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Tankless Water Heater Gpm Calculator

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A tankless water heater GPM (Gallons Per Minute) calculator is a vital tool for homeowners and installers aiming to choose the appropriate tankless water heater for their needs. It calculates the flow rate required to supply hot water to multiple fixtures simultaneously without running out.

Purpose and Functionality of the GPM Calculator

The GPM calculator for tankless water heaters helps determine the flow rate needed based on the total demand of all household fixtures that use hot water at the same time. This ensures the water heater can handle the peak demand efficiently.

Formula and Inputs for the Tankless Water Heater GPM Calculator

Inputs Needed:

  • Flow Rates of Fixtures (F): This includes the GPM for fixtures like showers, kitchen faucets, dishwashers, and washing machines.

Calculation Formula: Total GPM Required=𝐹1+𝐹2+𝐹3+…+𝐹𝑛Total GPM Required=F1​+F2​+F3​+…+Fn​ Where 𝐹1,𝐹2,𝐹3,…,𝐹𝑛F1​,F2​,F3​,…,Fn​ are the flow rates in gallons per minute for each fixture used simultaneously.

Step-by-Step Example Calculation

Scenario: Calculate the required GPM for a home with the following simultaneous fixture use:

  • Shower: 2.5 GPM
  • Kitchen Faucet: 1.5 GPM
  • Dishwasher: 1.0 GPM
  • Washing Machine: 2.0 GPM

Calculation: Total GPM=2.5+1.5+1.0+2.0=7.0 GPMTotal GPM=2.5+1.5+1.0+2.0=7.0GPM

This calculation indicates that the tankless water heater needs to provide at least 7 GPM to meet the peak demand efficiently.


  • Temperature Rise: The need to heat water from its incoming temperature to the desired temperature can affect the required GPM.
  • Simultaneous Usage: Realistically estimate which fixtures might be used simultaneously to avoid oversizing the water heater.
  • Energy Efficiency: Consider a unit that balances water needs with energy consumption to optimize operational costs.

Relevant Information Table

FixtureFlow Rate (GPM)
Kitchen Faucet1.5
Washing Machine2.0
Total GPM7.0


The tankless water heater GPM calculator is an essential tool for ensuring you select a water heater that meets your home’s hot water demands without inefficiency or wastage. By calculating the total GPM required based on actual usage scenarios, homeowners and installers can make informed decisions, leading to better satisfaction and reduced utility bills. This tool not only aids in selecting the right heater but also helps in planning for optimal energy consumption and cost-effectiveness.

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