Work out fixture unit demand instantly with clear inputs, formula shown and shareable results.
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.
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)
Preliminary estimate only. Use the fixture unit and demand tables in the plumbing code adopted locally for design and permit submission.
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.
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.
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.