What size tankless water heater do I need for my home

Before you dive into our list of the best tankless water heaters on the market, you need to know EXACTLY what size tankless water heater you would need for your home.

And here’s why:

If you choose a too small tankless water heater, you may end up taking cold showers when somebody else in the house is using hot water. But if you buy a too big tankless water heater, it won’t come cheap. You’ll overpay for what you probably will never use.

So you may wonder, how do I know how big of a tankless water heater would you need? The answer is in this article! Just keep reading.

In this article, I’ll explain how to choose the right size of a tankless water heater in 3 easy steps so you don’t overpay for excessive performance.

Here’s how you do it:

1

Calculate the combined flow rate (in GPM)

How many gallons per minute (GPM) do I need for a tankless water heater?
Go to step 1

2

Determine the temperature rise in your region

Use our groundwater temperature map to determine the temperature rise in your region
Go to step 2

3

Put it all together

Pick the brand and the model that meets all the requirements
Go to step 3

Before getting into the details, consider checking out this water heater as one of the best “fit-for-all” models.

Rinnai V65iN Gas Indoor Tankless Water Heater Features and Specifications:
  • UEF (Efficiency Rating): 0.8
  • Product dimensions: 13.9 x 8.8 x 23.6 inches
  • Flow rate: 6.5 GPM
  • Activation Flow Rate: 0.26 GPM
  • Energy Star® certified
Read Verified Customers Reviews

Or this electric alternative:

EcoSmart ECO 27 Electric Tankless Water Heater:
  • Max. Flow Rate: 5 GPM
  • Activation Flow Rate: 0.25 GPM
  • Dimensions: 17″H x 17″W x3.75″D
  • Service Requirements: 113 amps (3 double pole x 40 amp breakers)
  • 99.8 Energy efficient
Read Verified Customers Reviews

Step 1: Determine the combined flow rate (in GPM).

With traditional storage water heaters, you could calculate the size of it based on the number of family members. Whereas with tankless water heaters it’s more important to know how many fixtures it’s required to serve simultaneously.

It doesn’t matter how much people are going to take a shower if they use it one by one. With tankless water heaters, you’ll have an endless supply of hot water. However, if several people are going to use two or more showers at the same time, an undersized tankless water heater may not be able to handle the demand.

That’s why you’ll need to calculate how many gallons of hot water per minute it should heat to supply the peak demand.

Although it may sound too “professional” it’s very easy to do:

  1. Simply count the maximum number of fixtures and appliances that you want to run simultaneously. Make a list of them.
  2. Now find out their flow rates. That is how much hot water they use per minute.
  3. Add up all numbers to get the total amount of hot water per minute.

If you don’t know the actual flow rates for your shower or faucets, you can use our table of the average flow rates for most types of fixtures:

FixtureAverage flow rate (in GPM)
Lavatory faucet0.5-1
Kitchen faucet1.5-2.2
Showerhead2-2.5
Tub faucets4-7
Washing machine2
Dishwasher1.5

Or you could apply a scientific method and measure the flow rate by yourself. Take a 1-gallon bucket, open the fixture (showerhead, faucet, bathroom taps, etc.) and measure how long it will take to fill the bucket. Then apply this formula:

Flow rate = 60 / seconds a fixture took to fill the bucket

If you don’t want to wast several gallons while doing the test measure, fill in a quarter of the bucket and apply this formula:

Flow rate = 15 / seconds a fixture took to fill a quarter of the bucket

Now let’s see how you determine the total required flow rate by looking at the following example:

Let’s say, you have 2 bathrooms. And you want to be able to use both of them simultaneously. Plus a kitchen sink and a washing machine will be used too.

If we take the table above as a reference, we’re going to end up with this equation:

(2 showerheads * 2.5 GPM) + 1.5 GPM for a kitchen sink + 2 GPM for a washing machine = 8.5 GPM for the whole house.

So if you want to use all the fixtures and appliances at the same time, your tankless unit should be able to provide 8+ GPM of hot water.

Keep in mind, that you should only count the fixtures and appliances that you want to run simultaneously. Don’t count them just for the fact that you have them.

You could have noticed that we took the maximum possible flow rate number from the chart which is rarely true in real life. But it’s always better to overestimate a little than underestimate. Just to be safe.

