The maximum gpm rating your water heater needs depends on how many fixtures and appliances will be drawing hot water at the same time. Input the gpm of your fixtures and then check off the fixtures and appliances that will be in use simultaneously.

How to calculate gpm


You now need to calculate your temperature rise.

The ratings of tankless water heaters depend on how many degrees they must heat the groundwater to a desired temperature (usually 120º F). This can be done by subtracting your groundwater temperature (assume 50º F if you don't know) from your desired water temperature.

For example, if your temperature rise is 70º F and your maximum gpm is 6.8, you would then order a tankless heater rated to produce at least 6.8 gpm at a 70º F rise.

To reduce flow rates, install low-flow fixtures.

• * Experts stress that a detailed consultation with an experienced plumber about your water-use habits is the best way to make the most accurate estimation of your heater size. These calculations are intended to provide a basis for such consultation by illustrating how particular incidents of water usage affect your total needs.
• ** Except for clothes washers and dishwashers, the events above will consist of 80 percent hot water. Multiply your gpm figure by 0.80 to get the gpm of hot water needed for each event.
  
  
• ¹Typical shower and bath faucets flow at 2.5 gpm (gallons per minute), which is typically a mix of hot and cold water.
• ²The typical Consumer Reports-tested clothes washers set for hot-water wash and rinse cycles could draw up to 3.3 gpm of hot water during use.
• ³The typical kitchen and bathroom faucet will flow up to at 2.2 gpm, which is typically a mix of hot and cold water.
• ⁴The typical Consumer Reports-tested dishwasher draws 1.3 gpm of hot water during use.