Drainage Fixture Unit Values
Reference data and engineering information about drainage fixture unit values for sanitary drainage applications.
Overview
Engineering reference data for Drainage Fixture Unit Values in sanitary drainage.
Key Formulas
Manning's Equation
Open channel flow.
Slope
Hydraulic gradient.
Variables
| Symbol | Description | Unit |
|---|---|---|
| Flow rate | m³/s | |
| Manning roughness | — | |
| Flow area | m² | |
| Hydraulic radius | m |
DFU Reference Table
The following table provides standard Drainage Fixture Unit values for common plumbing fixtures and appliances, based on the Uniform Plumbing Code (UPC).
Fixture | Min. Trap Size(inch) | DFU (Private) | DFU (Public) | Typical (Europe)(l/s) |
|---|---|---|---|---|
| Automatic clothes washers, commercial | 2 | — | 3 | — |
| Automatic clothes washers, residential | 2 | 2 | — | — |
| Bathroom group (WC, lavatory, bidet, tub/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 | — |
| 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 | 0.6 |
| 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 valve | 3 | 3 | 4 | — |
| Water cooler | 1 1/4 | 0.5 | 0.5 | — |
Source: Uniform Plumbing Code (UPC)
Understanding DFU and System Load
A Drainage Fixture Unit (DFU) is a design factor, not a direct flow rate. It represents the probable discharge into the drainage system by considering the rate of discharge, duration of operation, and average frequency of use for a specific fixture type.
The fundamental conversion based on the UPC definition is:
This corresponds to the flow of one cubic foot of water discharged through a 1-1/4 inch pipe over one minute.
Design Consideration: Intermittent Use
When sizing drainage systems, the total DFU load for all connected fixtures is not simply the sum of their individual values. This is because all fixtures in a system are never operated simultaneously. The calculated total must be adjusted (typically reduced) to account for this intermittent use pattern to determine the realistic peak drainage load for the system.