Peak Demand GPM Calculator (Fixture Units)
Turn your total water-supply fixture units (WSFU) into a probable peak flow in GPM. The Hunter curve accounts for the fact that fixtures rarely all run at once, so the demand is far below the arithmetic sum — the figure you use to size a meter, service and main.
Calculator
About 30.0 WSFU corresponds to roughly 18.0 GPM of peak demand on a standard Hunter-curve band. Use it to size the meter and main; your local code and a licensed plumber set the final design.
Peak demand is the hinge between counting fixtures and sizing pipe. It answers “how many gallons per minute will this system realistically ever need at once?” — and because plumbing systems enjoy diversity (not every fixture runs together), the honest peak is a fraction of the theoretical maximum. Get this number right and the meter, service line and main all follow.
Formula
Peak demand is read from a probability (Hunter) curve, not a sum:
peak_GPM = Hunter_band(total_WSFU)
The curve reflects diversity — the more fixtures a system has, the smaller the fraction running at any instant. So demand rises with WSFU but flattens out: 10 WSFU is about 8 GPM, 30 WSFU about 18 GPM, 100 WSFU about 44 GPM. The band here is the flush-tank version of the classic Hunter curve, labeled as a planning estimate.
Worked example
A two-bath home with a kitchen, a laundry and a couple of hose bibbs totals about 30 WSFU.
- 30 WSFU → roughly 18 GPM of peak demand on the Hunter-curve band.
Notice that 30 fixture units is only about 18 GPM — nowhere near the sum of every fixture’s full flow, because they don’t all open at once. Feed that 18 GPM into the supply pipe-size tool to pick the line, and into meter and service sizing.
The Hunter curve, briefly
The Hunter curve (fixture-unit method) is the standard way plumbing engineers estimate probable simultaneous demand. Each fixture is weighted in WSFU, the total is read off the curve, and the resulting GPM sizes the water meter, the service line from the street and the building’s main. Flush-valve fixtures (common in commercial buildings) draw more heavily than the flush-tank fixtures in a typical house, so they use a separate, higher curve — the band here is the residential flush-tank version.
Because the curve flattens as fixtures multiply, adding a bathroom to a big house barely moves the peak demand, while adding one to a small house moves it more. Use the resulting GPM as a planning input, not a code determination: real designs also weigh pressure, elevation, pipe material and continuous-flow loads (like irrigation) that sit outside the fixture curve. Confirm the meter and service sizing with a licensed plumber and your water utility.
Reference table
| Total WSFU | Peak demand (band) | Supply size |
|---|---|---|
| 5 WSFU | 5 GPM | 1/2 in |
| 10 WSFU | 8 GPM | 1/2 in |
| 20 WSFU | 14 GPM | 3/4 in |
| 30 WSFU | 18 GPM | 3/4 in |
| 50 WSFU | 26 GPM | 1 in |
| 75 WSFU | 34 GPM | 1-1/4 in |
| 100 WSFU | 44 GPM | 1-1/4 in |
Labeled Hunter-curve planning band (flush-tank fixtures) mapped to a copper supply line at a typical velocity. Confirm with a licensed plumber and your local plumbing code.