Need More Help? Use the Form Below or Call 1-888-559-6352

The email address and phone number are good ways to reach me.

How big of tankless water heater do I need?

Nov 09, 2021

Black sink in a home kitchen that is running with hot steam coming from the running water.


When the time comes to choose a new tankless water heater for your home, size is actually one of the main things you need to consider. A heater that’s too small won’t handle your hot water demand. It will run out before all your showers are complete and limit your appliances that are reliant on hot water. One that’s too large, will offer more than you need.


Read on to the steps you can take to figure out what size tankless gas water heater you need.

How Big of a Tankless Water Heater Do I Need?

To determine what size tankless gas water heater you need, you have to figure out the flow rate and temperature rise needed by your home water use. Never attempt to save on your budget by selecting an undersized tankless water heater. This will only end up causing you problems down the road.


Here are some rough estimates to help you figure out how big of a tankless water heater you need. Approximate hot water heater size and required gallons per minute (GPM) for a:


  • Family of 2: 6-8 GPM, 10-18 kW
  • Family of 3: 7-9 GPM, 15-23 kW
  • Family of 4: 8-10 GPM, 20-28 kW
  • Family of 5: 9-11 GPM, 25-34 kW

Steps for Determining Ideal Water Heater Size

1. Number of Devices and Flow Rates

Count the number of devices you intend to run off your hot water heater, then total up their flow rates in gallons per minute. Make sure you calculate the maximum number of devices you’ll use, then the final flow rate is the demand necessary. Include showers, dishwashers, and even hot water needed while running the garbage disposal. If you want to reduce your flow rate, you can install low-flow water fixtures to save water and lower the toll on your water heater.

2. Temperature Rise

To calculate your temperature rise, you must subtract the incoming water temperature (what’s coming into your home from your water source—either a well or the city) from your desired output temperatures. For example, if the incoming water temperature is 50°F but your desired water temperature is 100°F, then the temperature rise is 50°F. That’s what you’ll need from your water heater.

3. Calculate Sizing

In this process, you must calculate how much water you’ll need at a given time. On average, shower temperatures are between 100-105°F, using about 2.6 gallons. For a single household, that may mean needing a tankless water heater that can produce 55°F temperature rise at 5.2 gallons per minute. Homes with more residents will need a larger hot water heater.

Tankless Water Heater Professionals

If you’re having difficulty figuring out what size tankless gas water heater you need for your home, consult the experts at CroppMetcalfe. We can handle the job from the calculations stage all the way through installation.



What Our Customers Say...

"I've been a CroppMetcalfe customer for 9 years and you have always provided outstanding service. I just had my semi-annual maintenance done, and I wanted to thank George for the truly outstanding service, professionalism, and excellent advice. He is definitely one of the best 5-Star Technicians I've had! Thank you CroppMetcalfe!"

Bill H