Question 1

How do you calculate water pressure drop in a pipe?

Accepted Answer

Pressure drop in a pipe comes from two things — friction loss (water rubbing against the pipe wall) and elevation changes (gravity). You calculate friction loss using the Hazen-Williams formula, which factors in pipe length, diameter, flow rate, and the pipe material's roughness (C-factor). Then add or subtract 9.81 kPa for every metre of height change. The total gives you the pressure drop over that pipe run.

Question 2

What is the minimum water pressure at a fixture in Australia?

Accepted Answer

AS/NZS 3500.1 requires a minimum of 50 kPa at any outlet, but practically you want at least 150 kPa. Most instantaneous hot water systems (Rinnai, Bosch) need 140-150 kPa minimum just to fire up. If your calculator shows anything under 150 kPa remaining, consider upsizing the pipe or fitting a booster pump.

Question 3

How much pressure do you lose per metre of pipe height?

Accepted Answer

Exactly 9.81 kPa per metre of vertical height. This is pure physics — the weight of the water column fighting gravity. So running a pipe up 6 metres to a second floor costs you about 59 kPa of pressure before friction even enters the picture.

Question 4

Why is pressure so bad on old galvanised pipes?

Accepted Answer

Galvanised steel corrodes from the inside out. After 20-30 years the bore gets choked with rust and calcium scale, cutting the effective internal diameter and dropping the C-factor from 120 down to 80 or lower. The friction loss through old galv can be three to four times worse than new copper or PEX of the same nominal size. If you're troubleshooting low pressure on a house built before the 1980s, the galv mains is almost always the culprit.

Question 5

What is the Hazen-Williams C-factor for PEX pipe?

Accepted Answer

PEX pipe (Auspex, Rehau, Roth) has a C-factor of 150. That's the smoothest rating you'll find in domestic plumbing — the plastic bore doesn't corrode, scale up, or get rough over time. It's one reason PEX delivers better long-term flow performance compared to copper, which sits at 140.

Question 6

Does pipe diameter affect pressure drop?

Accepted Answer

Massively. Pipe diameter is raised to the power of 4.87 in the Hazen-Williams formula, so small changes in diameter cause huge changes in friction loss. Going from DN20 (16.92mm ID copper) to DN25 (22.96mm ID copper) roughly halves the friction loss for the same flow rate. On long runs, that difference can save you 40-60 kPa of pressure.

Question 7

How do I account for elbows and fittings in pressure drop?

Accepted Answer

The simplest method is to add 10-20% to your straight pipe length to account for fittings. Each 90-degree elbow adds roughly 0.5-0.8 metres of equivalent pipe length, and each tee adds about 1.0-1.8 metres depending on the pipe size. For critical jobs, you can add up the equivalent lengths of each fitting individually, but the 10-20% rule of thumb works well for most residential installs.

Question 8

What flow rate should I use for a standard 3-bedroom house?

Accepted Answer

A typical 3-bed, 1-bath house has about 22-26 loading units, which converts to a probable simultaneous demand of roughly 0.6-0.7 L/s under AS/NZS 3500.1. That's the worst-case scenario where the shower, toilet, and a tap are all going at once. Use our Fixture Unit Calculator to get an exact number for your job.

Question 9

Can water pressure be too high in Australia?

Accepted Answer

Yes. AS/NZS 3500.1 says the pressure at any fixture shouldn't exceed 500 kPa. Anything above that and you risk blowing off hose connections, damaging tap cartridges, and causing water hammer. If your mains pressure is over 500 kPa (common in low-lying suburbs near reservoirs), you need a pressure limiting valve (PLV) at the meter.

Question 10

What causes water hammer and how does pressure drop relate to it?

Accepted Answer

Water hammer happens when fast-moving water is suddenly stopped by a valve closing quickly — usually at a solenoid valve on a dishwasher or washing machine. It's directly related to velocity, not pressure. If your pipes are undersized and water velocity is high (over 2.5-3.0 m/s), water hammer gets worse. Upsizing the pipe reduces velocity, which kills the hammer and reduces friction loss at the same time.

Question 11

How do I check my mains water pressure?

Accepted Answer

Screw a pressure gauge onto your front garden tap or the tap closest to the water meter, and turn it on. That gives you the static supply pressure. Do this in the morning when demand is low to get the true reading. Standard Australian mains pressure sits between 400-600 kPa, but it varies by suburb and water authority. Hills districts can be as low as 200 kPa.

Question 12

Is PEX better than copper for pressure drop?

Accepted Answer

PEX has a higher C-factor (150 vs 140 for copper), so it creates slightly less friction per metre. But PEX also has thicker walls, meaning the internal diameter is smaller than copper of the same nominal size. For DN20 pipe, copper has a 16.92mm internal bore while PEX only has 14.4mm. That smaller bore more than cancels out the smoother surface. So for the same nominal size, copper actually wins on pressure drop.

Question 13

What is the difference between static pressure and dynamic pressure?

Accepted Answer

Static pressure is what you measure when no water is flowing — just the weight of the water column from the reservoir or tank above you. Dynamic pressure is what you get when taps are open and water is moving through the pipe. Dynamic pressure is always lower than static because friction eats into the pressure as water flows. The pressure drop this calculator shows is the difference between static and dynamic.

Question 14

How far can you run 20mm pipe before pressure drops too much?

Accepted Answer

Depends on the flow rate and material. For a typical domestic flow of 0.3 L/s through DN20 Copper Type B, you lose about 0.8 kPa per metre of pipe. Over a 30-metre run, that's roughly 24 kPa — manageable if you started with 500 kPa. But at 0.5 L/s through the same pipe, friction loss jumps to about 1.9 kPa per metre — and 30 metres costs you 57 kPa.

Question 15

Do I need a booster pump for a two-storey house?

Accepted Answer

Usually not, if your mains pressure is decent (400 kPa+). Going up to a second floor (about 3 metres) costs 29 kPa of static head loss. Add maybe 20-40 kPa of friction loss for a typical pipe run, and you're still left with 330-350 kPa. You only need a booster if mains pressure is already low (under 250 kPa) or if the pipe run is very long (50+ metres).

Pipe Material	C-Factor	Typical Application & Surface Condition
PEX (Auspex, Rehau, Roth)	150	Ultra-smooth plastic bore, zero corrosion scale
PVC Class 18	150	Smooth plastic main lines & cold water services
Copper Type B	140	Standard Australian domestic hot/cold supply lines
Copper Type A	140	Thick-walled copper for underground mains
Galvanised Steel (New)	100–120	New steel lines (high initial wall drag)
Galvanised Steel (Old 20+ yrs)	70–80	Severely corroded, scaled interior bore

Water Pressure Drop Calculator

Water Pipe Sizing

Fixture Unit Calc

Understanding Pipe Pressure Drop in Australian Plumbing

How Friction Loss Works in Water Pipes

Hazen-Williams Formula (Metric)

Hazen-Williams C-Factor by Pipe Material

Worked Example: Pressure Drop to a Second-Floor Bathroom

1 Input Parameters

2 Calculate Friction Loss

3 Calculate Static Elevation Loss

4 Determine Residual Pressure

When You Need to Upsize or Fit a Pressure Booster

Cross-Checking Pipe Velocity

Frequently Asked Questions