Peak Water Demand Estimator (FU Method)
Calculated Plumbing Demand
What is a Water Demand Calculator?
A Water Demand Calculator is a digital tool that estimates the peak water usage of a building by combining fixture counts, fixture units, and flow demand equations.
The calculation is based on the Fixture Unit (FU) Method, which comes from Hunter’s Curve, a plumbing standard that predicts how many fixtures will be used simultaneously. Each fixture—like sinks, showers, toilets, dishwashers, and washing machines—has an assigned fixture unit value.
By multiplying fixture counts with their FU values, the calculator totals demand and converts it into peak GPM (gallons per minute) using an approximation formula:
[
\text{Estimated GPM} = C \times (\text{Total FU})^E
]
Where:
- C = Constant (in this calculator, 3.0)
- E = Exponent (commonly 0.5 for residential plumbing)
Why Use a Water Demand Calculator?
Accurate Pipe Sizing – Avoid undersized or oversized plumbing layouts.
Cost Savings – Prevent overbuilding expensive piping or pumps.
Code Compliance – Stay aligned with plumbing standards like Hunter’s Curve.
Better Planning – Plan hot water systems, tankless units, and water heaters effectively.
Convenience – Quickly estimate peak flow instead of manually calculating fixture units.
How the Calculator Works
The Water Demand Calculator lets you input the number of common plumbing fixtures in your project. Each fixture type is automatically weighted with its industry-standard fixture unit (FU) value:
- Showers / Bathtubs = 2.0 FU
- Lavatories (Sinks) = 1.0 FU
- Water Closets (Flush Tank) = 2.5 FU
- Water Closets (Flush Valve) = 4.0 FU
- Washing Machine = 3.0 FU
- Dishwasher = 3.0 FU
The tool then:
- Adds up total fixture units.
- Applies the GPM conversion formula (Constant × FU^Exponent).
- Displays peak water demand in GPM.
For example:
- 2 showers + 3 sinks + 2 flush tanks + 1 washing machine + 1 dishwasher = 19.0 FU
- Estimated peak demand ≈ 13.1 GPM
This number guides your plumbing system design.
Step-by-Step: How to Use the Calculator
- Enter fixture counts – Input how many showers, sinks, toilets, and appliances you have.
- Click Calculate Demand – The tool instantly sums fixture units and calculates peak GPM.
- Review Results – See both the Total Fixture Units and the Estimated Peak Flow.
- Adjust as Needed – Change fixture numbers to see how demand shifts.
- Plan System Design – Use results to size pipes, pumps, or water heaters.
Example Use Case
Imagine you’re designing plumbing for a 3-bathroom house:
- 3 showers (6 FU)
- 4 sinks (4 FU)
- 3 toilets with flush tanks (7.5 FU)
- 1 washing machine (3 FU)
- 1 dishwasher (3 FU)
Total Fixture Units = 23.5 FU
Estimated Peak Flow = 14.6 GPM
With this information, you know your main water supply and distribution lines should handle at least 15 gallons per minute during peak demand.
Important Notes & Disclaimer
- The calculator uses a simplified version of Hunter’s Curve.
- Results are approximations and should be verified against local plumbing codes.
- Commercial and multi-family projects may need more advanced methods.
- Always consult a licensed plumber or engineer for final designs.
