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Leaking cast iron drain pipe, Poughkeepsie NY © Daniel Friedman at InspectApedia.comDrain Fixture Units: DFUs

This article defines Drain Fixture Units or DFUs and explains how this measurement is used when designing or installing plumbing systems.

DFUs are a unit of measure of the wastewater volume or loading of a drain waste or vent system.

DFUs are used to specify the size of waste piping, vent piping, or related drain devices such as air admittance valves or Studor valves.

Here we provide a helpful table giving the number of DFUs for various plumbing fixture types.

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Definition of Plumbing Vent DFU Ratings: Drain Fixture UNITS

Multiple plastic drain traps - how many DFUs are here? (C) InspectApedia.com Definition of Drain Fixture Unit or Definition of DFU:

The Uniform Plumbing Code defines Drainage Fixture Unit as follows:

Drainage (DFU). A measure of the probable discharge into the drainage system by various types of plumbing fixtures.

The drainage fixture-unit value for a particular fixture depends on its volume rate of drainage discharge, on the time duration of a single drainage operation and on the average time between successive operations. - UPC 2006 

Drain Fixture Unit, or DFU, is a plumbing design factor, or a relative measure of the drain wastewater flow or load for various plumbing fixtures.

Here are two quantitative measures of DFUs:

How Drain Fixture Units are Used

A plumber or other designer or installer planning the plumbing drain system for a building adds together the DFU value for all of the plumbing fixtures that will be served by an individual plumbing drain line in order to be sure that the drainage is big enough. An undersized plumbing drain can lead to annoying slow drainage or in the worst case, to plumbing fixture overflows that flood and damage the building.

By adding the DFU load rating of all of the individual fixtures on a single drain to be served by a single air admittance valve (AAV), the plumber or designer can select an AAV with sufficient capacity.

As we discuss separately at AIR ADMITTANCE VALVES AAVs, Oatey, an AAV manufacturer, provides the helpful DFU Load Table given just below. Notice in the table below that the DFU factor for a plumbing fixture will vary depending on the drain and trap size or diameter.

Drain Fixture Unit (DFU) Table for Common Plumbing Fixtures 1
Plumbing Fixture Type
Drain Fixture Unit
Load Rating
PRIVATE
(DFU)
Drain Fixture Unit
Load Rating
PUBLIC
(DFU)
Drain Fixture Unit
(DFU)
Load Rating
EUROPE
(Liters/Second)
Trap
Diameter


(Inches)
Bathroom Group
Traditional 2
6
    
3
Bathroom Tub
2
 
0.9
1.5
Bathtub with Shower
2
2
  
1.5
Bidet
2
 
0.3
1.5
Bidet
1
   
1.25
Dishwasher
2
    
1.5
Drinking fountain  
0.5
0.1
1.25
Floor drain
6
6
  
3
Floor drain
8
8
  
4
Garbage grinder  
3
  
2
Mobile home
main trap
12
    
3
Shower stall
2
2
  
1.5
Sink, bar
1
2 (?)
  
1.5
Sink, kitchen,
commercial
w/ food waste		  
3
  
2
Sink, kitchen
2
2
  
1.5
Sink, laundry tub
2
2
  
1.5
Sink, lavatory
1
1
  
1.25
Sink, medical
clinic		  
2
  
1.5
Sink, mop  
3
  
2
Sink, residential
2
    
1.5
Sink with Garbage
Grinder (Disposal)
2
3
  
1.5
Toilet - WC Flushometer
3
4
  
3
Toilet - WC gravity flush 3
3
4
  
3
Urinal
2
2
0.3
2

Washing Machine
Clothes
2
3
  
2
Water cooler
0.5
0.5
  
1.25

Notes to the table above

1. Oatey Corporation, "Oatey Sure-Vent® Air Admittance Valves Technical Specifications", Oatey® Corporation, - retrieved 2016/05/08, original source: http://www.oatey.com/doc/aavtrifoldlcs420c101812lr.pdf The company provides AAVs rated at 6, 20, 160, and 500 DFUs.

2. 1 toilet at 1.6 gpf, 1 bathtub with shower, 1 sink

3. 1 toilet at 1.6 gpf

Watch out: While it is acceptable to oversize a Sure-Vent®; however, an undersized Sure-Vent® (Oatey) or Studor Vent (like the Studor Mini-Vent®) or other AAV product will not allow the plumbing system to breathe properly.

