Fixture Unit Calculator Formula

Understand the math behind the fixture unit calculator. Each variable explained with a worked example.

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Formulas Used

Total Fixture Units

total_wsfu = toilets * 2.5 + lavatories * 1 + bathtubs * 2 + dishwashers * 1.5 + washing_machines * 2

Estimated Peak Demand

est_gpm = 5 * sqrt(toilets * 2.5 + lavatories * 1 + bathtubs * 2 + dishwashers * 1.5 + washing_machines * 2)

Variables

VariableDescriptionDefault
toiletsToilets (Flush Valve)2
lavatoriesLavatories / Sinks3
bathtubsBathtubs / Showers2
dishwashersDishwashers1
washing_machinesWashing Machines1

How It Works

Fixture Unit Method

Each plumbing fixture is assigned a unit value based on its flow demand. The total WSFU determines minimum pipe size per code tables. A quick estimate for peak demand in GPM uses the Hunter curve approximation: GPM = 5 x sqrt(WSFU).

Worked Example

2 toilets, 3 sinks, 2 showers, 1 dishwasher, 1 washer.

toilets = 2lavatories = 3bathtubs = 2dishwashers = 1washing_machines = 1
  1. 01Toilets: 2 x 2.5 = 5
  2. 02Sinks: 3 x 1 = 3
  3. 03Showers: 2 x 2 = 4
  4. 04Dishwasher: 1 x 1.5 = 1.5
  5. 05Washer: 1 x 2 = 2
  6. 06Total = 15.5 WSFU
  7. 07Peak GPM = 5 x sqrt(15.5) = 19.7 GPM

Frequently Asked Questions

What is a fixture unit?

A fixture unit (FU) is a relative measure of the probable hydraulic demand of a plumbing fixture. One fixture unit originally equaled about 7.5 GPM, but modern codes use tables rather than a direct conversion.

Are these values code-compliant?

This calculator uses typical residential WSFU values. Always verify against your local plumbing code (IPC, UPC, or local amendments) for the final design.

Learn More

Guide

Plumbing Pipe Sizing Guide: Water Supply and Drain Lines

Learn how to size plumbing pipes for water supply, drainage, and venting. Covers fixture units, pipe materials, code requirements, and pressure considerations.

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