Colorado Becomes the 12th State to Approve IAPMO’s Water Demand Calculator for Pipe Sizing

The way the plumbing industry sizes water supply piping is changing, one state at a time. On August 14, IAPMO announced that Colorado’s State Plumbing Board voted unanimously to adopt the Water Demand Calculator (WDC) into Chapter 6 of the state plumbing code as an alternative method for sizing water supply piping in single-family and multifamily buildings. The change is effective immediately for voluntary use statewide.

The WDC replaces assumptions that date back to Hunter’s curve in the 1940s with a statistical model of peak demand built around today’s low-flow fixtures and how people actually use water. According to IAPMO, the practical results are smaller pipe sizes and lower construction cost, reduced water and water-heating energy use, faster hot water delivery at the tap, and less stagnation in oversized lines, which is a recognized factor in Legionella risk.

Colorado joins California, Hawaii, Montana, Nevada, New Jersey, New Mexico, North Dakota, Oklahoma, Oregon, Washington, and Wisconsin as the 12th state to allow the calculator. Cities including Houston, San Jose, Seattle, and Vancouver, British Columbia, also permit it. IAPMO’s Christina Kaeini credited the board’s leadership and the support of stakeholders and industry partners for the adoption.

IAPMO has been steadily building the technical case for the WDC over the past decade, with field studies in multifamily buildings showing that actual peak flows run well below what traditional fixture-unit methods predict. Each state adoption adds to the body of installed projects that engineers and code officials in other states can point to when they consider the same change.

Why it matters to JFN

On a 300-bed student housing building or a 250-unit apartment community, right-sizing the domestic water mains, risers, and meters can cut copper or PEX diameters by a size or two, and that is real material and labor savings a plumbing subcontractor can bring to a general contractor as a value-engineering proposal. Two of Arizona’s neighbors, Nevada and New Mexico, already allow the WDC. We are watching Arizona’s jurisdictions closely, because the first contractor ready to propose it on a Phoenix-area multifamily project will have a meaningful advantage for its clients.

Source: IAPMO, August 14, 2026