by Patti Dees

Determining the water supply fixture unit (WSFU) is the first step in sizing pipes that connect local water services to a building. The WSFU is not a measurement unit, though it is used to estimate load demand for intermittent-use fixtures, making it easier to design, install and optimize plumbing systems.


The Idea Behind WSFUs

When designing plumbing systems, many elements can impact how much water a building needs: the number and type of fixtures, their distance from the main water supply line, the pressures required at different points within the system, available pipe diameters and materials and which fixtures require continuous use or intermittent use. Because there are so many different parts, plumbing calculations can be complex, especially for fixtures like sinks that are only used off and on throughout the day.

The WSFU is a sizing tool that helps estimate peak water demand for intermittent-use fixtures. It is an assigned value, not a measurement, that factors in how often the fixture is likely to be used, as well as the amount of water required to function, and how that water is used by the fixture. This value, once converted to a volumetric flow rate, can then be added to the water demand calculated for continuous-use fixtures. The total flow rate can then be used in subsequent design calculations.

The WSFU value is defined by the plumbing code used for a project. The most common codes are the International Plumbing Code (IPC) and the Uniform Plumbing Code (UPC). Both codes provide tables that list the WSFU for individual fixtures divided by type, use and temperature.



The type of fixture – sink, toilet, washer, etc. – directly impacts how much water is required for its basic function.



Even across a single type of fixture, how it is used can differ when installed in residential or commercial buildings. Within commercial installations, different industries can require different factors for the same type of fixture. For instance, a sink in a hospital surgical suite is likely to see more frequent use than a sink in a small office building.



The main piping running into a building supplies cold water. Hot water requires branch piping to divert cold water to heating equipment. WSFU tables often list factors for cold and hot water when fixtures use both.


The Importance of Fixture Units

The WSFU greatly simplifies determining the sizing for supply pipes. Without the use of WSFUs, each individual fixture would need to be evaluated for volumetric flow rates, pressure requirements, the impact of friction and pipe material on the pressure and flow downstream, how often it is likely to be used, the time between uses, etc. The calculations would be a massive undertaking, even for a small single-family home. Since it is safe to assume that similar fixtures within similar scenarios will have similar solutions, aspects of individualized calculations can be applied to the type of fixture instead.

With standardized values, the use of WSFU also provides consistent results across all projects that fall under a particular set of codes. This can make purchasing, installation and inspection streamlined. WSFU takes an incredibly complex system and makes at least part of the calculation and installation processes more manageable.


Using WSFU

The use of WSFU is straightforward. A value from the relevant table is given to each fixture that connects to a piping section. These are summed together. Within the same plumbing code, additional tables are used to convert the total WSFUs for the section into a volumetric flow rate, usually gallons per minute (GPM). If the total WSFU value falls between the values listed in the table, interpolation is used to find the GPM. At this point, the volumetric flow rates for any continuous-use fixtures are added to the flow rates calculated from the WSFU.

However, the WSFU only goes so far in simplifying pipe sizing calculations. Getting the most efficient design requires knowing which details, beyond the number and type of fixtures, will impact a facility’s plumbing. Therma provides owners and managers with design and maintenance plans based on years of valuable experience shared across its knowledgeable staff.

Patti draws on her background as a chemical engineer to share information with readers on technology, manufacturing, and construction.



UpCodes – Appendix B: Sizing the Building Water Supply and Distribution Piping Systems, New Jersey Plumbing Code 2018

Engineering Pro Guides – Domestic Water Piping Design Guide, How to Size and Select Domestic Water Piping

Engineering Toolbox – Water Supply Fixture Units (WSFU)

International Code Council (ICC) – International Plumbing Code (IPC)

UpCodes – Uniform Plumbing Code 2021 (UPC 2021)