Water demand calculator

Calculate fixture units and GPM demand

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The Water Demand Calculator helps plumbing engineers determine water supply requirements for buildings using fixture unit methodology. Calculate total fixture units and convert to gallons per minute (GPM) for water supply sizing.

Fixture unit method is the standard approach for sizing water supply systems in buildings. Each plumbing fixture is assigned fixture units based on its flow rate and usage pattern. Total fixture units are converted to peak demand flow rates.

Enter fixture counts by type to calculate total fixture units and peak water demand in GPM. The calculator uses standard fixture unit values and demand curves to determine water supply requirements for residential and commercial buildings.

Features

Everything you need

Fixture unit calculations
Multiple fixture type support
Peak demand determination
GPM conversion from fixture units
Residential and commercial support
Demand curve applications
Water supply sizing

How it works

Simple workflow

1
Enter fixture counts by type
2
Calculator assigns fixture units
3
Total fixture units calculated
4
Convert to peak GPM demand
5
Size water supply accordingly

Use cases

Built for real-world scenarios

Water supply design

Size water supply pipes and equipment based on calculated peak demand.

Fixture planning

Determine water supply requirements during space planning and fixture selection.

Code compliance

Verify water supply systems meet plumbing code requirements for fixture units.

System upgrades

Evaluate existing water supply capacity for building additions or renovations.

FAQ

Frequently asked questions

Fixture unit (FU) is a measure of probable water demand. One FU = 7.5 GPM for supply systems. Fixtures have different FU values: Toilet 4 FU, Lavatory 1 FU, Shower 2 FU, Kitchen Sink 2 FU.

Use demand curves: For 1-10 FU: GPM = FU × 0.75. For 10-100 FU: GPM = 10 + (FU-10) × 0.5. For 100+ FU: GPM = 55 + (FU-100) × 0.25. Peak demand is less than sum of all fixtures.

Peak demand is the maximum simultaneous flow rate, which is less than the sum of all fixture flows because not all fixtures operate simultaneously. Demand curves account for this probability.

Typical: 2-4 FU per person for residential, 1-2 FU per person for commercial offices. Higher for hotels, restaurants, and healthcare facilities with more fixtures per occupant.

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