.4mm ID): Flow velocity hits 2.76 m/s (below 3.0 m/s limit), but friction loss over 45m equals 228 kPa.
- 2. Evaluate Residual Pressure: 300 kPa supply - 228 kPa drop = 72 kPa remaining. This fails minimum fixture requirements (mixer taps require 150 kPa).
- 3. Step Up to DN25 PEX (18.0mm ID): Flow velocity drops to a quiet 1.77 m/s, and friction loss drops significantly to 77 kPa.
- 4. Final Pressure Verification: 300 kPa supply - 77 kPa drop = 223 kPa remaining, easily exceeding minimum fixture operating pressure.
Maximum Flow Rates Before Hitting 3.0 m/s Limit
Reference table for maximum allowable flow rate (L/s) through common domestic pipe sizes before reaching the AS/NZS 3500.1 velocity cap of 3.0 m/s:
| Nominal Pipe Size | Copper Type B (L/s) | PEX Pipe (L/s) | PVC Class 18 (L/s) |
|---|---|---|---|
| 15mm (DN15/16) | 0.27 L/s | 0.31 L/s | 0.78 L/s |
| 20mm (DN20) | 0.68 L/s | 0.48 L/s | 1.32 L/s |
| 25mm (DN25) | 1.24 L/s | 0.76 L/s | 2.10 L/s |
| 32mm (DN32) | 2.02 L/s | 1.26 L/s | 3.47 L/s |
| 40mm (DN40) | 2.99 L/s | 1.98 L/s | 4.47 L/s |
Dealing With Static Head Loss (Elevation)
When running pipework vertically up into a ceiling cavity or multi-storey building, gravity exerts a constant backpressure of 9.81 kPa per metre of vertical rise (static head loss).
For instance, feeding a second-floor bathroom 6 metres above meter level incurs a 58.9 kPa static pressure loss before accounting for pipe friction loss. Always include elevation change when calculating residual pressure.