DFU are used to calculate the drainage flow from fixtures and the required capacities of sewer service systems.
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 Fixture||Minimum Size of Trap||Drainage Fixture Unit Values|
|Typical Values used in Europe|
|(inch)||Private Installations||Public 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||-|
|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|
|Laundry tub||1 1/2||2||2|
|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|
|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.