Now you can decide whether you want to buy one big whole house tankless water heater (like Rinnai RUC98iN) or use a smaller one (like this) and buy additional smaller units for bathroom, shower or appliances.

The decision may depend on one factor: the length of the pipes. The further your fixtures are from the tankless water heater, the longer you’ll have to wait before the hot water gets to the fixture.

A small point of use tankless water heaters solves this problem by boosting up the water temperature right where they’re used.

Use low flow showerheads to reduce the total flow rate and save money!

If you use low flow showerheads, your hot water demand is going to be lower and you won’t need a huge and expensive tankless water heater. Modern low flow showerheads provide the same water pressure and give you the same shower experience as higher flow models.

But there’s also a downside in these showerheads. If the flow rate is too low, the tankless water heater may not be activated, since they have different activation flow rates. That’s something to keep in mind when choosing one for yourself. The lower the activation flow rate, the better.

Step 2: Determine the temperature rise in your region

Now that you calculated how much water you need to heat per minute, you should determine how hot the water should be. In other words, you have to determine the temperature rise.

Temperature rise is the difference between the temperature of the incoming cold water and the outgoing hot water

It’s very important to know since the ACTUAL flow rate of a specific tankless water heater largely depends on the temperature rise. You can’t get an 11 GPM flow rate with Rinnai RUR199iN in Boston in winter. But you surely can in Miami.

So just in case you didn’t know this, the flow rate that manufacturers put in the description of tankless water heaters is the maximum possible flow rate for that particular model. The number is only true for warmer climates with a very small temperature rise (where the groundwater is warmer). So if you live in the northern states where the groundwater is colder, you’re going to get a couple of gallons of hot water per minute less with the same tankless water heater.

The groundwater temp varies in different states throughout the USA. Using our groundwater temperature map you can easily determine the temperature rise. Just hover over the map to see the groundwater temperature for your region.

Desired Outlet Temperature (F)
Ground Water Temperature (F)
Required Temperature Rise (F)
enter the groundwater

If you’re still confused, do this:

  1. Find your state on the map.
  2. Determine the groundwater temperature by the color. Or just hover over your region.
  3. Now subtract this number from the desired temperature of outgoing hot water (it’s usually around 110-115°F) and you’ll get your temperature rise.

For example, if you live somewhere in Kansas, your average incoming cold water temperature is about 52°F. So to get 110°F hot water in your shower you’ll have to heat the water by 110 – 52 = 58°F. So the temperature rise for your region is 58°F.

Now that you’ve calculated the temperature rise for your region, you’re ready to go to the next step:

Step 2.5: Consider peak time usage

This section should have been a part of step 1 as it’s about the flow rate. It’s just a quick reminder. So if you haven’t considered the peak time usage on step 1, please go back and recalculate the number.

So what’s it all about? The main point is that when you’re calculating the flow rate when trying to size your tankless water heater, you should consider the peak load and not the average demand. I hope I made it clear.

You don’t want to end up getting a cold shower in the busy hours because your tankless couldn’t keep up. You should be prepared for the worst. It wouldn’t hurt much to overestimate your average hot water demand a little bit.

Step 3: Put it all together

Ok, at this point you know your hot water demand and the required temperature rise.

Your next move is to go and choose a tankless water heater that meets the requirements.

Let’s say you want to find the best tankless water heater to heat the water for a shower, a kitchen sink, and a washing machine all at the same time. And you live in South Carolina.

How big of a tankless water heater do I need?

The answer is simple: you’ll need a tankless water heater that can provide a temperature rise of 50°F for about 6 gallons of water per minute. The best gas option, in this case, would probably be Rinnai V-series.

Remember, the maximum flow rate that manufacturers mention in the description is not what you’ll get at your place. The lower the incoming water temperature, the lower flow rate a tankless unit would be able to provide.

Keep in mind that it’s always better to oversize the unit than buy the one that can’t supply enough hot water.

You can check the actual flow rate for your temperature rise in the specifications of a model or in the review section on our main page.

Also, feel free to ask anything in the comments. Our experts will try to help you with sizing if you provide maximum information.

