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Fixture Units to (gpm)

Fixture Units to (gpm)

Hi all please help me with a question. I am vary much junior to the industry so please bare with me if I am annoying.

I am able to convert Fixture Units (Fu) to Gallons per mint (gpm) on a table.
What is or where can I find the simplest mathematical calculation to do the same conversion?


RE: Fixture Units to (gpm)


Mike Halloran
Pembroke Pines, FL, USA

RE: Fixture Units to (gpm)

Are you serious?  

Sounds like a straight proportionality problem.  You should be able to figure it out from the table values that you have.  


RE: Fixture Units to (gpm)

I had to Google "fixture units".  No simple answer found.

"Drainage Fixture Units

A DFU is a measurement unit for potential water use for a given plumbing fixture or appliance, as provided for in the Building Officials & Code Administrators (BOCA) National Plumbing Code. "

A water closet of 1.6 GPF or less gets a DFU of 3.0 in Virginia Beach, where the DFU sum is computed as part of an inspection fee.  A bathtub is 4.0 DFUs.

A water closet of 1.6 GPF gets a Water Supply Fixture Unit of 2.5 in Seattle, where they are used for sizing pipes.  A bathtub is 10.0 WSFUs.

Apparently WSFUs are not the same as DFUs.  It's not clear if they're the same everywhere, or how they are actually derived.

Maybe the cited code will provide a clue, but it's clear that there isn't going to be a simple correlation between FUs and gallons, because the FU is time- averaged and adjusted for duty cycle of each individual water sink or drainage source, and I'd bet there's been some political input toward encouraging use of low volume water closets, which are big water wasters, especially if there's a lot of fiber in your diet.

Mike Halloran
Pembroke Pines, FL, USA

RE: Fixture Units to (gpm)

1 WSFU = 1 GPM

charts from ashrae reprinted here:

If you have a copy of ASHRAE Applications, it's in the service water heating section (48).  There is a full description of how to use size appropriately but no relationships are given, just the charts.

As you can see, it all depends on the intended application. (ie. the frequency of use)  You could curve fit approximate solutions to all of them but probably just easier to tabulate or read from the chart.


RE: Fixture Units to (gpm)

I suppose I should have also mentioned that the 1 WSFU = 1 GPM conversion is only applicable to flow rates of about 5 GPM then the relationship diverges considerable.


RE: Fixture Units to (gpm)

Thanks for the help all

Many lessons learned


RE: Fixture Units to (gpm)

There is no catch-all equation for this. The tables are sequential and inclusive ranges of straight line functions with decreasing slopes.  It is known as Hunter's curve and is based on probability of simultaneous fixture use.  For sizing a pipe to an individual fixture, the probability is 100% that all fixtures on the pipe will be used at the same time.  For a main that serves 100 fixtures, the probability is far less, so the design load is reduced to a fraction.  

Using a spreadsheet to enter the table values (e.g. from ASPE data or code tables),  I use the Indirect and Vlookup functions to locate the correct WSFU range to convert intermediate table values to GPM.

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