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Fixture Unit Demand Calculator

Work out fixture unit demand instantly with clear inputs, formula shown and shareable results.

About Fixture Unit Demand Calculator

Fixture units weight each appliance by how heavily and how often it draws water. Because fixtures rarely operate together, total units convert to probable peak flow along Hunter's curve, approximated here as 2.5 x WSFU^0.6, which flattens sharply as the fixture count grows.

Formulas

Fixture units

WSFU = 2.2 per flush tank WC + 0.7 per lavatory + 1.4 per shower + 1.4 per sink

Probable demand

GPM ≈ 2.5 x WSFU^0.6 (Hunter curve approximation)

Worked example

With the default values below, Fixture Unit Demand Calculator returns Estimated peak demand of 14.4. Change any input above and every figure here is recalculated in your browser.

Inputs used

Flush tank water closetsminimum 0
4
Lavatoriesminimum 0
4
Showers or bathtubsminimum 0
3
Kitchen or service sinksminimum 0
2

Results

Estimated peak demand
14.4 GPM
Total water supply fixture units
18.6 WSFU
Demand in litres
54.7 L/min
Suggested meter size
3/4 in

Step by step

  1. 1Values used
  2. 2Fixture units
  3. 3Probable demand
  4. 4Estimated peak demand= 14.4 GPM
  5. 5Total water supply fixture units= 18.6 WSFU
  6. 6Demand in litres= 54.7 L/min
  7. 7Suggested meter size= 3/4 in

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 1 decimal place, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Preliminary estimate only. Use the fixture unit and demand tables in the plumbing code adopted locally for design and permit submission.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Preliminary estimate only. Use the fixture unit and demand tables in the plumbing code adopted locally for design and permit submission.

Frequently Asked Questions

How is the Fixture Unit Demand figure worked out?

WSFU = 2.2 per flush tank WC + 0.7 per lavatory + 1.4 per shower + 1.4 per sink. Fixture units weight each appliance by how heavily and how often it draws water.

Why not just add up the fixture flows?

Adding maximum flows would size the main for every fixture running at once, which never happens. Fixture units capture the statistics of simultaneous use instead.

What values do I need to enter?

This calculator takes 4 inputs: Flush tank water closets, Lavatories, Showers or bathtubs, Kitchen or service sinks. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

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