Fixture Unit Demand Calculator
Computes the total plumbing demand load in fixture units for toilets, sinks, showers, and appliances, then converts it to a peak flow estimate for pipe sizing. Use it when designing or auditing a building's water supply system.
About this calculator
Fixture units (FU) are a dimensionless weighting system standardized in codes like the Uniform Plumbing Code to represent the relative water demand of each fixture type. The raw fixture unit total is: FU_total = (waterClosets × 4) + (lavatories × 1) + (showers × 2) + (appliances × 3). Because not all fixtures run simultaneously, the formula applies a usage factor (a probability-based concurrency multiplier, typically 0.25–1.0) to scale peak demand. The estimated peak flow is then: Flow = √(FU_total × usageFactor) × 3, where the square-root relationship reflects the statistical nature of simultaneous use — doubling fixture count less than doubles peak demand. The output can be compared against pipe capacity tables to select the correct supply pipe diameter for the building.
How to use
Assume a small office with 3 water closets, 4 lavatories, 2 showers, 1 appliance (dishwasher), and a usage factor of 0.5. Step 1 — weighted FU total: (3 × 4) + (4 × 1) + (2 × 2) + (1 × 3) = 12 + 4 + 4 + 3 = 23 FU. Step 2 — apply usage factor: 23 × 0.5 = 11.5. Step 3 — take square root: √11.5 ≈ 3.391. Step 4 — multiply by 3: 3.391 × 3 ≈ 10.17 GPM peak demand. A 1-inch supply main (rated ~15 GPM at typical pressure) would comfortably serve this load.
Frequently asked questions
What are plumbing fixture units and how are they used to size pipes?
A fixture unit is a standardized measure — originally defined by Roy Hunter in the 1940s — that represents one cubic foot of water per minute (roughly 7.48 GPM) as a baseline demand unit. Each fixture type is assigned a weight: toilets carry 4 FU, showers 2 FU, lavatories 1 FU, and so on, reflecting their individual peak flow rate and frequency of use. Engineers sum the fixture units for an entire building, apply a probability-based demand factor, and then look up the resulting GPM in pipe sizing tables to select the correct diameter. This method avoids the gross oversizing that would result from assuming every fixture runs simultaneously.
How does the usage factor affect the fixture unit demand calculation?
The usage factor accounts for the statistical probability that all fixtures will run at the same moment, which decreases as building occupancy patterns become more predictable and spread out. A residential home might use a factor of 0.7–1.0 (high probability of simultaneous morning use), while a large office building might use 0.25–0.4 because occupants stagger usage throughout the day. Using a factor that is too high wastes money on oversized pipes; using one that is too low risks pressure drops and inadequate flow at fixtures during peak periods. The appropriate factor is often specified in local plumbing codes or engineering guidelines.
Why do water closets have a higher fixture unit value than sinks or showers?
Water closets (toilets) carry 4 fixture units because a single flush demands a high instantaneous flow rate — typically 1.6 gallons released in under 30 seconds — and because the fill cycle that follows keeps a solenoid or ballcock valve open for up to two minutes. Lavatories and showers use water more slowly and continuously, so their FU weights (1 and 2, respectively) are lower despite sometimes running longer. The weighting system therefore captures peak instantaneous demand rather than total daily volume, which is what determines pipe sizing for pressure and velocity constraints.