WATER PIPE and PUMP SIZE CALCULATORS to The Australian Plumbing Code AS/NZS 3500.1

EXPLANATION: This is a very simple and quick method of calculating all the water pipe, and pump sizes, in any plumbing project, with only 3 clicks. It will also satisfy the Australian Plumbing Code.
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- Calculate the available Hydraulic grade
and the Capacity of all Pipe sizes with this hydraulic grade.
(Optional, but req'd for pump calcs)

Num of Dwellings
and/or Number of LU's
and/or Fixed Flow (L/s)
Flow Total(L/s)
Copper Dia (mm)
Plastic Dia (mm)

Start Pressure (kPa)
Length to worst case(m)
Height diff from start(+/-m)
Max Desired Vel(m/s)
Min Residual Head(m)
Hydraulic Grade

Hydraulic Grade Diag

A value is not required in the velocity or residual head fields if you wish to accept the default values.
Enter the remaining fields where required and press calculate. The capacity of all pipes sizes in the project will be shown in the table. You may then select your size based on the number of dwellings that pipe will serve, or the number of loading units. or the flow.

Should you wish to be told the pipe size without using the table, the program will need to know the flow, and/or dwellings, and or loading units. Use the optional fields to enter the required information.
The fixed flow field is useful when this is a constant and is known, as in a fire hose reel, or a shower block that must cater for all to be operational at once, etc.

The calculator in step 2 can be used to calculate the loading units, and flows, for individual fixtures.

If the pressure falls so low that a single dwelling may require about a 32mm diameter pipe or larger, the program will calculate a pump size. Use the pump input section to add the necessary details to do this.

Start pressure cannot be lower than -60kpa as most pumps cannot suck any higher than about -30kPa (3 metres).

Probable simultaneous flow
The flow can be anything from zero in the middle of the night, to having all taps turned on at once.
So how do we arrive at a "design" flow. Fortunately the Plumbing Codes allocate a number to each fixture, called a 'loading unit' (LU) or a 'fixture unit'(FU).
This number started out as gals/min (or something) but over the years this number has been tweaked and also takes into account a likely frequency of use.
These numbers are added up along the pipeline as each fixture is added, and a formula (or graph) in the Plumbing Code derives the 'Probable Simultaneous Flow' at that point.

If there is only one fixture, then the pipe must be sized for 100% of this flow requirement. However as the number of 'Loading Units' builds up, it becomes less likely that all fixtures will be operating at the same time. So as the number of LU's increase, the likely percentage of fixtures operating at any one time decreases. (to a certain minimum). This is called the Probable simultaneous flow rate (PSFR).

Generally all pipelines in a project are designed to the PSFR, unless there is a known flow required somewhere, as in a hose reel, or there is a situation where all fixtures are likely to be turned on at once, as in a shower block at a football field.

Dwellings and Loading Units:
A typical dwelling as used in the code is 1 bathroom, one kitchen, and one Laundry. (30 loading units, 0.48 L/s).
However from table 3.2, - 0.48 L/s equates to 31 loading units.
So, this program uses 31 LU's for a single dwelling.

That was alright in the good old days when houses had only one bathroom, but nowadays houses have at least two. So you can decide, does having two bathrooms make the occupants go to the toilet twice as often, and have twice as many showers?

It depends on the number of occupants, doesn't it? so use you own judgement, or use the loading unit method where you can't go wrong.

Another interesting thing happens with the How water heater. The code give this a loading Unit value of 8 LU. But a hot water heater in itself doesn't use any water. So if using this method, there is no need to add hot water fixture units, as the value is taken as 8. So if we don't have to calculate LU's for hot water, how do we size the pipes?

I prefer the following approach:- A fixture like a sink or a bath can operate with either full cold, or full hot water flow, therefore both the hot and cold pipework can be calculated with the same loading units. If there is a mixture of hot and cold, the total flow does not necessarily double, or even increase, as there are (or should be) flow limiting devices on these fixtures, ie the code stipulates the maximum allowable flow for most fixtures.

Anyway when in the shower, I believe a user has a certain desirable flow, whether that be full cold, hot, or a mixture. The bottom line is, hot and cold water pipes can be sized on LU's, but the flows are not additive. If you believe this is a correct approach, the 8 loading units for the hot water heater is not added. and the loading units for the fixtures are counted only once.