Sizing Guide For Tankless Water Heaters: Equivalents Table

How big is your familyHow big was your old water heaterCapacity required (in gallons per minute – GPM)
1-2 people30, 40, 50 gallons3, 4, 5 GPM
2-3 people40, 50+ gallons4, 5, 6 GPM
3-4 people50, 75, 80 gallons6, 7, 8 GPM
5+ people75, 80+ gallons8, 9, 10, 11 GPM
Rough estimation of how big of a tankless water heater would you need for your situation

Please, keep in mind that this is just a rough estimation. There’s more to the sizing process than just a table. You would need to calculate the demand and the temperature rise in your state to properly size a tankless water heater.

Tankless water heater sizing calculator

We thought it would be easier for you to calculate the flow rate and the temperature rise if you could have a simple calculator. So here’s how you can determine GPM for a tankless water heater. Just select the number of fixtures and appliances that you are going to use simultaneously and we’ll calculate total GPM (based on average numbers).

Showers
Bathroom faucets
Kitchen faucets
Dishwasher
Washing machine


What size tankless water heater do I need to replace a 50 gallon water heater?

If you're looking to replace your old tank 50 gallon water heater, you would need a good tankless equivalent for it. So here it is:

Rinnai RL Series HE+ Tankless Hot Water Heater: Indoor Installation:
  • Fuel Type: Natural Gas or/and Liquid Propane
  • UEF (Efficiency Rating): 0.81
  • Product dimensions: 14 x 9.3 x 23 inches
  • Flow rate: 7.5 GPM
  • Energy Star certified (may be available for rebates)
Read Verified Customers Reviews

But before you go and buy it, you need to learn something:

You can't size a tankless water heater by the standards of the storage water heater. They don't work that way. With a tankless, you can have endless hot water, so it doesn't matter how big your storage was.

If you want to upgrade your water heater size from 40- or 50-gallons storage water heater or replace it, you won't necessarily need a big tankless water heater. The right size is a tankless water heater that can heat enough gallons of water to a specific temperature per minute to keep up with the demand. It's the flow rate and the temperature rise that matter.

Now that may sound confusing or/and complicated. Let me put it simple: the size totally depends on the flow rate and the temperature rise.

Count all the hot water fixtures in your house that you want to run simultaneously. 

That's pretty easy to do. Remember that it doesn't matter how many bathrooms your house has. What matters is how many of them you're going to use at the same time.

Add up their flow rates. 

The flow rate is the amount of water that goes through the fixture per minute in gallons. If you don't know the exact numbers, run a test.

  1. Take a 1-gallon bucket. 
  2. Measure how fast a faucet, showerhead, bathtub, or any other fixture is going to fill a quarter of the bucket.
  3. Count the flow rate by the formula:
FLOW RATE = 15 / Seconds to fill a quarter of the bucket

Determine the temperature rise for your region.

To determine the temperature rise subtract your groundwater temperature from the temperature you want to have in the shower.

For details refer to our guide here.

Pick the right model that meets the requirements

It's not an easy thing to do, I must say. With so many trusted brands and models on the market, you can easily get confused. 

Knowing what requirements it should meet helps to narrow down the list to several brands and models for different power sources and various budgets, of course.

We've tried and come up with a list of the best models for most use cases. But if none fits your situation, let us know in the comments, please.

A sizing example

For example, you live in Texas. You want to be able to run two showers plus let a washing machine do its job. And all of this should be running at the same time.

Now, let's do the math. 

The flow rate of an average showerhead is 2-2.5 GPM (gallons per minute). Two of them would be 4-5 GPM. A washing machine consumes about 2 GPM on average.

The groundwater temp in your region is between 60°F and 67°F. That results in a temperature rise of about 110°F - 60°F = 50 °F.

Now go to our list of the best tankless water heaters and see which one fits your needs best. It should be able to provide 6 to 7.5 gallons of hot water per minute at the temperature rise of 50°F.

What size tankless water heater do I need to replace an 80 gallon water heater?

Again, if you want to choose the right tankless water heater to replace an 80 gallon tank water heater, you would need to determine the GPM and temperature rise first.

If you have a large house and you want to buy a big whole house tankless water heater, go for Rinnai RUR199. It's probably the most powerful tankless water heater on the market. And it's able to supply a large 3 bathroom house even if you are planning to use them all at the same time plus a washing machine.

