Fixture Unit Calculator - WSFU to GPM with Hunter's Curve

How It Works

1. Select Fixture Rows

   Choose the local fixture rows available in the screen, including common private-use fixtures and clearly labeled flushometer planning rows.

2. Enter Fixture Quantities

   Input the count of each fixture type. The shared-state loader sanitizes malformed quantities and ignores unknown fixture IDs.

3. Review WSFU Totals

   The screen tallies total WSFU, hot-water WSFU, and cold-water WSFU separately and shows the source note attached to each selected row.

4. Screen GPM Demand

   The total is converted to estimated peak demand with a local Hunter-style interpolation. Flushometer-heavy screens receive extra warning text.

5. Take It to Code Review

   Use the output as a planning worksheet, then verify adopted code edition, local amendments, pressure-loss design, meter/service limits, velocity, and AHJ expectations before final sizing.

Built For

• Plumbers building a preliminary WSFU worksheet before adopted-code lookup
• Mechanical designers screening peak domestic-water demand for early layout work
• Facilities teams checking whether a renovation needs a deeper plumbing review
• Plumbing contractors documenting fixture-count assumptions during bid preparation
• Plan reviewers and owners spotting fixture rows that need code-source confirmation
• Apprentice plumbers learning WSFU, hot/cold branch tallies, and Hunter-style demand concepts

Features & Capabilities

Source-Warning Boundary

The app states where values are public source-backed, where they are local screens, and where adopted-code review is required.

Corrected Private Fixture Rows

Common private-use rows such as bathtub, lavatory, shower, clothes washer, dishwasher, tank water closet, and drinking fountain are aligned to the public CPC/UPC source pointer where clear.

Hot/Cold Branch Split

Fixtures served by both hot and cold connections use three-quarters of the listed total WSFU for each branch, matching the cited table note.

Local Hunter-Style Demand Screen

Converts total WSFU to a planning GPM with local interpolation and highlights flushometer-heavy or out-of-range screens.

Rough Pipe-Size Bands

Shows broad pipe-size planning bands while warning that final sizing requires pressure, length, elevation, material, meter, valve, fitting, velocity, and residual-pressure checks.

Assumptions

• Clear private-use fixture rows are seeded from the publicly accessible CPC/UPC Table 610.3 supplement source pointer
• Fixtures with both hot and cold connections use three-quarters of the listed total WSFU for each branch, matching the cited table note
• Flushometer rows remain local screening values because the cited UPC/CPC table directs flushometer systems to separate sizing provisions
• The Hunter-style curve and flushometer multiplier are local planning curves that require code-edition and accepted-engineering-practice review
• Pipe-size bands ignore developed length, pressure budget, elevation, pipe material, fittings, meter/service size, and velocity limits

Limitations

• Not a complete IPC, UPC, CPC, or local-code table reproduction
• Does not decide whether Hunter-style tables or an alternative method such as the IAPMO Water Demand Calculator is allowed or required
• Does not account for fire sprinkler demand on combined domestic/fire systems
• Pipe sizing does not model pressure loss, water hammer, thermal expansion, or pressure regulator requirements
• Does not calculate backflow preventer sizing or pressure loss through specific backflow devices
• Building-specific factors like recirculation loops, booster pumps, and pressure zones are not modeled
• Does not address drainage fixture unit (DFU) calculations for waste and vent pipe sizing
• Local code amendments may modify standard fixture unit values or adopt alternative demand curves

References

1. IAPMO 2022 California Plumbing Code July 1 2024 Supplement - Table 610.3 public source pointer
2. IAPMO 2024 Uniform Plumbing Code source pointer
3. ICC 2024 International Plumbing Code source pointer
4. Hunter, Roy B. - "Methods of Estimating Loads in Plumbing Systems" (NBS Building Materials and Structures Report BMS 65, 1940)
5. IAPMO Water Demand Calculator source pointer

Frequently Asked Questions