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

Calculate Water Supply Fixture Units and Convert to Peak Demand for Pipe Sizing

Free fixture unit calculator for plumbers, mechanical engineers, and plan reviewers. Select fixtures from IPC/UPC standard tables, tally water supply fixture units (WSFU), and convert the total to peak GPM demand using Hunter's Curve. Determines minimum pipe size for water supply mains, risers, and branches based on available pressure and developed length. Supports hot-only, cold-only, and total fixture unit calculations.

Pro Tip: Hunter's Curve was developed in the 1940s based on flush-valve toilet fixtures in commercial buildings. Modern low-flow fixtures use significantly less water, so Hunter's Curve overpredicts peak demand for residential and modern commercial buildings by 20-40%. Some progressive jurisdictions have adopted modified fixture unit values that reflect modern fixtures. If your calculated GPM seems high, it probably is. But the code tables are the code tables, and inspectors enforce what the book says. Size to the code, note the conservatism, and move on.

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Fixture Unit Calculator
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How It Works

  1. Select Fixture Types

    Choose from standard fixtures: water closet (flush valve or flush tank), lavatory, bathtub, shower, kitchen sink, dishwasher, clothes washer, hose bibb, and more. Each fixture has a preset WSFU value per IPC Table 604.3 or UPC Table 610.3.

  2. Enter Fixture Quantities

    Input the count of each fixture type. For mixed-use buildings, separate public from private fixtures because flush-valve water closets have higher WSFU values than flush-tank models.

  3. Calculate Total WSFU

    The calculator tallies total WSFU, hot-water WSFU, and cold-water WSFU separately. Hot and cold totals are needed to size the hot and cold water mains independently.

  4. Convert to GPM via Hunter's Curve

    The total WSFU is converted to peak probable demand in GPM using the Hunter's Curve lookup. This is the design flow rate for sizing the water main, meter, and distribution piping.

  5. Determine Pipe Size

    Enter the available street pressure, elevation loss, meter and valve friction losses, and developed length. The calculator determines the minimum pipe diameter that delivers the required GPM at the most remote fixture with adequate residual pressure.

Built For

  • Plumbers sizing water service and distribution piping for new residential construction per IPC/UPC
  • Mechanical engineers calculating peak water demand for light commercial buildings, restaurants, and offices
  • Plan reviewers verifying fixture unit counts and pipe sizing on submitted plumbing plans
  • Plumbing contractors estimating pipe sizes during bid preparation for tenant fit-out projects
  • Facilities managers evaluating whether existing water infrastructure can support added fixtures in a building renovation
  • Fire protection engineers verifying domestic water demand to confirm adequate pressure for combined fire/domestic systems
  • Apprentice plumbers learning fixture unit methodology and Hunter's Curve application

Features & Capabilities

IPC/UPC Fixture Tables

Pre-loaded fixture unit values from both IPC Table 604.3 and UPC Table 610.3. Select your applicable code and the calculator uses the correct WSFU values. Public vs private fixture distinction is handled automatically.

Hunter's Curve Conversion

Converts total WSFU to peak probable GPM demand using the standard Hunter's Curve for predominantly flush-tank systems and the flush-valve curve for commercial buildings. The converted GPM is the design flow rate for pipe sizing.

Hot/Cold Split

Separately calculates hot-water and cold-water fixture units using the 75% rule (hot-water WSFU = 75% of total for fixtures served by both hot and cold). This is needed for sizing the hot and cold water mains independently.

Pipe Sizing with Pressure Budget

Factors in available street pressure, static elevation head (0.433 PSI per foot of elevation), meter loss, backflow preventer loss, and friction loss per 100 feet of developed length to determine the minimum pipe diameter.

Custom Fixture Entry

Add non-standard fixtures with custom WSFU values for specialized equipment like commercial kitchen equipment, laboratory fixtures, or medical gas outlets.

Frequently Asked Questions

A WSFU is a dimensionless unit that represents the probable demand for water flow from a single plumbing fixture. It was developed by Roy Hunter at the National Bureau of Standards in the 1940s as a way to convert diverse fixtures into a common unit for demand calculation. A lavatory is typically 1 WSFU, a flush-tank water closet is 2.5 WSFU, and a flush-valve water closet is 5 WSFU. The values reflect both the flow rate and the probability of simultaneous use.
Use Hunter's Curve, which is a graphical or tabular relationship between total WSFU and peak probable GPM. The conversion is not linear because of probability theory: as you add more fixtures, the probability of all of them running simultaneously decreases. For example, 20 WSFU converts to about 15 GPM, but 100 WSFU converts to about 38 GPM, not 75 GPM. The curve flattens as fixture count increases.
IPC (International Plumbing Code) and UPC (Uniform Plumbing Code) assign slightly different WSFU values to some fixtures. The UPC tends to be more conservative. Most eastern and midwestern US states use IPC. Western states, including California, use UPC. The differences are generally small enough that pipe sizing results are similar, but you should use the code adopted by your jurisdiction for permit compliance.
Yes, significantly. Hunter's original research was based on 1940s flush-valve toilets that used 5-7 gallons per flush. Modern fixtures use 1.6 GPF toilets, 1.5 GPM showerheads, and 0.5 GPM lavatory faucets. Several studies have shown that actual peak demand in modern buildings is 20-40% below what Hunter's Curve predicts. Some jurisdictions have adopted modified curves, but most inspectors still enforce the original values from the IPC/UPC tables. Size to code even if you know it is conservative.
Static pressure loss from elevation is 0.433 PSI per foot of height. A four-story building with 40 feet between the water service entry and the top-floor fixtures loses 17.3 PSI to elevation alone. If street pressure is 60 PSI and you lose 17 PSI to elevation plus 8 PSI to meter, backflow preventer, and friction, you have 35 PSI available at the top floor. If that is not enough for adequate fixture performance (typically 8-15 PSI minimum at the fixture), you need a booster pump or larger pipe to reduce friction losses.
Disclaimer: This calculator provides fixture unit estimates based on IPC/UPC standard tables and Hunter's Curve methodology. Pipe sizing depends on local water pressure, pipe material, and building configuration. All plumbing system design must be verified by a licensed professional engineer or plumber and must comply with local plumbing codes. ToolGrit is not responsible for system design or installation outcomes.

Learn More

Industrial

Fixture Units Explained: WSFU, Hunter's Curve, and Modern Plumbing Load Calculations

What water supply fixture units are, how Hunter's Curve converts them to GPM, why the method overpredicts for modern fixtures, and how to size water supply piping from code tables.

Read Guide →

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