Question 1

What is a good water flow rate for a house in Australia?

Accepted Answer

A typical Australian home requires a mains flow rate of at least 20 to 30 L/min at the meter to comfortably run multiple fixtures at once. A standard 3-star WELS shower uses 9 L/min, so if your mains supply drops below 15 L/min, you'll definitely notice pressure drop when someone flushes the toilet.

Question 2

How do I convert L/s to L/min?

Accepted Answer

To convert Litres per second (L/s) to Litres per minute (L/min), simply multiply the L/s value by 60. For example, a flow rate of 0.2 L/s is exactly 12 L/min. To go the other way from L/min to L/s, divide by 60.

Question 3

What is the maximum allowable water velocity in AS/NZS 3500?

Accepted Answer

According to AS/NZS 3500.1, the maximum water velocity in a water supply system shouldn't exceed 3.0 m/s. Pushing water faster than this drastically increases the risk of water hammer, noisy pipes, and physical erosion of the pipe walls and fittings.

Question 4

Why is water velocity lower for recirculating hot water systems?

Accepted Answer

In a heated water flow and return system, the pump runs constantly. If the velocity is too high, the continuous flow of water will slowly scour and erode the inside of the copper pipes, leading to pinhole leaks. The recommended velocity for these systems is usually capped around 1.2 m/s.

Question 5

Does increasing pipe diameter increase flow rate?

Accepted Answer

Increasing the pipe diameter increases the capacity of the pipe and reduces friction loss, which allows for a higher flow rate at the end fixture, assuming your mains pressure remains constant. It also slows the water velocity down, which makes the plumbing quieter.

Question 6

What is the internal diameter of DN20 copper pipe?

Accepted Answer

In Australia, DN20 Type B copper tube has an outside diameter of 19.05mm and a wall thickness of 1.02mm, giving it an internal diameter of exactly 17.01mm. This internal diameter is the measurement you must use when calculating flow rate and velocity.

Question 7

What's the difference between water pressure and flow rate?

Accepted Answer

Water pressure (measured in kPa) is the physical force pushing the water through the pipes. Flow rate (measured in L/min) is the actual volume of water coming out of the end of the pipe over a specific time. You can have high static pressure but terrible flow rate if your pipes are undersized or blocked.

Question 8

How do you manually measure flow rate from a tap?

Accepted Answer

Grab a standard 9-litre or 10-litre bucket and a stopwatch. Turn the tap on full hot or cold, start the timer, and see how long it takes to fill the bucket. If a 10-litre bucket fills in 30 seconds, your flow rate is 20 L/min.

Question 9

Why do my pipes whistle when the shower is on?

Accepted Answer

Whistling pipes are usually caused by excessive water velocity forcing water through a narrow restriction, like an undersized branch line or a partially closed isolation valve. Upsizing the pipe or removing the restriction will slow the velocity and stop the noise.

Question 10

What causes water hammer?

Accepted Answer

Water hammer (hydraulic shock) happens when fast-moving water is suddenly stopped by a valve closing quickly, like a washing machine solenoid or a flick mixer. The momentum of the water creates a shockwave. Keeping pipe velocity under 1.5 m/s is the best way to prevent severe water hammer.

Question 11

Is PEX smaller inside than copper?

Accepted Answer

Yes, standard PEX piping has thicker walls than copper to achieve the same pressure rating. For example, DN20 PEX usually has an internal diameter around 16.0mm, whereas DN20 Type B copper is 17.0mm. This slight reduction means PEX will have a slightly higher velocity for the exact same flow rate.

Question 12

What is the standard flow rate for a fire hose reel?

Accepted Answer

Under AS 2441, a standard fire hose reel in Australia must be capable of delivering a minimum flow rate of 0.33 Litres per second (which is 19.8 L/min). The supply pipes must be sized to ensure this flow rate can be achieved while maintaining the required pressure.

Fixture / Outlet	Typical Flow Rate (L/min)	Equivalent in L/s
Standard Shower (WELS 3 Star)	9 L/min	0.15 L/s
Kitchen Sink Mixer	6 - 9 L/min	0.10 - 0.15 L/s
Toilet Cistern (Fill rate)	6 L/min	0.10 L/s
Bath Spout	15 - 20 L/min	0.25 - 0.33 L/s
Garden Hose Tap (Mains)	15 - 18 L/min	0.25 - 0.30 L/s

Water Flow Rate Calculator

Water Pipe Sizing

Pipe Volume

Getting Pipe Flow and Velocity Right

The Core Formula Explained

Q = A × v

Australian Standard Velocity Limits (AS/NZS 3500)

Worked Example 1: Checking Velocity for a Shower Run

1 Step 1:

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4 Result:

Worked Example 2: Sizing a Pipe for a Commercial Washdown

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3 Step 3:

4 Result:

Frequently Asked Questions