Drainage Fixture Units (DFU) and Water Supply Fixture Units (WSFU) serve similar purposes but for different systems. DFUs are used to size drainage pipes and are based on the rate and volume of discharge, taking into account the probability of simultaneous use. WSFUs, on the other hand, are used to size water supply piping and account for flow rate, pressure requirements, and usage patterns. The values are not interchangeable, and fixtures often have different DFU and WSFU values. For example, a water closet (toilet) might have a DFU value of 3-4 for drainage purposes but a WSFU value of 2.5-5 for water supply calculations, depending on the specific code and toilet type.

Plumbing codes do provide for some combinations and reductions in specific situations. For example, a “bathroom group” (consisting of a toilet, lavatory, and bathtub or shower) is assigned a single DFU value that is less than the sum of the individual fixtures. This accounts for the low probability that all fixtures in the group will discharge simultaneously at full capacity. Similarly, as the total number of fixtures increases in a building, codes often apply reduction factors to the total DFU value when sizing the building drain and sewer. These reductions account for the decreasing probability of simultaneous discharge as the number of fixtures increases. However, these combinations and reductions must be applied strictly according to code provisions—you cannot arbitrarily reduce DFU values based on your own assessment of usage patterns.

Continuous flow fixtures such as HVAC condensate drains, drinking fountains with chillers, or certain industrial process drains require special consideration in DFU calculations. Instead of using standard DFU values, these fixtures are typically calculated based on their flow rate: 1 DFU is generally equivalent to a flow rate of 1 cubic foot per minute (approximately 7.5 gallons per minute). For example, if you have a process that continuously discharges at 15 GPM, this would equate to approximately 2 DFUs. These continuous flow fixtures are then added to the DFU total from intermittent fixtures. Some codes have specific provisions for continuous flow, so always check your local code for the exact requirements. Continuous flow is particularly important when sizing horizontal branches that serve only these types of fixtures.

Undersizing drainage pipes can lead to serious problems, including slow drains, frequent backups, gurgling sounds, sewer gas odors, and even sewage overflows during peak usage. Undersized pipes cannot handle the volume of waste and may become more easily clogged with debris. On the other hand, oversizing drainage pipes, while less problematic from a functional standpoint, can create other issues. Oversized pipes cost more to install, take up more space in walls and floors, and may operate less efficiently because the reduced flow velocity can lead to solids settling in the pipes rather than being carried away. Additionally, oversized venting can fail to properly balance air pressure in the system. The goal is to size pipes according to code requirements, which are designed to provide adequate capacity while maintaining proper flow characteristics. When in doubt, it’s better to slightly oversize than undersize, but extreme oversizing should be avoided.

Local amendments to plumbing codes can significantly impact DFU calculations and drainage system design. Many jurisdictions adopt a base plumbing code (such as the IPC or UPC) but then modify specific provisions to address local concerns, historical issues, or regional preferences. These amendments might include different DFU values for certain fixtures, modified pipe sizing tables, additional requirements for specific applications, or alternative venting methods. For example, a jurisdiction in a cold climate might require larger minimum pipe sizes to reduce freezing risk, or an area with water conservation mandates might have different DFU values for low-flow fixtures. Before starting any plumbing project, obtain the current adopted code with all local amendments from your building department. Our calculator provides a good starting point based on standard code provisions, but always verify the results against your local requirements and consult with a licensed plumber familiar with local codes for final designs.