What Size Tankless Water Heater Do I Need?

Tony Carrick
Written by Tony Carrick
Updated May 18, 2023
Mom washes hands with toddler in the bathroom
Photo: kate_sept2004 / E+ / Getty Images


  • The flow rate of your home’s water fixtures is one factor in sizing your water heater

  • Temperature rise is also crucial when sizing a tankless water heater

  • Most tankless water heaters have a capacity of 5 to 10 GPM

  • You can use multiple water heaters throughout your home to improve efficiency

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If you’re looking for a way to reduce your utility bills and save on energy costs, you might want to consider a tankless water heater. Tankless water heaters help reduce energy consumption because they only heat the amount of water that you need at any given time. But to make your tankless water heater as efficient as possible, you should make sure that it’s the right size for your house. What size tankless water heater you need depends on the number of fixtures in your home and the temperature of the water coming into it. 

Factors That Determine Tankless Water Heater Size

Sizing the hot water heater system properly can help you to make your heaters as efficient as possible. Depending on the size of your home, the number of heaters, and your household size, you may need to install a tankless water heater with a capacity of 2 to 12 gallons per minute (GPM). Most tankless hot water heaters have a capacity between 5 and 10 GPM. The keys to determining the best tankless water heater size for you are flow rate and temperature rise. 

Flow Rate

Flow rate refers to the amount of water the tankless unit can deliver at one time. To ensure that you don’t run out of hot water at an inconvenient time, your unit's flow rate should match how much water your household needs. For instance, if you're installing a tankless water heater in the kitchen, you should have a flow rate high enough to heat water for multiple appliances and water sources simultaneously, like running the dishwasher and washing your hands at the same time. 

Calculate Your Flow Rate

The first step to determining what size tankless water heater you need is calculating the maximum flow rate of your plumbing. To do this, add together the flow rate of all the fixtures that will connect to the tankless water heater. 

For example, if you have a shower that uses 2.1 GPM, a kitchen faucet with a flow rate of 2.2 GPM, a washing machine that uses about 1.5 GPM, and a dishwasher that uses 1 GPM connected to the tankless water heater, the total maximum flow rate would be 6.8 GPM. Remember, you’re only including fixtures that use hot water, so don’t add dishwashers that heat their own water or toilets that don’t use hot water.

Temperature Rise

Temperature rise refers to how hot you need your tankless water heater to heat the water. The temperature rise of the tankless units installed in your home may differ depending on your needs. While the water temperature coming into your home will generally be around 50 degrees Fahrenheit, the desired temperature of the water leaving your sinks, appliances, and shower may be different from one another.

Determine Your Temperature Rise

Start by determining your target temperature, which is the water temperature you want the water in your home to reach. Most plumbing pros recommend setting the unit to 120 degrees, which is enough heat to shower comfortably, wash dishes, and do laundry. 

Next, determine how much your hot water heater will have to increase the temperature of the incoming water to reach your target temperature. You can figure this out by looking at a U.S. groundwater temperature chart, which tells you the temperature of the water coming into your home based on where you live. Once you have that number, subtract the groundwater temperature from your target temperature to determine the minimum rise for your tankless water heater. 

For example, the chart shows that your groundwater is 72 degrees if you live in Orlando, Florida. In that case, your water heater needs to warm the water by 48 degrees to reach a target temperature of 120 degrees. If you live in Boston, where groundwater is a chilly 47 degrees, your water heater will need to raise the heat by 73 degrees to reach that target temperature. 

Remember that the harder the tankless water heater has to work to reach your target temperature, the lower its flow rate.

Multiple Units

For optimal energy efficiency, it’s a good idea to employ multiple water heaters in different locations throughout your home. With multiple heaters, each heater will be closer to the faucet or appliance, so the hot water has less distance to travel. While installing multiple tankless hot water heaters may be more expensive up-front, it can help you save on energy over time. Plus, if you install multiple units, you’ll typically need smaller units than you would if you were only installing one.

How Many People Are in Your Household?

If you’re having trouble determining the flow rate of the hot water taps in your home, you can use the size of your household to come up with a ballpark figure of the flow rate you need out of your tankless hot water heater. Small households of two people need a tankless heater with a flow rate of 6 to 8 GPM, while a larger household of four people requires a flow rate of about 8 to 10 GPM. Remember to factor in temperature rise when deciding how big of a tankless water heater you need for your household.

Choosing the Right Tankless Water Heater Size

person stepping into bath with water running
Photo: Maridav / Adobe Stock

Now that you know your home’s flow rate and the temperature of your water, you can use those numbers to size your tankless water heater. Luckily, tankless water heater manufacturers make it easy for you to select the right unit by providing you with the flow rate you can expect to get based on your temperature rise. 

For example, the Rinnai Super Efficiency Plus natural gas tankless water heater lists a flow rate of 8 GPM for 35 degrees of temperature rise, 6.8 GPM at 45 degrees of temperature rise and 4.7 GPM at 65 degrees of temperature rise in its specifications. 

If you’re struggling with these calculations, a local water heater installer can help you estimate the water needs based on where you live and the number of water fixtures you have. Plus, if you choose incorrectly, you run the risk of running out of hot water at an inopportune time. It’s a good idea to work with a water heater pro to determine what size tankless hot water heater is best for your needs.

Margaret Wack contributed to this piece.

Frequently Asked Questions

Ordinary water heaters heat water in advance and store it in a tank so that it’s ready whenever you need it. Tankless water heaters don’t hold water in a tank—instead, they heat water quickly and efficiently whenever you need to use hot water. This makes tankless water heaters very efficient. Tankless systems also tend to last longer than any other types of water heaters.

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