Fixture Unit Calculator - WSFU to GPM with Hunter's Curve
Calculate Water Supply Fixture Units and Convert to Peak Demand for Pipe Sizing
Free source-aware fixture unit planning screen for plumbers, designers, and facilities teams. Select local water-supply fixture rows, tally water supply fixture units (WSFU), and screen peak GPM demand with a local Hunter-style curve. The output includes hot, cold, and total WSFU totals, rough pipe-size planning bands, and warnings for adopted-code review. It does not reproduce full IPC, UPC, CPC, or local-code tables and is not a permit-ready pipe schedule.
Size the water heater for your fixture load
Water Heater Sizing Calculator →Calculate pipe pressure drop for supply piping
Pipe Pressure Drop Calculator →Screen drain, waste, and vent prompts for the same fixtures
Plumbing DWV Sizing Calculator →Read the complete fixture unit guide
Fixture Units Explained →How It Works
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Select Fixture Rows
Choose the local fixture rows available in the screen, including common private-use fixtures and clearly labeled flushometer planning rows.
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Enter Fixture Quantities
Input the count of each fixture type. The shared-state loader sanitizes malformed quantities and ignores unknown fixture IDs.
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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.
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Screen GPM Demand
The total is converted to estimated peak demand with a local Hunter-style interpolation. Flushometer-heavy screens receive extra warning text.
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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
- IAPMO 2022 California Plumbing Code July 1 2024 Supplement - Table 610.3 public source pointer
- IAPMO 2024 Uniform Plumbing Code source pointer
- ICC 2024 International Plumbing Code source pointer
- Hunter, Roy B. - "Methods of Estimating Loads in Plumbing Systems" (NBS Building Materials and Structures Report BMS 65, 1940)
- IAPMO Water Demand Calculator source pointer
Frequently Asked Questions
Learn More
Fixture Units Explained: WSFU, Hunter's Curve, and Code Review Boundaries
What water supply fixture units are, how Hunter-style curves estimate demand, and why adopted code, local amendments, and pressure-loss design control final water supply sizing.
DWV Pipe Sizing: How to Size Drain, Waste, and Vent Pipes
Drain fixture-unit prompts, slope-dependent drain review, vent method boundaries, stack and branch-interval prompts, trap-arm and cleanout gaps, IPC/UPC differences, AHJ review, and product-data limits.
Backflow Preventer Sizing: RPZ, DCVA, and PVB Selection
How to select and size backflow prevention assemblies. Hazard levels, assembly types, pressure loss, and annual testing requirements per AWWA and local codes.
Grease Trap Pumping Records and Schedule Planning
How grease trap and interceptor pump-out planning depends on measured grease and solids, local thresholds, product instructions, manifests, and sewer-authority review.
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