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 (DFU) | 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 | - | |
| 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.