• Home
  • |
  • Blog
  • |
  • What Size Tankless Water Heater Do I Need: Advice on How to Buy the Right Heater For Your Needs

What Size Tankless Water Heater Do I Need: Advice on How to Buy the Right Heater For Your Needs

As an Amazon Associate we earn from qualifying purchases. Learn More.

Tankless Water Heater Calculator

Use our handy Tankless Water Heater Calculator to determine which size appliance best meets your household needs.


Tankless Water Heater Sizing Calculator

Use our  Sizing Calculator to determine your: 

Flow Rate  and Temperature Rise

Where are you located?

Ground Water Temperature Map

If your old water heater met your capacity needs, then there may not be a reason to change. But before you purchase another, you may want to consider the different options available. The best time to make changes to your water heater set-up is when you're buying a new one!  

Here are some questions to ask: What types of water heaters are on the market to choose from? What size tankless water heater do I need? Do I need a large or a small water heater? How do I even calculate what my household uses or needs? Read on to learn more!

Replacing Your Water Heater

Replacing your water heater is rarely how you want to spend your time or money, and there's many things to consider beyond how you'll dispose of your old unit. 

How Do You Know When to Replace a Water Heater?

Tank-style water heaters don't last forever, and as they age they tend to have common signs that show they are nearing the end of their service life.

If your heater is between 8 to 10 years old, it's probably time to start preparing for a new one. You may be able to squeeze a few more years of use, but we recommend keeping a close lookout for leakage.

Consult with a Local Plumber

Get FREE Quotes Today

One common sign to replace your water heater is lukewarm or cold water. Although, this could be a fixable issue (such as a thermostat or heating element), it could also be an indication that your tank simply isn't large enough for your household hot water needs. When this is the case, it's time to upgrade to a larger heater.

Leaking Water Heater Tank

However, the number one reason to replace a water heater leakage. Some leaks can be repaired, but when the steel storage tank springs a leak your only option is to replace the entire water heater.

5 glasses with varies amounts of water

How to Determine Your Tank Size

If you plan to replace your tank-style water heater with another tank-style unit, you should know ​your current tank size. This way you'll be able to adjust the size accordingly when you purchase your new heater. 

A 50-gallon tank that's not meeting your household hot water demands should be replaced with a larger water heater. In addition, knowing this infomation will also help you get a more accurate quote.

Finding the Tank Size

All water heaters have a manufacturer sticker, which is typically located on the side or top of the appliance. The sticker will list, among other things, the heater's serial number, model number, and manufacture date. But it will also list the capacity of the tank.

Once you know the capacity you'll be able to decide if your old water heater met your household hot water demands. Did your showers run cold during peak periods? This is a quick and easy way to size a new tank.

Water Heater Style Type Options

Silver shower head in a blue shower

Tank-Style Water Heaters

Tank-style is the most common type of water heater. They're very versatile and come in models that can operate on electricity, natural gas, and liquid propane. Some are even designed to use solar panels.

Tank-style heaters have an insulated tank where the water is heated and stored until there's a hot water demand within the house. When a faucet is opened, hot water flows from the tank to the faucet. Cold water then fills the tank back up to capacity where it's then heated and held until another hot water faucet is opened.  

Consult with a Local Plumber

Get FREE Quotes Today

Tank capacity sizes vary from 28 gallons to over 100 gallons. If your old water heater wasn't able to keep up with your household hot water demands, there's a good chance that you either out grew your water heater, or it was incorrectly sized from the beginning.

A correctly sized water heater will provide a nice balance between delivering enough hot water and operating in an energy efficient manner. Sizing is simply determining your household's hot water needs. It sounds hard, but it's actually not. ​

Red paper cut out people on a dark background

How to Size a Tank-Style Water Heater

The simplest method is to follow this rule-of-thumb chart. Remember, this is not an exact science and you should adjust accordingly to account for your family's individual hot water needs. 

Household Members

Tank Size Needed

2 People

45 to 55 gallons

3 People

55 to 65 gallons

4 People

65 to 75 gallons

5 People

75 to 85 gallons

6 People

85 to 100 gallons

7 People

100+ gallons

Recovery written in yellow blocks on a red background

Recovery Rate

When selecting a tank-style water heater it's important to consider the rate at which the unit is capable of recovering. 

Recovery rate = Number of gallons of water heated in an hour​

The recovery rate is frequently overlooked when choosing a water heater, however, it plays a big role in the unit's performance. If your household has a high demand for hot water, you'll want to select a water heater with a high recovery rate.

Consult with a Local Plumber

Get FREE Quotes Today

First Hour Rating

Another important factor to consider is the First Hour Rating (FHR). The FHR is how many gallons of hot water the appliance can deliver in a single hour, starting with a full tank of hot water.