If this model is too expensive for you, Rinnai RUR160 is an equivalent for 80 gallon tank water heater (there are both natural gas and propane options). It's cheaper but still very powerful.

And if that unit also doesn't fit into your budget, give a look at Takagi model that's capable of providing up to 10 gallons of hot water per minute. It's much cheaper but it also works worse than Rinnai in cold climates.

What Size Tankless Water Heater Do I Need For A Family Of 2, 3, 4, 5

As we've mentioned earlier, with tankless water heaters it doesn't really matter how many people are there in your house. What matters more is how many of them are going to use hot water at the same time.

Let's talk about this in detail.

What's the difference?

First of all, you need to understand the difference in sizing methods between tank water heaters and tankless water heaters.

With old storage water heaters, your showers and hot water usage were limited to the amount of the preheated water in the tank.

Tankless water heaters provide endless hot water on demand. And I literally mean it. Once the hot water reaches the fixture, you and your family don't have to worry that it's ever going to run out. It's heated on the go. So as long as you have electricity on and the groundwater keeps flowing in, your hot water is going to be there.

But as great as it sounds, there are some limits to tankless water heaters. Their performance totally depends on the amount of heat they can provide to heat the water on the go. In other words, the more BTUs, the better.

But two similar tankless water heater models may have the same BTU, although one of them may be more efficient than the other.

That's why it's more convenient for a consumer to look at the flow rate at the given temperature rise. It shows how much hot water it can supply per minute.

And the temperature rise is defined by the difference between the output water temp and groundwater temp.

Sounds way too complicated, isn't it? Let me make it easier for you.

How big of a tankless water heater I need?

We've already covered most of the necessary info earlier. If you have time, we recommend reading the whole guide.

If you don't, I'll try to explain everything really short.

So, to determine the right size of a tankless water heater for a family of 5 you'll need to calculate the peak flow rate in your house. And find a tankless model that would be able to handle the load at the needed temperature rise.

Let me show how to do it on an example:

Say, you live in California or Texas. With a big family of yours, you'll probably want to be able to use two or three showers simultaneously. Also, you'll want to leave some additional capacity for unpredictable situations. Like using a washing machine or a kitchen sink.

The average flow rate for showerheads is 2 GPM. The total flow rate is:

(2-3 showerheads * 2 GPM) + 2 GPM for washing machine or a faucet = 6-8 GPM

And the groundwater temp at your region is somewhere between 57°F and 67°F. So the temperature rise will be 110-57/67= 53°F to 43°F.

Now let's get back to the original question about sizing a tankless water heater. A family of 5 people will require at least a 6 GPM tankless water heater. Here are our recommendations for the best tankless water heaters:

In my opinion, the best gas option would be this:

Rinnai RUC80iN Ultra Series Natural Gas Tankless Water Heater:
  • UEF (Efficiency Rating): 0.96
  • Product dimensions: 10 x 18.5 x 26 inches
  • Activation Flow rate: 0.26 GPM
  • Max Flow rate: up to 7.7 GPM at 43°F temp rise
  • Energy Star certified (may be available for rebates)
Read Verified Customers Reviews

If you're looking for electric tankless water heaters options, consider this one:

Ecosmart ECO 36 36kw 240V Electric Tankless Water Heater:
  • Max. Flow Rate: up to 6.5 GPM at the 43°F temp rise
  • Activation Flow Rate: 0.25 GPM
  • Dimensions: 17 x 19 x 3.8 inches
  • Service Requirements: 150 AMPS DEDICATED! (4 x 40A breakers)
  • 99.8 Energy efficient
Read Verified Customers Reviews

But REMEMBER: electric tankless water heaters almost always will be more expensive in the long run. They usually require serious electric service upgrades and cost more than gas tankless water heaters. So, if the gas or liquid propane are available options, I'd go for them.

What Size Tankless Water Heater Do I Need For A 2, 3, 4 Bathroom House

You just want a quick answer, don't you? If you're in a hurry, mind checking out the start of this page. I've put my product recommendations there.

For the rest of you, who really want to understand what size tankless water heater would be the best choice for their house, - keep reading.