As an aside, it is desirable to have equal pressure at both the hot and cold taps. This serves to eliminate a lot of undesirable temperature fluctuations. Therefore try to make the hot and cold water not only the same size, but the same length and number of bends etc.

Length to furthest or highest fixture:
If a 3 story block is next to the start location, and a 1 story house is 50m away it is difficult to tell which is the worst case. In this situation try both alternatives to see which gives the smallest hydraulic grade (or biggest pipe size). That will be the worst case.

If the furthest fixture is lower than the start location, enter the height difference as a negative value (ie use a minus sign).

Pipe Size:
Pipe sizes are calculated using the Colebrook-White formulas in AS 2200 Design charts for Water Supply and Sewerage. This program will calculate copper and Plastic water pipe sizes up to 200mm dia.
Plastic pipe details are based on:- PE100, Pn16, SDR11
Copper pipe sizes are taken from AS1432:2000, Copper tube type B.
The internal diameters used in the programs are shown below

Copper Pipe Type B
Nominal Dia
Dn (mm)
Actual I.D.
15 10.88
18 13.84
20 17.01
25 22.96
32 29.31
40 35.66
50 48.36

Copper Pipe Type B
Nominal Dia
Dn (mm)
Actual I.D.
65 61.06
80 72.94
90 85.64
100 98.34
125 123.74
150 148.34
200 199.14

Plastic PE100 SDR11
Nominal Dia
Dn (mm)
Actual I.D.
16 13
20 16
25 20
32 26
40 32
50 40
63 57
Plastic PE100 SDR11
Nominal Dia
Dn (mm)
Actual I.D.
75 61
90 73
110 89
125 101
140 114
160 130
200 162
Note: Although Australia and new Zealand have the same plumbing code, the Copper Pipes in NZ are manufactured to different standards. So this calculator is not suitable for use in New Zealand.

The recommended maximum velocity is 2.4 m/sec. (Note that the Australian code allows 3m/s.) Any desired max velocity may be used in the program. However it is recommended that velocities exceeding 3m/s should only be used where piping noise will not be a problem and on straight runs with no branches, or valves.

Residual Pressure:
This program recommends a minimum residual pressure of 11m at the furthest (or highest) fixture. The code AS/NZS 3500.1 allows 5m. 11m is the manufacturers recommended minimum for tempering and mixing valves. However any value may be entered

If the hydraulic grade is less than 1:33 the program will suggest a pump is required.
The Pump duty (pipelind loss) is calculated by using this grade to calculate the head loss in the longest (or highest pipe length). Entering the suction and delivery sizes as required, only calculates the head loss around the pump. This heas loss is added to the pipeline loss. An extra 2m loss is added for any unknowns.

Head Losses allowed in Pump room.
When calculating the pump duty, extra head losses have been allowed in the pump room.
Pump Room Layout This is made up of:-
  • 2 * butterfly valves (k=5),
  • One duo check valve( k=1.8),
  • 5 * 90 deg short radius bends (k=1.5)
and as shown in the figure. Total 'k' = 19.3

Different valves and fittings will have different losses.
The coefficients of friction "k" are as per AS 2200 Design Charts for Water Supply and Sewerage.
Head loss = k*V2/2g. where 'V' is the water velocity.

When a pump is required, the table of pipe sizes is calculated using the pump.

If connecting to an already installed pumped system, you should be aware that the pump pressure will fall off with increasing flow. Therefore measuring the pump pressure to use as a start pressure, will not be correct unless the pump is operating at the required design flow.

Best Method for big Jobs:
Enter the parameters to determine the worst case. This will produce the table of pipe size capabilities.
On your plan (or diagrammatic) work from the end of each branch, and draw on the running total of loading units for each pipe section, until the start is reached.
Then read off from the table, the required pipe size for each pipe section.

Hint: Memorise the LU's from the table below and do the adding in your head. With practise you can size a whole highrise building in under half an hour, and that's with a coffee in the middle.
Don't believe me? Well normally most dwelling units are similar plumbing wise, so when you have done one, you have done them all. And once you have done one floor, you have done them all etc, Just a little bit of joining things up around the place and Bob's your Uncle.

Fixture Loading Units
WC 2
Bth 8
Bn 1
Shr 2
Sk 3
LT 3
WM 3
DW 3
HC 4