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RED Calc Free Help Water Flow Rate
Water Flow Rate Tool User Guide

Water Flow Rate
Red Calc Tool User Guide

What this tool can do for you

This is a "solve-all" tool that relates the volume of water drawn, the duration of the water draw, and the flow rate. The primary expected use case is determining flow rate using a test container with known volume and a stop watch. The tool was developed as an aid in using the DHW Volume per Use tool when flow rate is used and must be determined, but may also be used for more general purposes.

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Source: US EPA

Calculated values

  • Draw volume - the volume of water accumulated by the end of the time interval.
  • Draw duration - the time interval of the flow.
  • Flow rate - the water flow rate.


  • Clicking the label for any input or result will cause a popup help box to appear. This help box includes the allowed and normal values (for inputs). Read more.
  • This is a "solve-all" tool. Select the radio button to the left of the label for which you wish to solve. This label will turn to blue and the input box will disappear.

Inputs and field measurements

  • Draw volume - the measured volume of water accumulated by the end of the time interval.
  • Draw duration - the measured time interval of the flow.
  • Flow rate - the measured water flow rate.

Related tools

  • DHW Systems Comparison: This tool allows you to calculate the annual cost of heating domestic hot water. In addition, you may compare the annual costs of up to three systems to a base system. Four water heating fuel types are available; electricity, natural gas, LPG, and #2 oil. You can use this tool for any system for which you have an energy factor (EF), including electric resistance and heat pump systems, and various types of oil- and gas-fired systems.
  • DHW Average Daily Usage: This tool calculates average daily hot water use based on the amount of hot water used by various appliances and use incidents. Default usage values are included.
  • DHW First Hour Rating: This tool helps you determine the First Hour Rating for the purpose of sizing a storage water heater. This method is based on guidance from the Air-Conditioning, Heating and Refrigeration Institute (AHRI), the industry standard for sizing storage water heaters.
  • DHW Instantaneous Sizing: This "solve-all" tool is used for sizing an instantaneous (tankless) hot water heater. It can also be used to determine the maximum hot water flow rate for a given system and temperature rise.
  • DHW Volume per Use: This tool determines the volume of water used given an amount of mixed (hot + cold) water used and the temperatures of the hot, cold, and mixed water. The amount of mixed water can be specified by volume directly, or from a flow rate and duration.

Related external resources

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