Hot Water Demand Calculator (Fixture Units)
Estimate a home’s peak hot-water flow in GPM by adding up fixture units and applying the demand (Hunter) diversity curve.
How it works
WSFU = Σ (fixture count × hot-side fixture-unit value) · Peak GPM ≈ 1.456 × WSFU^0.74 (Hunter demand curve)
You never run every hot tap at once, so plumbing design uses fixture units and a diversity curve rather than a naive sum of every fixture’s flow. Each fixture gets a water-supply-fixture-unit (WSFU) weight; the total is converted to a probable peak flow using the Hunter demand curve, which levels off as a building grows because simultaneous use becomes proportionally rarer. This peak GPM is what sizes the water heater’s recovery and the hot-water piping.
Worked example
A home with 2 lavatories (0.5 each), 1 shower (1.0), 1 kitchen sink (0.75), 1 dishwasher (0.75) and 1 clothes washer (1.0) totals 4.5 hot-water fixture units. The demand curve gives about 1.456 × 4.5^0.74 ≈ 4.4 GPM of probable peak hot-water flow.
Sources: Hunter’s curve / fixture units
Frequently asked questions
What is a fixture unit?
A water-supply fixture unit (WSFU) is a weighting that captures how much water a fixture draws and how often it runs. A busy shower rates higher than a rarely-used lavatory, letting designers combine mixed fixtures into one demand number.
Why not just add up every fixture’s GPM?
Because they almost never all run at once. Summing raw flows badly oversizes the system. The Hunter demand curve estimates the realistic simultaneous peak, which is far lower and rises slowly as more fixtures are added.
Is this exact enough for a permit?
It is a solid planning estimate using standard fixture-unit values and a curve fit. For permitted work, follow your local plumbing code’s official fixture-unit tables and demand chart, which may assign slightly different values.
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This calculator is for educational purposes. Double-check important results before acting on them.