In this article, I explain in simple words (and images) how to find the best tankless water heater for your needs and not overpay.

How to read the specifications

First of all, you need to know what are the most important specifications of tankless water heaters. What you should look at when choosing one.

There's only one most important thing. It's the flow rate at given temperature rise.

The greater the flow rate, the more hot water a tankless can provide per minute.

To find out what's your preferred flow rate, you need to determine the peak load for your house and family and find out the temperature rise in your region. Then just choose a tankless accordingly.

An undersized tankless water heater would not be able to provide enough hot water for your needs. An oversized unit is going to cost you a fortune.

We've covered the sizing process in our guide earlier. Mind checking it out.

In short, you determine the peak demand by adding up the flow rates of all the fixtures you're planning to use at the same time.

Most likely, you have no clue how to do it. If that's the case, use the calculator below. Enter the number of fixtures and you'll get the total flow rate. Feel free to ask any questions in the comments section, if you still don't know what to do.

Fixture TypeQuantity
Showerheads
Showerheads
Bathroom Faucets
Bathroom Faucets
Hand Washing Sink
Hand Washing Sink
Bathtub Faucet
Bathtub Faucets
Kitchen Faucet
Kitchen Faucets
Washing Machine
Washing Machine
Dishwasher
Dishwasher
Total Flow Rate
Total Flow Rate
0 GPM

Now that you know your demand, you need to determine the temperature rise.

The temperature rise is the difference between your set output water temperature and inlet water temperature. Subtract the groundwater temp from the preferred shower temp and you'll get your temperature rise.

The shower temperature is usually about 110-115°F. You can find the groundwater temperature in your region on our map that we presented earlier. Alternatively, just check out the numbers.

Desired Outlet Temperature (F)
Ground Water Temperature (F)
Required Temperature Rise (F)
enter the groundwater

Great! You know the two most important things you'll need to choose the best tankless water heater for a 2 (or 3, or 4) bathroom house.

Now just go to our main rating and find a model that would fit your requirements.

Keep in mind, that the flow rate you see in the table is the maximum load a tankless can handle in perfect conditions. Use the table for navigation, but make sure that the model you choose can supply enough hot water at your temperature rise.

Example. Let's say you have a one bathroom house, where you'd want to run one shower and one other appliance at a time. So, if you have a one bathroom house, you will need a tankless water heater that can heat at least 3 gallons of hot water per minute. We recommend Rinnai or Ecosmart.

Best Tankless Water Heater For 2 Bathroom House

Again, it all depends on your requirements. My recommendation may not be the right choice in your situation.

But just to give you a starting point for further research, if you you have a 2 bathroom house (or 2 bedroom house) and you want to run 2 showers simultaneously, you will need a tankless water heater that can provide at least 5 gallons of hot water per minute. I recommend Rinnai V65iN:

Rinnai V65iN Indoor Gas Tankless Water Heater

Rinnai V65iN Gas Indoor Tankless Water Heater Features and Specifications:
  • UEF (Efficiency Rating): 0.8
  • Product dimensions: 13.9 x 8.8 x 23.6 inches
  • Flow rate: 6.5 GPM
  • Activation Flow Rate: 0.26 GPM
  • Energy Star® certified
Read Verified Customers Reviews

Rinnai V65iN can provide astonishing for its price range of 6.5 GPM. It is a perfect tankless water heater for 2 bathroom house. It would best fit for middle-sized homes in states with a warm or moderate climate.

For homes in the northern states, I'd recommend a more powerful model from this series (click the link to check it out).

If the natural gas is not an option in your area and you need an electric unit (although I always prefer gas/lp over electric), I would recommend this one:

EcoSmart ECO 27 Electric Tankless Water Heater

EcoSmart ECO 27 Electric Tankless Water Heater:
  • Max. Flow Rate: 5 GPM
  • Activation Flow Rate: 0.25 GPM
  • Dimensions: 17"H x 17"W x3.75"D
  • Service Requirements: 113 amps (3 double pole x 40 amp breakers)
  • 99.8 Energy efficient
Read Verified Customers Reviews

Best Tankless Water Heater for 3 Bathrooms

In a 3 bathroom house (3 bedroom house?), you'd typically want to use at least 2 showers simultaneously. But let's not forget about other appliances like washing machine or dishwasher which may require up to 2 gallons per minute of hot water each.

