Hey guys. I'm new to plumbing and am learning through a book right now. I have a table given to me that they say will help me determine how many fixture units and what the minimum branch pipes sizes are in a building.

First off, the book failed to explain what exactly is 1 fixture unit. I've been on Google for hours a day and all I found is that 1 DFU is equal to 1 GPM, is that correct? But that still doesn't help me to understand the table I have in front of me.

An example from the table: The appliance is a bathtub, the minimum fixture branch pipe size is 1/2", and for private it says 4.0 and public is also 4.0.

My questions:
What is a fixture unit? Is it a measurement?
What does the 4.0 for a bathtub or 1.5 for a kitchen sink mean and where did this calculation/number come from? Where does the .5 even come from?
Do fixture units apply only to drainage but not supply or does it apply to both?
Is a branch pipe only for draining or can the term be used for water supply as well?

thats a good one, i am an older master plumber and i can't actually answer your question, it has always been simply a unit of measurement. i am not sure about that 1 to 1 ratio on the gpm to dfu either, i think that is actually false as the two are complete different units of measurement.

Here's a table I found online similar to the one I have in my book

[www.engineeringtoolbox.com]

your first question is like asking "what is an inch?" it is a unit of measure as holland replied..
i use the term 'branch' interchangably between drains and water pipe.

It doesn't matter what it is you will be required to follow whatever the code book says for the inspector to pass your job. When sizing water or gas you will be required to size the piping so that it will handle the demand of the fixtures or appliances in the case of gas. You should take a code class if you have not attended and passed a recognized apprentice class. There you will learn how to make drawings that allow you to see the distances and size of various pipes which will determine how many fixture units are allowed for the size of pipe or tube. Pressure is also taken into account for water lines. You will learn about velosity and friction loss. Proper installations will prevent a lot of noise in your walls and attic from water moving through the tubing. It is all very interesting and makes sense once you get involved in it. Take your time to learn, you will enjoy your trade more if you are proficient at it. Good luck.

In plumbing, a Fixture Unit (FU) is equal to one cubic foot of water drained in an 1 1/4 pipe over one minute. A Fixture Unit is not a flow rate unit but a design factor. One cubic foot of water is roughly 7.48 gallons ( 6.25 Imperial Gallons) A Fixture Unit is used in plumbing design for both water supply and waste water.

Different fixtures have different flow requirements. In order to determine the required size of pipe, an arbitrary unit is used for pipe sizing which takes into account the likelihood that all the fixtures will not be used at the same time. This is called "fixture unit" (FU). The relationship between gallons per minute (gpm) and fixture unit is not constant, but varies with the number of fixture units. For example, 1000 FU is equivalent to 220 gpm, but 2000 FU is not double that, but is only 1.5 times as much, or 330 gpm.

Fixture unit values can be determined using charts from the International Plumbing Code or similar codes in local jurisdictions.

There are situations where a design provides for more FUs being discharged than being supplied. This occurs in situations where liquids may infiltrate or are added to a draining system, such as might happen in a large sports venue. Examples of how this could occur include rain water infiltration.

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