■ MEP BIM INSIGHTS — MEP ENGINEERING

Plumbing Fixture Units & Pipe Sizing (IPC)

Plumbing pipe sizing looks simple from the outside — bigger building, bigger pipes. In practice it follows a specific method defined by the plumbing code, and getting it wrong produces either undersized pipes (low pressure, poor drainage) or oversized ones (wasted cost, and in drainage, poor scouring velocity). This article walks through how fixture units drive pipe sizing under the International Plumbing Code (IPC).

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The Core Idea: Fixture Units

You cannot simply add up the flow rate of every fixture in a building and size the pipe for that total. Not every fixture runs at once — a 300-unit apartment building never has all toilets, showers, and sinks operating simultaneously. The plumbing code handles this with the fixture unit: a weighted value assigned to each fixture type that represents its probable load on the system.

There are two separate fixture unit systems, because supply and drainage behave differently:

• WSFU — Water Supply Fixture Units, for sizing supply piping
• DFU — Drainage Fixture Units, for sizing drainage and vent piping

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Step 1: Assign Fixture Units

Each fixture type has a code-defined fixture unit value. The IPC provides tables for both supply (WSFU) and drainage (DFU). Representative values:

Fixture	WSFU (supply)	DFU (drainage)
Water closet (flush tank)	2.2	3
Water closet (flushometer)	5.0	4
Lavatory	0.7	1
Shower	1.4	2
Kitchen sink	1.4	2
Floor drain	—	2

Values vary by fixture type, whether the supply is hot, cold, or both, and by occupancy (private vs public). Always use the values from the applicable code edition and its amendments.

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Step 2: Sum and Convert to Flow

Once every fixture is counted and assigned units, the units are summed for each pipe segment — the total fixture units downstream of that point.

For supply piping, that total WSFU is converted to a probable peak flow in gallons per minute (GPM). This is where the demand factor lives: the conversion is not linear. The relationship — historically Hunter’s curve — recognizes that as more fixtures are added, the probability of all running at once drops. A hundred fixtures do not demand a hundred times the flow of one.

For drainage piping, the total DFU maps directly to a required pipe size in the IPC drainage tables, along with the slope required.

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Step 3: Size the Pipe

Supply piping

With peak GPM known for a segment, the pipe is sized to keep velocity and friction loss within limits. Typical constraints: velocity under about 8 ft/s (higher velocities cause noise and erosion), and friction loss low enough that the pressure at the most remote fixture still meets the minimum required. Both must be satisfied.

Drainage piping

Drainage is sized by DFU total, pipe slope, and whether the pipe is horizontal or a stack. Drainage has a lower velocity limit consideration too — but in the opposite direction: pipes must maintain enough velocity (scouring velocity, typically 2 ft/s minimum) to carry solids. An oversized drainage pipe can actually drain worse.

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What This Means for the BIM Model

In a Revit plumbing model, pipe sizes are the result of these calculations — and they must be right before coordination:

• Model the calculated size. Pipe diameters in the model should match the fixture-unit sizing, not be placed at a convenient default and adjusted later.
• Drainage slope must be modeled. A flat drainage pipe in the model doesn’t reflect reality — slope consumes vertical space and is a real coordination constraint against structure and other systems.
• Fixture unit data can live in the model. With shared parameters, fixture unit values can be attached to fixture families, supporting schedule-based verification of the sizing.
• Stacks and horizontal runs size differently. The model must reflect that a stack and a horizontal branch carrying the same DFU total may be different diameters.

Our Approach at GEOMETRY-S

We size plumbing systems by fixture-unit method per the applicable IPC edition — WSFU for supply, DFU for drainage — and model the calculated sizes with correct drainage slope so the pipes in the model are the pipes that get installed. Sizing is completed before coordination, so slope and diameter are real constraints in the federated model, not adjustments made after clash detection.