Studor Mini-Vent® DFU sizing chart at InspectApedia.com

Illustration: Studor's Mini-Vent® DFU (Drain Fixture Unit) sizing chart from the

See details and more AAV DFU charts at

...


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Reader Comments, Questions & Answers About The Article Above

Below you will find questions and answers previously posted on this page at its page bottom reader comment box.

Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

How many fixture units is a trench drain like the Zurn Z886?

Zurn Z886 Perma-Trench Linear Trench Drain System cited & discussed at InspectApedia.comHow many fixture units is a trench drain (interior applications). Specifcally Zurn Z886 8601 (rated at 93 GPM). - by Ryan

Reply by (mod) - the Zurn 886 is a zero DFU system that can handle 93 GPM

Ryan,

From my reading of the company's specifications at zurn.com, these are trench drain inserts, not building wastewater disposal system components.

Shown here is the Zurn Z886 Perma-Trench linear trench drain system. This is not a plumbing drain it's an intercept drain that would be used, for example, at the bottom of a sloped driveway that gives access to the entry to a garage. By intercepting water running towards the garage entry abnd sending that water around the building to an acceptable destination we avoid flooding the garage and its contents.

The Zurn 886 series refers to the 6" Zurn Z886 Trench Drain - a modular drain trench channel - that handles flowing wastewater or runoff at 93 gpm (6 LPS) (0.2 CFS).

You could convert the Zurn drainage capacity (93 gpm) to Drain Fixture Units by dividing Zurn's number (93 gpm) by 7.48 (gpm per 1 DFU) = 12.4 DFUs but as you would not be connecting this drain to your building's plumbing system I'm not sure why we'd do that.

Other products in the Zurn series accept up to 530 gpm.

Put another way, since this drain does not connect to your building's plumbing drain system, the Zurn 886 is a zero DFU system. These products have nothing to do with sewage or wastewater processing and thus are not related to drain fixture units discussed in the article above.

It's worth noting here that the specifications and instructions below give water handling capacity for these intercept drains but say nothing at all about the piping required to carry and dispose of the drainage water to an acceptable location.

For that information you should see the Uniform Plumbing Code citation that I'll include below, using New York State's 2025 Adoption of the Uniform Plumbing Code as an example. - Daniel F.

Zurn Z886 References

 

How Many DFUs is a Commercial Kitchen Floor Sink

How many DFU'S on a floor sink in a commercial kitchen (L.A. County)? Frank Lee, 5/5/2020

Answer by (mod)

Frank

A typical commercial kitchen sink is rated for 3 DFUs assuming it includes a waste grinder. - Dan

 

How do we Compute DFUs for a Shared Kitchen Sink with Dishwasher?

A residential sink with garbage disposal is 2 DFU's and a dish washer is also 2 DFU's. The dishwasher typically drains into the garbage disposal, so is this combination 2 DFU's or 4 DFU's? = Bret Marquis 2020-8/12

Answer by (mod)

Bret: since the dishwasher might indeed be running while someone is also running water into the sink, it makes sense that you'd add those respective DFU's: use 4 DFUs. - Daniel F

 

Do Whirlpool Bathtubs Affect the DFU Rating/Value?

Does a whirlpool bathtub effect the DFU rating/Value ? Henry - 2/9/2022

Answer by (mod)

My guess would be "no" although the counter-argument would be that the greater size of some jetted or whirlpool tubs might mean a greater volume of water even if the drainage rate isn't more than an ordinary tub.

Here's a plumbing code citation with a good answer:

P3004.1 DWV System Load

The table of DFUs does not single out a whirlpool tub for a different rating

The code gives: Bathtub (with or without a shower head or whirlpool attachments) = 2 DFUs- Daniel

 

How many DFUs do we count for a furnace?

How many fixture units is a drain on a furnace by Fred

Reply by (mod) - For a condensing gas furnace we count 2 DFUs or less

What an excellent question.

Since a furnace is a forced air heating system it won't have a drain.

Or are you asking about a condensing gas fired furnace that produces a small amount of condensate? If so we'd look at air conditioner condensate drain DFUs.

My OPINION is that the condensate produced in that case is less than the standard 2 DFUs used for "semi continuous condensate drain systems"

as per

Values for Continuous Flow.

For a continuous or semi-continuous flow into a drainage system, such as from a pump, ejector, air-conditioning equipment or similar devices, two DFUs shall be considered to be equal to each gallon per minute (gpm) of flow. Amended at 38 Ill. Reg. 9940, effective April 24, 2014)

But perhaps I'm being too fussy and you mean a hydronic boiler or hot water heating system.

