Water Supply Pipe Size Calculator (Fixture Units)
Enter your total water-supply fixture units (WSFU) and this tool estimates the peak demand in GPM (a labeled Hunter-curve band) and the smallest supply pipe that covers it — the same logic a plumber uses to size a main or a branch.
Calculator
About 20.0 WSFU maps to roughly 14.0 GPM of peak demand (Hunter-curve band), which a 3/4 in supply line covers at a typical velocity. Sizing follows standard conventions — your local code and inspector govern; confirm with a licensed plumber.
Undersized supply pipe is the classic cause of pressure that sags when a second faucet opens: the water is there, but the pipe can’t deliver the flow fast enough without a big friction penalty. Oversized pipe wastes money, holds more cooling water and lengthens the wait for hot. Fixture-unit sizing threads that needle by estimating a realistic simultaneous demand rather than the theoretical maximum.
The workflow is simple: total your fixtures in WSFU, read the peak GPM, then choose the pipe. This tool does both steps and shows the reference band so you can see how the answer would change if you added a fixture or two.
Formula
Sizing a supply line is a two-step lookup, not a single equation:
- Fixture units → peak demand. Total WSFU is converted to a probable peak flow with a Hunter-curve band — because not every fixture runs at once, 20 WSFU is only about 14 GPM, not the sum of every fixture’s full flow.
- Peak GPM → pipe size. Pick the smallest nominal diameter whose planning capacity (GPM at a safe velocity of roughly 5–8 ft/s) meets or beats that demand:
peak_GPM = Hunter_band(total_WSFU)size = smallest pipe where capacity_GPM ≥ peak_GPM
Capacity bands (labeled): 1/2 in ≈ 9 GPM, 3/4 in ≈ 18 GPM, 1 in ≈ 33 GPM, 1-1/4 in ≈ 53 GPM, 1-1/2 in ≈ 83 GPM.
Worked example
A modest single-family home totals about 20 WSFU (a couple of bathrooms, a kitchen, a laundry and a hose bibb).
- 20 WSFU → roughly 14 GPM of peak demand on the Hunter-curve band.
- 14 GPM fits inside the 3/4 in capacity band (about 18 GPM), but exceeds 1/2 in (about 9 GPM).
- Result: a 3/4 in supply line at a typical velocity.
That is why most homes run a 3/4 in cold-water main to the manifold and drop to 1/2 in at individual fixtures. A long run or high demand pushes you up to 1 in.
How fixture-unit sizing works
Water-supply fixture units (WSFU) are a labeled IPC/UPC planning convention that weights each fixture by how much water it draws and how often: a flush-tank water closet is about 2.2 WSFU, a lavatory 0.7, a shower 1.4, a hose bibb 2.5. Because household fixtures rarely all run at once, the total is mapped through a probability (Hunter) curve to a realistic peak flow rather than a straight sum.
The capacity bands here assume copper at a conservative velocity to limit noise and erosion. PEX and CPVC have different inside diameters and friction, so a plumber may size a run one step larger for a long or high-friction path. Pressure matters too: a house on 45 psi needs more generous pipe than one on 70 psi to hit the same fixture flow. Treat the size here as a starting point and confirm the design against your local plumbing code and static pressure.
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.