Work out the internal diameter a water pipe needs to carry a given flow at a sensible velocity — a quick starting point ahead of a full AS/NZS 3500 sizing.
Indicative only. Pipe sizing under AS/NZS 3500 also considers pressure loss, loading units, simultaneous demand and the material’s actual bore — size real jobs with the full method.
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Start free trialDividing the flow rate by the design velocity gives the cross-sectional area the pipe bore must have.
The internal diameter is √(4 × area ÷ π). Keeping velocity sensible avoids noise, water hammer and erosion.
Round the calculated bore up to the next nominal pipe size, remembering the actual internal diameter varies by material and class.
Pick a design velocity, then divide the flow rate by it to get the pipe’s cross-sectional area, and convert that to an internal diameter. For 0.5 L/s at 2 m/s you need about 18 mm bore, so a DN20 pipe. Real sizing also checks pressure loss and demand under AS/NZS 3500.
Typically around 1–2.4 m/s. Too slow and pipes are oversized and sluggish; too fast and you get noise, water hammer and erosion of the pipe wall, especially in copper. The plumbing standard sets maximum velocities to protect the system.
Nominal size (DN) is a label, not the exact internal diameter. The actual bore depends on the pipe material, class and wall thickness — a DN20 copper, PEX and PVC pipe all have slightly different internal diameters, which matters for accurate flow and pressure work.
No — it’s an indicative aid. Compliant pipe sizing uses loading units, probable simultaneous demand and pressure-loss calculations to AS/NZS 3500, by a licensed plumber or hydraulic designer.
This calculator is an indicative aid based on flow, velocity and bore. Compliant pipe sizing must follow AS/NZS 3500 accounting for loading units, demand, pressure loss and material bore, by a licensed plumber or hydraulic designer.
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