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 Cold and Hot water pipe, and pump sizes, in any plumbing project, with only 3 clicks. It will also satisfy the Australian Plumbing Code.
video: How to Design Plumbing Water Pipes
video: How to Design Hot water circulation Systems
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NOTE: to calculate hot water recirculating loop and pump sizes use STEP 3.


- Calculate the available Hydraulic grade
and the Capacity of all Pipe sizes with this hydraulic grade.

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(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 Diagram

Find the approx pressure at any point along the "Index line"

STEP 3 -

Calculate hot water recirculation pipe and pump sizes. Learn More?
Note: This calculation uses the hydraulic grade calculated above.
However the Probable Simultaneous flow (PSF) and the required maximum velocities are different for hot water circulation loops. (refer to the notes below)

The first set of tables calculates the pipe sizes without the recirculation flow added.
This is necesary to get an idea of the heat losses, and hence the recirculation flow required, so it can be added to the total flow.
This is a requirement of AS/NZS 3500.4 to ensure that the allowable velocity is not exceeded.

Heat loss through a hot water recirculating pipe
Heat is being lost from the recirculating pipework.

The object is to replace the total heat lost in the loop by circulating the exact amount of hot water.

This means we need to enter each pipe dia, length and insulation thickness in the table below.

Learn More? and other instructions.

Convert HW recirculation loop Loading Units to a flow just for fun.

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, or any combination of these.

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.

The calculator in step 3 will calculate the pipe sizes required in a hot water recirculation loop, along with the required recirculation pump size.
The pipe sizes in step 1 are for hot and cold branch lines only.

The hot water recirculation loop is Designed in accordance with AS/NZS 3500.4-2021 Heated water services, and uses different allowable velocities and slightly different probable simultaneous flows (PSF).

If the pressure falls so low that a single dwelling may require about a 32mm diameter pipe or larger, the program will suggest that a pump is used. 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 (PSF)
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.

This program calculates the Probable simultaneous flow for Hot and cold branch lines by using AS3500.1 table 3.2.4, and the equation in table 3.2.3 note2.
For hot water circulatory systems the program uses table O1 in AS 3500.4.
Because of rounding allowances the conversion is usually within a fraction of a litre of the table values.

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).

Hot water circulation loops
Hot Water recirculation diagrammatic
The object is to reduce the wait times for hot water to arrive at any fixture.
This is done by continuously circulating hot water around a loop that passes as close as possible to the hot water fixtures.

The hot water loop piping is sized on loading units, and index length, just like the cold water. It is not sized as a ring main. However the Code applies different flows and velocities to those used for cold water. Large velocities in hot water can cause pipe erosion and other problems and are to be avoided. Hot water recirculation pipes are always insulated.

The circuits can be any shape or location. They can be vertical, or horizontal, or both as shown in the diagram.
The heater can also be anywhere, however it is probably best to keep it at low level for ease of access. fuel delivery, and of course hot water rises.

There can be any number of secondary circuits. A Secondary circuit may be on every floor. The important thing is to join the secondary return line back to the main return line, and not to the main riser.

Balancing Valves
The balancing valves are not balancing on anything. They are an attempt to balance the flow, or more accurately balance the heat distribution equally through each circuit.
Hot water balancing valve from Reliance Manufacturing

Balancing valves come in all shapes and sizes, with or without all sorts of testing or adjusting plugs. The one shown is from "Reliance Manufacturing" It is thermostatically controlled and adjustable, and has a removable temperature gauge.

Water will always follow the path of least resistance. In this case it could be the shortest length from the heater and back to the heater. So there is the possibility of short circuiting the whole thing, and have very little recirculation along the longest loop.

Sometimes if is difficult to know which is the circuit of least resistance, as this also depends on the required flow and the pipe sizes through that circuit.
This program allows you to find out which loop is the worst case, ie which has the most resistance (friction). This is the loop that the pump should be designed for.

