What is a fixture unit (FU)?

A fixture unit is a dimensionless number assigned to each plumbing fixture that represents its hydraulic load relative to a lavatory (wash basin), which is defined as 1 fixture unit. Toilet = 4 DFU, shower = 2 DFU, etc. Summing fixture units allows engineers to size pipes using code lookup tables without detailed flow calculations for every fixture.

What is the difference between DFU and WSFU?

Drainage Fixture Units (DFU) measure the load on the drain, waste, and vent (DWV) system and are used to size drain pipes, stacks, and building sewers. Water Supply Fixture Units (WSFU) measure demand on the cold and hot water supply system and are used to size water service lines, mains, and branches.

What is the difference between IPC and UPC fixture unit values?

The International Plumbing Code (IPC) and the Uniform Plumbing Code (UPC) assign slightly different fixture unit values to some fixtures. IPC is adopted in most eastern US states while UPC is common in western states and internationally. Always verify which code your jurisdiction has adopted before using these values for a permitted project.

Do fixture units account for diversity (simultaneous use)?

Fixture unit tables already embed an empirical probability factor for simultaneous use, derived from Hunter's Curve (Roy B. Hunter, 1940). You do not apply an additional diversity factor — simply sum the fixture units and look up the pipe size in the applicable code table.

Can I use these values for a permit submittal?

These values are based on IPC Table 709.1 and UPC Table 702.1 and are accurate for standard residential and commercial fixtures. Always cross-check against the current adopted code edition in your jurisdiction. The engineer of record is responsible for final sizing.

Fixture Unit Calculator

Name: Fixture Unit Calculator
Availability: InStock
Author: Nham Vu

Select fixture types, enter counts, and calculate total drainage and water supply fixture units for plumbing system sizing per IPC/UPC standards.

Fixture Schedule

Fixture	DFU	WSFU	Quantity

Enter fixture quantities
and click Calculate.

Total Drainage Fixture Units

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DFU — use for drain pipe sizing

Total Water Supply Fixture Units

—

WSFU — use for water service sizing

Breakdown by Fixture

Fixture	Qty	DFU	WSFU

Code reference: IPC Table 709.1 / Table 604.3. Verify values against the current adopted edition in your jurisdiction.

Summary

Select fixture types, enter counts, and calculate total drainage and water supply fixture units for plumbing system sizing per IPC/UPC standards.

How it works

Choose the plumbing code standard: IPC (International Plumbing Code) or UPC (Uniform Plumbing Code).
Enter the quantity of each fixture type in your building or system.
The table shows the per-fixture drainage unit (DFU) and water supply unit (WSFU) values from the selected code.
Click Calculate to sum all fixture units across all entered quantities.
Review the total DFU and WSFU values and use them to size your drain pipes and water service per the applicable code tables.

Use cases

Sizing the building drain and sewer for residential and commercial projects.
Selecting the correct water meter and service line size for a new building.
Verifying that an existing drainage system can handle a proposed tenant improvement.
Calculating fixture unit loads for multi-family residential buildings.
Checking plumbing submittals against code-minimum pipe sizing tables.
Teaching plumbing engineering fundamentals to students and apprentices.
Performing quick feasibility checks during the schematic design phase.
Supporting permit applications that require fixture unit totals.

Frequently Asked Questions

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Last updated: 2026-05-23 · Reviewed by Nham Vu