For this level of demand, you'd want something really powerful. Especially at the times when you need all 3 baths or showers. The peak demand is probably between 7 GPM and 9 GPM.

Considering the great performance and the maximum flow rate of about 9.4 GPM and relatively low price, Rinnai RL94iN - a natural gas tankless water heater is the best choice for 3 bathrooms and additional house appliances.

Rinnai RL Series HE+ Tankless Hot Water Heater

Rinnai RL Series HE+ Tankless Hot Water Heater:
  • Fuel Type: Natural Gas or/and Liquid Propane
  • UEF (Efficiency Rating): 0.81
  • Product dimensions: 14 x 10 x 23 inches
  • Max. Flow rate: 9.4 GPM
  • Energy Star certified (may be available for rebates)
Read Verified Customers Reviews

Don't be fooled by the max. flow rate number. Don't forget that it's the maximum possible score for very warm climates. In real life, it would handle just about what you need. That is between 7GPM and 9GPM, depending on the temperature rise.

It's also not the most efficient model. But considering the price, it's probably the best you can get for your needs. If you're in doubt, check out our reviews of the best tankless water heaters and choose what you think would fit best for you.

Best Tankless Water Heater for 4 Bathrooms

It's not very easy to choose tankless water heaters for large homes. Especially if you live in the north of the USA. The climate is cold, the groundwater temperature is low. The right choice must have a great performance score or it would not be able to handle the heavy load of 4 bathrooms.

In my opinion, your best option here is Rinnai RUR199iN. It's very efficient and one of the most powerful tankless water heaters on the market. It's not cheap though.

Please, recheck twice or even three times if it meets your requirements. Otherwise, you may spend a couple of thousands of bucks for lukewarm showers, since the unit may not be able to heat enough water and handle all the load.

If you live in the south or you don't plan on using all of your four bathrooms at the same time (but still be able to use them whenever you need), Rinnai 199iN would be a 100% fit.

🥇 Rinnai RUR199iN Tankless Water Heater for 4 Bathroom House

Rinnai RUR199iN Features and Specifications:
  • Fuel Type: Natural Gas
  • UEF (Efficiency Rating): 0.93
  • Product dimensions: 22.1 x 14.8 x 33.9 inches
  • Max flow rate: 11 GPM
  • Built-in recirculation pump
Read Verified Customers Reviews

5 thoughts on “What size tankless water heater do I need for my home”

  1. Hi, Rex!

    Thanks for the guide! It’s really helpfull, but I still can’t figure out what’s the right size for me.
    We live in Wyoming. The winters are freezing cold here. And we have two bathrooms. We’re a family of 4.
    It seems like no tankless water heater can handle the load at out temperature rise.

    What would you recommend for our situation?

  2. (1) how many kw would i need for a 2 person family 1650 square foot house, the unit will set in about the center of the house
    (2) is there a limit to the amount of water pressure it takes to operate the tankless system

    • Hi Kenneth!
      As I’ve already said, it all depends on where do you live and what temp groundwater is in your region.

      If I’m right and you live in West Virginia, you’ll probably be fine with any 27kW+ electric tankless water heater like this one. It can handle one shower and one faucet or a piece of appliance working simultaneously at 59 degrees temp rise (which is the case for WV).

      But please, be aware that you may need a service upgrade.
      If there’s such an option, I’d recommend taking a look at natural gas water heaters.

      As for your second question, yes. There are limits to minimum required and maximum allowed operating pressure. For Ecosmart 27 that I recommended its 25 PSI and 150 PSI respectively. Note that it activates at 0.25 GPM which is relatively low. For Stiebel Eltron 36 kW you’d need twice as much (0.5 GPM).

  3. OK maximum G p.m. for me would be 14.5 and I live in Knoxville Tennessee and therefore the temperature rise would need to be 58°. What system do I need?

    • That’s pretty high. You might want to utilize several point of use units to enhance the performance of the main one.
      It’s also possible to install a couple of the biggest tankless water heaters and make them work in one system.

      Or you may be interested in commercial systems. They are pretty expensive though

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