The program allows you to select which pipes are in the loop in question, and hence calculate the "head loss" through that circuit.

If we put a balancing valve in all the circuits, we can restrict the flow to force more water through the other loops. Hence "balance" the flow.
Balancing valves can restrict the flow as mentioned, or they can be heat operated as in a tempering valve, or thermostatic mixing valve.
In either case it would be nice if the designer could indicate what flow (or temperature) to regulate to.

This is easily done. We know the start temperature of the heater, and we know the return temperature is 5 deg less. (Code Requirement) We can assume that the temperature loss along the worst case loop is linear, therefore we can calculate the approximate temperature at any point along that main loop.
For example if one of the secondary circuits joins back to the main loop at about half way along its length, the temperature drop at this point should be about half the total temperature drop of 5 deg. ie 2.5 deg. That is 2.5 deg less than the start temperature from the heater.

Petes Plug for water temperature and Pressure testing

Therefore we need a way of reading temperatures. This can be done by inserting a temperature gauge just after the balancing valve or, it may be more economical to insert a Petes Plug. This device allows you to plug or unplug your temperature/pressure measuring device.

If we adjust the secondary circuit to achieve this temperature just before the junction point, then we have balanced the temperature to be the same as the temperature of the main loop at that point.
Any hotter and we are starting to short circuit, ie depriving the other loops of heating water.

Air Valves
There is always air dissolved in water. When water is heated it has a tendency to UN-dissolve. ie come out of solution. The air will normally rise to the top, unless the velocity is fast enough to drive it all the way through. In either case it will eventually pass through the high points.

Air in the pipework acts like a blockage because it reduces the diameter, and this is not good. It also stuffs up all of our flow calculations, and that's not good either, so best to let it out through an air valve. An air valve will let air out (and sometimes let air in), but it won't let any water out.

An air valve should be placed at all high points.

Expansion Offsets & the plumbing Code.
Hot Water expansion method 1
Hot Water expansion method 1

With hot water there is a need to take expansion seriously. For this please refer to the Hot water Code AS/NZS3500.4.
It would be difficult to design anything to do with hot water without constant reference to this document.

Expansion is a big deal, and there is quite a lot of calculation involved. It requires calculating the amount of expansion for different materials, fixing point location, and offset lengths etc.

Heat Loss
Heat lost through pipe insulation
Even though the pipework is insulated, heat is still being lost. The amount of heat lost depends on The heat loss is calculated in watts or kilowatts. the object is to calculate the total heat loss from everything in the recirculation pipe network.
Knowing this, we can calculate how many litres/sec of hot water we need to pump around the circuits to replace all the heat lost.

Thermal Conductivity
Thermal Conductivity is a way of comparing different materials by measuring how many Watts of heat can get through 1sqm of material of thickness 1m, if there is 1 degC temperature difference from one side to the other.

It is interesting to note that the thermal resistance "R-Value" is the opposite, and as such is the reciprical of the Thermal conductivity.

comparing heat transmission to water flow
Heat is measured in Watts. Watts are actually joules/sec.
If we use a water analogy, imagine that heat (in Watts) flowing through insulation, is like water in litres/sec flowing through a pipe filled with sand.

To know how many L/s can get through a pipe, we need to know the inlet and outlet pressure, the area, and the length of the pipe. This allows us to calculate the hydraulic grade, we then use a formula to calculate the L/s.

It is the same with heat, we need to know the start and end temperature, the area, and the length of the material it has to go through (insulation thickness). This allows us to calculate the temperature gradient (analogous to the hydraulic grade).
We then use a formula to calculate how many watts are getting through the insulation.

The R-Value is a measure of thermal resistance of a material of a particular thickness. When analogous to a sand filled water pipe, its like calculating the resistance of the sand.

Its a bit like Mannings 'n' in the Mannings fromula, or the Colebrooke-White 'k' in the Colebrooke-White formula for calculating pipe flow. But its spread out across the whole pipe, and not just the wetted perimeter.