Nobody counts a heating boiler drain among the building's DFUs since you would not normally be draining it at all, certainly not regularly, and when you do need to drain a boiler for service or repair, you'd typically run a hose outside to a ditch rather than draining into a septic system.

 

Reader disagrees with the standard calculation of water volume of 1 DFU

"1 DFU ≈ (approximately) 7.48 US GPM or ≈ 0.47 liters/second"

I think this statement is wrong. Per Chart A 103.1, typically gpm is less than DFU number not greater by a factor of 7.48. See Chart A 103.1 in plumbing code.


Reply by (mod)

@Anonymous,

Thank you so much for the comment and Demand load charts for drain systems - please do give us the source for your charts.

The calculation is simple and is one reported in the DFU definitions and references cited in this article: one cubic foot of water = 7.48 U.S. gallons or 28.32 liters. That's a figure cited in pretty much every authoritative online source you'll find.

1 DFU = 1 cubic foot of water drained through a 1 1/4" diameter pipe in one minute.

1 DFU ≈ (approximately) 7.48 US GPM or ≈ 0.47 liters/second

Note: 1 cubic foot = 7.48 US Gallons. So from the first definition one can calculate the second number or GPM or LPM

Note also that as reflected in your demand charts (which I think may be being mis-applied in the instance of your comment) the relation of DFUs to flow rate in a drain system is not a constant, but varies by the fixture unit drain load on the plumbing drain system.

See the DFU charts and water volume data in these example sources

I agree that this topic can be confusing - which is why the plumbing code simply gives the plumber some clear charts of DFUs per fixture type so that she can add up the drain load on a given system.

Comment by Disappointed

@InspectApedia-911, restating your numbers, advertising them as "exquisitely simple" (and, thereby, indirectly insulting @Anonymous), giving your source as "various engineers", and throwing in a stray statement from an uncited Wikipedia article (hardly a reliable source) is not the path to a helpful answer to @Anonymous' challenge.

Your numbers (repeatedly) state:

1 DFU = 1 cubic foot of water drained through a 1 1/4" diameter pipe in one minute.

1 DFU ≈ (approximately) 7.48 US GPM or ≈ 0.47 liters/second

In the first statement we are talking about 1 cubic foot of water, in the second we are talking about gallons, providing the conversion factor (~0.134gal/cu-ft) and remarking that it seems a bit small (I don't think I could fit 7+ gallon-sized jugs of water in milk crate [those are a little under 1 cubic foot]) is both more informative and a kinder, gentler response.

Doing the math (1 gal /(0.134gal/cu-ft = almost 7.48 gal) would also help, as would suggesting an experiment (get something like a milk crate, line it with plastic, and see how many gallons of water it takes to fill it).

Arrogance rhymes with ignorance for a reason.

Answer by (mod)

Thank you for your suggestions, I'll review the material here and edit appropriately when we migrate the Q&A up into the discussion when it's preserved in the page above or on a FAQs page.

Certainly my intent is to welcome and respect all readers. Occasionally someone makes a comment that to me seems confusing or well off the mark: I need to be careful not to fall into the same quagmire of ego and irritation. So I appreciate that you took the time to write this.

About DFUs I'm using standard definitions that are consistent across quite a few reliable / respected sources.

Remember that the intent is to evaluate the ability of a particular drain system (size, length, elbows, heights, as well as wastewater nature and volume) to handle the anticipated wastewater load in a building. Or put in basic terms I like, how many sinks, tubs, and showers can I attach to a given drain of particular size and other parameters.

The DFU number that I gave for gallons in a cubic foot was correct. It's about 7.48 or 7.47 gallons (varying by rounding methods at different sources) gallons per cubic foot of volume. I agree that very often these numbers are a bit counterintuitive but we wouldn't just make this data up.

I did look at the references above on this page and you'll see that we cite authoritative sources including building codes and standards and fixture manufacturers. If you have additional authoritative sources for DFUs that you recommend we'll be grateful to see them and to add them here.

Daniel Friedman

...

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  • Oatey Corporation, "Oatey Sure-Vent® Air Admittance Valves Technical Specifications", Oatey® Corporation, - retrieved 2016/05/08, original source: http://www.oatey.com/doc/aavtrifoldlcs420c101812lr.pdf
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