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Drainage Fixture Unit Values - DFU

DFU are used to determine the drainage from fixtures and required capacity of sewer service systems

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DFU (Drainage Fixture Unit Values) is defined by the the Uniform Plumbing Code (UPC) and can be used to determine required drainage capacities from fixtures and their service systems.

A Fixture Unit is not a flow rate unit but a design factor based on the rate of discharge, time of operation and frequency of use of a fixture. A fixture unit is equal to one cubic foot of water drained in an 1 1/4 pipe over one minute (≈ 7.48 US gpm, ≈ 0.47 l/s).

Individual Appliance, Appurtenance or FixtureMinimum Size of TrapDrainage Fixture Unit Values
(DFU)
Typical Values used in Europe
(inch)Private InstallationsPublic Installations(l/s)
Automatic clothes washers, commercial 2 3
Automatic clothes washers, residential 2 2
Bathroom (water closet, lavatory, bidet and tub or shower) 3 6 -
Bathtub 1 1/2 2 2 0.9
Bidet 1 1/4 1   0.3
Bidet 1 1/2 2  
Clothes Washer 2 3 3
Dental unit, cuspidor 1 1/4 1
Dental lavatory 1 1/2 1
Dishwashing machine, domestic 1 1/2 2 2 0.6
Drinking fountain 1 1/4 0.5 0.5 0.1
Floor drain, 3" 3 6 6
Floor drain, 4" 4 8 8 2
Food waste disposer 2 3
Shower 2 2 2 0.4
Laundry tub 1 1/2 2 2
Lavatory 1 1/4 1 1
Bar sink 1 1/2 1  
Kitchen sink, domestic 1 1/2 2 2
Laundry sink 1 1/2 2 2
Mobile home, trap 3 12
Service or mop basin 2   3
Sink, bar 1 1/2 1 0.3
Sink, clinical 1 1/2 2
Sink, commercial with food waste 2 3
Sink, kitchen domestic 1 1/2 2 2 0.6
Sink, laundry 1 1/2 2 2
Sink, service mop basin 2 3 0.9
Urinal 2 2 2 0.3
Washfountain 1 1/2 2
WC - Water Closet, gravity tank 3 3 4 1.8
WC - Water Closet, flushometer tank, valve 3 3 4
Water cooler 1 1/4 0.5 0.5

Fixture Units are used in plumbing design for both water supply (WFSU) and waste water (DFU).

Since the fixtures in a system are never used all at the same time, the total units (capacity) achieved by adding the numbers for all fixtures must be compensated for intermittent use if we want a realistic estimate of the total drainage load.

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