Anyway it gives a figure that the Code can use to define what the R-Value of the pipe and insulation should be. The larger the value the greater the thermal resistance.

Knowing both the conductivity and the insulation thickness, a simple formula is used to calculate the R-Value.
This is formula 8.5 in AS/NZS 3500.4-2021. and is:-
R = (Insulation Thickness/Insulation Conductivity) + (Pipe Thickness/Pipe Conductivity)

This program uses a typical value of Thermal Conductivity for Plastic Pipe (PE-X) of 0.4 W/m.K.
and the Thermal Conductivity of insulation of 0.035 W/m.K. This value can be changed by the user.

The Code uses the R-Value as a means of stipulating the minimum insulation requirements. This requirement is dependent on the hot water situation ie circulating or not, associated with heaters or valves, pipework location inside or outside, and the climate region.
This program suggests the default of 0.6 which is for circulation hot water in all locations and Climate regions, except Alpine regions where it should be higher at 1.0.

The user can use table 8.2.1 and 8.2.2 in the Code to choose the the R-Value for other situations if required.
The values range from 0.2 for valves, 0.3 for pipes, and 0.6 for circulating systems.
The values are lowest for the hotter climate regions, and highest for the colder regions. There are 3 climate regions A, B and C.

Recirculation Pump Size
Hot Water expansion method 1
The recirculation pump must be sized to replace the heat lost from the system.

The interesting thing is, "What about the hot water that is going to the fixtures anyway?" and "How does this affect things?"
Well, when fixtures downstream are drawing water already, there is not much wait time, so no need to recirculate. Some systems work like this, ie the recirculation pump cuts in only when the temperature falls below a certain minimum.

However for the rest of us, we design the recirculation pump assuming no fixtures are operating. However in either case, here is the interesting bit, the recirculation flow must be added to the probable simultaneous flow to be able to calculate the final pipe size. But before we can calculate the recirculation flow to be added, we need to know the final pipe size.
A chicken and egg situation.

So there is a bit of trial and error involved. The trial and error involves calculating the pipe sizes first without the circulation flow.
Then calculating the circulation flow required with these pipe sizes. Adding this flow to the design flow for the fixtures. Then upgrading certain pipe sizes to meet the velocity requirements, and the heat loss "R" value. The program will advise you of this. Then do the calculation again with the upgraded pipe sizes etc.

"What about the pump head loss?", well we only need to calculate the friction loss, we don't need to take into account the building height, or the rise in the pipework height. Because the flow returns to the pump, which is at the same height that it left the pump, so no need to raise the water to a higher level, unless there is an air break in the system somewhere.

The program makes an allowance for bends and fittings by using the equivalent length method as recommended in the code, ie multiplying the total pipe length by 50% in the hope that the extra length will have an "equivalent" friction loss as all those bends and fittings.

However this does not work for things like valves and the heater.
The program adds 1m head loss for valves, and 1m head loss for the heater, in the hope that this will at least be in the ball park should the user not bother to read all the instructions.

However it would be best if you added the actual head loss for all extra fittings.
For instance the heater can have a head loss from practically nothing to about 5m.

By the way, don't worry if the smallest "off the shelf" pump is much larger than necessary, flows can always be adjusted by valves if necessary.

Code Requirements
From AS/NZS 3500.4 Heated Water Services:-
Hot Water Velocity requirements. (m/s)
Piping Copper Pipes Other materials
Circulatory (flow) 1.2 2
Circulatory return line 1.0 1.0
Non-circulatory (flow) 1.0 1.0

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.

Loading Units per fixture
Fixture Loading Units CW Loading Units HW
WC 2 -
Bth 8 4
Bn 1 1
Shr 22
Sk 3 3
LT 3 3
WM 3 3
DW 3 -
HC 4 -
HWS 8 -