Code Compliance: Finding and Sizing the Right Pipe

Here's an easy way to estimate peak water demand accurately and size your pipes accordingly. 

The Water Demand Calculator, which is found in Appendix M of IAPMO’s Uniform Plumbing Code, and in Chapter 5 of the IAPMO WE-Stand, provides an alternative methodology for estimating peak supply demand to enable right-sizing water pipes for single-family homes and multifamily buildings.

The calculator can also be used for remodeling projects that include additional bathroom or kitchen features. In 2024, California become the 10th state to adopt Appendix M into its codes and standards, joining Hawaii, Montana, Nevada, New Jersey, New Mexico, North Dakota, Oregon, Washington and Wisconsin, as well as such major cities as Houston, San Jose, Seattle and Vancouver, British Columbia.

“In some locations, such as Seattle, use of the Water Demand Calculator is mandatory to meet their water conservation and energy efficiency goals,” Lohr says.

The next step after using the Water Demand Calculator is to find the estimated demand for each pipe segment of a water distribution system. Builders or plumbing contractors must consult their plumbing code and find the appendix for sizing the water supply system. While all model codes have such an appendix, builders can refer to the IAPMO Uniform Plumbing Code, Appendix A for a good reference guide.

For example, the Seattle Plumbing Code includes IAPMO’s Appendix A, which has a chart that provides the required minimum fixture branch pipe size for each water supply fixture.

“Water is a very regional issue, so whether the focus is on water conservation or water quality will vary,” Lohr says. “Some jurisdictions will make the use of a Water Demand Calculator mandatory, while others will make it optional. In any case, builders and plumbing contractors can use our free calculator to make decisions that can reduce construction costs for them and water and energy bills for their customers.”

Click here to access IAPMO’s free calculator.