Water Supply Fixture Units (WSFU) Calculator
Count your fixtures and get a total in water-supply fixture units (WSFU) — the labeled convention that feeds peak-demand GPM and supply-pipe sizing.
Calculator
Your fixtures total 7.1 WSFU. Water-supply fixture units are a labeled planning convention — feed them into the peak-demand GPM and supply pipe size tools. Your local code governs the exact values.
Water-supply fixture units (WSFU) are the standard shorthand plumbers use to turn a list of fixtures into a demand number. Each fixture is assigned a unit value that reflects how much water it draws and how often — a toilet, a shower and a hose bibb do not all load the supply the same way. Adding the units gives a single total that stands in for the whole house.
That total is the entry point to sizing the water service and the main: you convert WSFU to a peak-demand flow in GPM using a Hunter-curve band, then pick the smallest pipe that carries that flow. This calculator does the first step — counting the units — from a plain fixture tally. The unit values are a labeled IPC/UPC planning convention you can rely on for estimating; your local code governs the exact figures.
Formula
Sum the count of each fixture times its WSFU value:
total WSFU = Σ (fixture count × WSFU per fixture)
The labeled convention values used here are: tank water closet 2.2, lavatory 0.7, shower 1.4, bathtub 1.4, kitchen sink 1.4, laundry 1.4 and hose bibb 2.5. A flush-tank toilet carries a high unit value because it refills sharply after every flush; a hose bibb is high because it can run wide open for a long time.
Worked example
Take a modest single-bathroom home: 1 tank toilet, 1 lavatory, 1 shower, 1 kitchen sink and 1 laundry connection:
1×2.2 + 1×0.7 + 1×1.4 + 1×1.4 + 1×1.4 = 7.1 WSFU
That 7.1 WSFU total feeds straight into the peak-demand GPM tool (about 6–7 GPM on the Hunter band) and then the supply pipe size tool, which would call for a 1/2-to-3/4-inch service on such a small load. Add a second bathroom, a dishwasher line and a couple of hose bibbs and the total climbs quickly — enter your real fixture counts to see it.
From fixture units to a pipe size
Fixture units are a modeling trick: you never actually run every fixture at once, so summing raw GPM ratings would badly oversize the pipe. The WSFU system, paired with the Hunter curve, estimates the realistic simultaneous peak instead. That is why 20 WSFU maps to roughly 14 GPM rather than the much larger figure you would get by adding flow rates — most fixtures sit idle at any instant.
Count every fixture that draws from the potable supply, including the water heater feed, the dishwasher and the clothes washer. Flush-valve (tankless) toilets carry higher unit values than the tank toilets counted here, and separate hot and cold demands are already folded into the combined fixture value for a home. If your project has commercial fixtures or flushometers, the values differ — that is where a licensed plumber and the local code table take over.
Use the total as the labeled starting point for sizing, not a final design. It flows into the peak-demand GPM and supply pipe size calculators, and the full convention is laid out in the fixture units table. The drainage side uses a separate count — see drainage fixture units (DFU) for sizing the waste and vent piping.
Reference table
Water-supply fixture units per fixture (labeled IPC/UPC planning convention):
| Fixture | WSFU |
|---|---|
| Water closet (tank) | 2.2 |
| Lavatory | 0.7 |
| Shower | 1.4 |
| Bathtub | 1.4 |
| Kitchen sink | 1.4 |
| Laundry | 1.4 |
| Hose bibb | 2.5 |
Values are labeled conventions — confirm with a licensed plumber and local code.
Frequently asked questions
What are water-supply fixture units (WSFU)?
WSFU are a labeled convention that assigns each fixture a demand value based on how much water it uses and how often. Summing them gives a whole-house demand figure that you convert to peak-flow GPM to size the water service and main.
How do I turn WSFU into a pipe size?
Feed the total into the peak-demand GPM tool, which uses a Hunter-curve band to estimate simultaneous flow, then into the supply pipe size tool to pick the smallest pipe that carries it. For example, 20 WSFU ≈ 14 GPM ≈ 3/4-inch copper.
Why does a toilet count more than a bathroom sink?
A tank toilet (2.2 WSFU) refills sharply after each flush and does so frequently, loading the supply more than a lavatory (0.7 WSFU) that runs briefly. The unit values reflect both draw and frequency, not just peak flow.
Is WSFU the same as drainage fixture units?
No. WSFU size the water supply; drainage fixture units (DFU) size the waste and vent piping, and the two use different values. Use the DFU calculator for the drain side.