The Rheem RTEX Tankless Electric Calculator is a convenient tool to help you determine the right size Rheem RTEX tankless electric water heater for your needs. This calculator uses two main factors to recommend a model: gallons per minute (GPM) and temperature rise.

Calculate Now:

1. 1. Determine GPM: To calculate your GPM, add up the flow rates of all fixtures you plan to use simultaneously. For example, if you want to run a shower and a faucet at the same time, you’d add their flow rates together. You can refer to the appliance flow rate chart for guidance on the average flow rates of various fixtures.

Average GPM Usage by Application

Application	Average GPM
Shower	2.0 - 3.0
Bathroom Faucet	0.5 - 1.5
Kitchen Faucet	1.0 - 2.5
Dishwasher	1.5 - 2.0
Clothes Washer	2.5 - 3.0
Bathtub	4.0 - 6.0



2. 2. Calculate Temperature Rise: This is the difference between the temperature you set on the water heater and the temperature of your incoming groundwater. To find the groundwater temperature, consult the groundwater chart, which provides average temperatures by region.

Average Inlet Ground Water Temperatures of the United States


77°F
72°F
67°F
62°F
57°F
52°F
47°F
42°F
37°F