This rating is especially important if you have a high demand period. For example, if each morning your household uses 55-gallons of hot water between 8 and 9am, you'll want to find a water heater with a FHR of at least 57.

Water falling into two cupped hands

Tankless Water Heaters

A tankless water heater actually doesn't store water, but rather heats the water when there's a demand. This is why they are frequently called on-demand or instantaneous  water heaters. 

The cold incoming water runs through a heat exchanger and within seconds the water leaves the unit hot. Tankless water heaters are capable of delivering a seemingly  endless flow of hot water since they're not limited by the size of a storage tank.

They're also more energy-efficient than their tank-style cousins because they're not holding and heating hot water. In fact, the electric tankless models frequently achieve an efficiency score of 99%!

However, there's also less margin for error when sizing a tankless water heater. Without a storage tank to draw from, if you exceed the units capacity, the tankless unit may not be able to deliver enough hot water.

Big fish in a small bowl and a little fish in a big bowl

How to Size a Tankless Water Heater

How do you determine the right size tankless water heater for your needs? To answer this question you need to know two things: Flow rate and temperature rise.

  • Flow Rate is basically the amount of hot water the heater is capable of delivering each minute, measured in gallons per minute (GPM). 
  • Temperature Rise is the difference between the incoming cold water temperature and the outgoing hot water temperature.

Consult with a Local Plumber

Get FREE Quotes Today

To determine the flow rate you'll need to figure out how many fixtures will be used during your peak demand. Peak demand is frequently in the morning when the household is showering. 

Calculating Peak Demand


Average GPM

Bathroom Sink

1.5 GPM


2.5 - 3.0 GPM


1.5 GPM


4.0 GPM

Washing Machine

2.0 GPM

Add each fixture used during the peak demand period. As an example: If your household typically uses the shower (2.5 GPM) and washes clothes (2.0 GPM) during peak demand, your flow rate requirement will be 4.5 GPM.

When sizing a tankless unit, there are 2 factors that you should consider:

  • The temperature rise is not the same year around. In the winter, the temperature rise will be higher than in the summer.
  • The flow rate is impacted by the temperature rise. In the winter, when the temperature rise is higher, your flow rate will be lower.

Consult with a Local Plumber

Get FREE Quotes Today

The most common tankless water heaters deliver 3.5 gallons per minute, which is generally enough hot water to service 2 points simultaneously. In other words, a shower and a faucet.

Tankless units are designed to operate on electricity, natural gas, or liquid propane. Although the electric units are far more energy efficient, there are often electrical upgrades that need to take place to the home in order to provide the amount of electricity necessary. 

Check out our detailed buyer’s guide where we look at our favorite tankless systems.

Bathtub hot water faucet

Point-of-Use Water Heaters

Point-of-use units are sometimes called utility water heaters. They are small tank-style water heaters that are frequently used in shops, garages or outbuildings, and they range in size from 2.5 to 19 gallons and come in various power sizes from high wattage models down to plugin 110V heaters.

A common use for these types of water heaters is to provide hot water to a secondary bathroom that is located too far from the main water heater, and therefore is poorly serviced. If a point-of-use water heater is installed, the bathroom will have near instantaneous hot water since the heater will be so close.

Mobile Home

Mobile Home Water Heaters

Mobile homes have special requirements for water heaters. A  traditional tank-style water heater is usually not an option, although, some tankless models are rated for mobile homes.

A mobile home water heater is designed to specifically be used in mobile homes. They are also available in both gas and electric models. ​

What's the Difference?

There are three main differences between a mobile home water heater and a regular tank-style heater.

  • The main reason mobile home water heaters are more expensive is because they have interchangeable propane and gas orifices. This allows the unit to be easily converted between propane gas and natural gas.
  • Mobile home water heaters have the water inlet and outlet connections configured differently than residential heaters. A residential water heater has both the inlet and outlet connections located on the top of the tank. Where the mobile home heater has the cold water inlet on the side, and the hot water outlet on the top.
  • Although many areas require residential water heaters to use earthquake straps, a mobile home heater includes a securing strap kit automatically.
Parked RV on a cloudy day

RV Water Heaters

Recreational vehicles require a specially designed water heater to fit their limited space. There are many different types of water heaters for RV's. They range from a basic design to the sophisticated. 

RV's are frequently used for extended periods of time. Many "snowbirds" live in their RV's 6 to 8 months a year, so RV water heater's are designed to be compact and yet meet the hot water demand. Check out our article on RV Water Heaters.

Consult with a Local Plumber

Get FREE Quotes Today

Related Posts