The Water Demand Calculator is IAPMO’s modern, statistically based method for estimating peak water demand in single-family and multi-family buildings. Developed using real-world data and contemporary usage patterns, it replaces traditional sizing approaches that are nearly 90 years old. The Water Demand Calculator enables building professionals to right-size water pipes, thereby reduce construction costs, improve water and energy efficiency, and potentially improving water quality.

Most residential plumbing systems in the U.S. are still sized using outdated methods created well before today’s efficient fixtures and appliances existed. These older approaches consistently oversize piping, increasing construction costs, wasting water and energy, and elevating public health risks associated with water aging. IAPMO, in conjunction with the American Society of Plumbing Engineers and the University of Cincinatti, created the Water Demand Calculator to address these challenges with a modern, validated method built on statistically sound research and field data.

Oversized pipes hold larger volumes of water for longer periods, increasing the likelihood of stagnation and bacterial growth. By right-sizing systems, the Water Demand Calculator helps reduce water age, which supports better water quality and reduces risks associated with waterborne pathogens, including Legionella.

The Calculator is designed for use across a wide range of residential applications, including:

• Single-family homes
• Townhomes and duplexes
• Small multi-family buildings
• Mid-rise and large multi-family buildings
• Residential portions of mixed-use occupancies

The method is already used in water-stressed jurisdictions and major cities such as California, Phoenix, Atlanta-Metro Area, Vancouver, and Seattle–King County.

Construction savings vary by project size and local conditions, but documented impacts include:

• Single-family homes: More than $2,000–$5,000 in combined piping and meter fee savings
•  6-unit buildings: Several thousand dollars per building
• 45-unit buildings: Often more than $100,000
• 200-unit high-rises: Up to $250,000 in plumbing system savings, plus $10,000+ per year in lower water and sewer charges

Savings result from reducing unnecessary pipe diameters, minimizing fittings and labor, and lowering utility meter connection fees.

Yes. Multiple independent studies show that using the Water Demand Calculator can save an average of 513 gallons per dwelling unit per year and often more. Faster hot water delivery reduces structural water waste, and because heating water is energy-intensive, right-sizing supports meaningful long-term energy reductions and decarbonization efforts.

Yes. The Water Demand Calculator appears in Appendix M of the Uniform Plumbing Code and is also published in Chapter 5 of the 2023 WE-Stand, IAPMO’s water efficiency and sanitation standard. Jurisdictions can adopt the method alongside any plumbing code without replacing their existing framework.

The core Water Demand Calculator spreadsheet is available at no cost and can be downloaded at: https://iapmo.org/we-stand/water-demand-calculator.

For professionals who need enhanced capabilities, IAPMO offers WDC-Pro, a subscription-based, web-powered version that significantly accelerates complex design workflows and can be access at: https://wdcpro.iapmo.org.

The Calculator is supported by a multi-year research partnership between IAPMO, the American Society of Plumbing Engineers (ASPE), the Water Quality Research Foundation, and the University of Cincinnati. Field studies show the older Hunter method can oversize design flow rates by up to 27 times, while the Water Demand Calculator closely aligns with measured real-world usage.

Adoption continues to grow across North America. Examples include:

• California (statewide for multi-family hot water systems)
• City of Phoenix
• Seattle–King County (proposed mandate)
• Vancouver, BC
• Charlotte, NC
• Castle Rock, CO
• Multiple regions experiencing drought or limited water availability

Adopting the Calculator helps communities improve water efficiency, manage infrastructure costs, and promote resilient, high-performance buildings.

Water Demand Calculator (Free):
IAPMO’s validated, code-recognized sizing method available as a downloadable spreadsheet. Ideal for single-family and small-to-medium multi-family projects.

WDC-Pro (Subscription):
A modern, web-based platform built on the same method but engineered for speed and scalability. WDC-Pro dramatically reduces calculation time and supports complex multi-unit and high-rise projects with professional-grade features.

A study conducted by Gary Klein and Associates in collaboration with 2050 Strategic Partners demonstrated that the Water Demand Calculator does not underestimate peak flow rates; in fact, it maintained at least a 2 times safety factor compared to measured real-world usage, giving users confidence in the results.