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The calculators
The water pipe sizing programs have been optimized for PC and mobile devices..
They calculate the hot and cold water pipe sizes for any plumbing installation.
The pipe sizes are calculated in accordance with the Plumbing codes shown.

The free calculators can calculate water pipe sizes for any job with only a few clicks. However they are limited to pipe sizes up to 50mm (2 inches). Although the free calculators are optimized for complete dwellings, the number of fixture units/Loading units for a dwelling is given, so by entering fractional dwellings, any value of loading/fixture units can be calculated.
A Dwelling is taken as roughly equivalent to 1 Bathrooms, a kitchen and a laundry.

The Proversion allows the opportunity to count the actual fixtures, known flows, and Loading Units as well.

The Australian Proversion calculates the available nominal pipe diameter for pipes made in Australia.
Refer HERE for more info on the Proversions.

Nominal Diameters
Pipes come in many different materials, strength grades, and diameters. To sort out the confusion, pipes are given a 'Nominal' diameter to which everyone refers.
Pipes of the same material and 'nominal' diameter generally have the same outside diameter. This is to allow different strength grades to be joined. Different strength grades mean different wall thickness. Therefore the internal diameter varies (For the same 'Nominal' diameter).

However for the purposes of flow calculations, it is the 'internal' diameter that matters. The internal diameter can vary from the 'nominal' diameter by as much as 5mm (3/16") for the smaller diameters, up to 30mm (1 1/4") for the larger diameters.
Also pipes are manufactured in various increments of diameter. For instance, at the smaller diameters, pipe diameters increment by approx 5mm (1/4"), and at the larger diameters (for pipes generally used in plumbing) the pipes increment by up to 50mm (2").
This also varies from country to Country.

The Australian Proversions display the required 'Nominal' diameter of pipes made in Australia. This is limited to pipe diameters up to 200mm. Domestic Pipe diameters above about 100mm (4 inches) are starting to get out of the realm of plumbing jobs, and maybe better suited for Municipal water pipe design.

Finding the Design Flow
The hard part with designing water pipe sizes is estimating the design flow, which varies all over the place, depending on the time of day, the type of building, occupancy rates etc.
Fortunately each Plumbing Code has a formula to help in estimating the 'design' flow.
This is done by allotting a "Fixture/Loading Unit" to each plumbing fixture. This number allows for the flow, and frequency of use.
The numbers are added up progressively as more and more fixtures are connected. The higher the number the greater the flow, but the percentage of fixtures operating together drops off (to a certain minimum).

The formula equates Loading Units to flow, in litres/sec, (or gals/min)
Basically, as the number of Loading Units increases, the percentage of all fixtures operating together decreases. (to a certain limit)
This is called the Probable Simultaneous Flow or Demand. Water Pipes for plumbing purposes are designed by the Loading Unit (or fixture unit) method.
The programs use the formula of the respective code.
Strangely this formula varies significantly in each Plumbing Code.

Some really interesting Stuff
The International Plumbing Code (and the British Plumbing Code) suggests we allow for almost twice the water flow as the Australian code. The Uniform plumbing Code as used in the United States recommends more again.

So which one is over designed and which is under designed? or do the different countries have a different water use culture? and what would be the effect anyway?
Australia being such a dry country puts a lot of emphasis on water restrictions, and water saving devices. Maybe this helps. Anyway the only noticeable effect to an end user could be having to adjust the tap (faucet) rotation, or to experience different pressure fluctuations as other fixtures are operated.

But if the pressure was still enough to work the fixture in question, does it really matter?.
It all comes down to risk analysis, how often are we prepared to put up with a possible pressure loss? once a day?, once a month? never?
A large hotel may have full occupancy, with all guests attending a conference, and all decide to take a shower at exactly the same time after the conference. And the wives decide to flush the toilet.
This would upset any Plumbing Code; but also very unlikely to happen. (very often, if at all)
The bottom line? Use the code that is allowed in your country.

However, strangely enough, if you follow the design rules in each code, the final pipe sizes are likely to be the same. Refer HERE to see how this works.

Finding the Pipe Size
Pressure (water head) We know that the more pressure we have, the more flow we will get out of a given water pipe. So our first step is to find out the start pressure. This could be the pressure in a street water main, a pump, or the pressure at the start of a branch take off.
However the pressure at the far end of the water pipe system will be reduced by friction along the way. So we must calculate the pressure that will be lost along the way by friction.

Also, if our house is much higher (or lower) than the street water main, this will make a big difference in the pressure. Therefore we must know this difference in elevation.

Some water fixtures require a certain pressure to operate properly. This is called the residual head or pressure. This figure can be anywhere from 5 to 15 metres. (49-147 kPa)(7-21psi).

We know the street pressure, we deduct the difference in elevation from the street to the highest fixture, we also deduct the residual pressure required, and what ever pressure is left we can say is available for friction.

Pipe Friction
The longer the pipe the more friction, so the length is important. The more bends and fittings the more friction, so the number of bends and fittings is important. Any other pressure losses are also important ie filters, softeners, meters etc.
However let's assume that the pressure loss through fittings etc can be the same loss as through a certain length of pipe. This length is called the 'Equivalent' Length. For instance it might be that the pressure loss through 50m of pipe is 'equivalent' to the pressure loss through 9 bends.

Anyway the Australian Plumbing Code says that this equivalent length can be taken as 50% of the total pipe length to the worst case. How good is that? no need to count bends and fittings, which is impossible to get right any way. One unforeseen obstacle to the pipe run can introduce 4*90 deg bends.

The International Plumbing Code suggests 50% as a first trial, however you must then add the pressure loss through other fixtures like the water meter, water softener, filters etc. and calculate the pressure loss in each major pipe section.

However this is starting to defeat the purpose of a quick and easy calculator, so to make life easier, the free versions uses the Australian Code idea. That is, the equivalent length is taken as 1.5*Actual length to allow for the head loss (pressure loss) due to bends and fittings. In practice this seems to be a good ball park figure.
Refer HERE for calculations to show how this works.

Also at the design stage it is impossible to estimate the correct number bends and fittings that the plumber will actually use.

Pipe grade
We know that the steeper the pipe grade the more flow. Yes, but does this work for pipes that go up and down and around and around like water pipes?.
Well yes it does, and we call it the hydraulic grade. The higher the reservoir the more flow. So the higher the pressure difference between the start and the end of the pipe the more flow. Now if we divide this pressure difference(measured in metres or feet) by the pipe 'equivalent' length, we get the 'Hydraulic grade'

Pipe size Formulas
Finally we are getting somewhere. Knowing the design flow, and the hydraulic grade, there are plenty of formulas to calculate the pipe size. eg Manning, Darcy, Colebrook-White, Hazen William. All these formulas use some form of pipe roughness coefficient to calculate the pipe size. As you can imagine an old rusty cast iron pipe is not going to flow as well as a new copper or plastic pipe.

The programs calculate the actual theoretical internal pipe diameter required to carry the design flow, under the given conditions of pressure and length, using the formula in the respective code.
The Australian version finds the nearest available (larger) pipe size in copper for pipes made in Australia.
The International versions display a theoretical pipe size. As pipes come in many different materials and sizes around the world, the user can look up the manufacturers pipe internal dimensions for the material of their choice, and find the next 'available' (higher) pipe size.

To go further and deeper Click here to LEARN MORE about the theory behind
'How to calculate the size of water pipes for buildings.'