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How Big of a Tankless Water Heater Do I Need: [Find Actual Size]

You’re buying a new water heater for your home but you don’t have any idea how big it should be.

Well, don’t worry. The answer to “how big of a tankless water heater do I need?” depends on many factors.

For example, your location, the GPM, and the number of people who live at your place will determine how big your on-demand water heater needs to be.

To help you choose the right tankless water heater, we’ve made a simple section with all the factors you need to know to size one. Let’s take a look to learn more.

Benefits Of Installing A Tankless Water Heater

Getting a tankless water heater has many benefits, including saving money, ease of use, and space.

But that’s vaguely scratching the surface. Here’s a quick overview of all the benefits:


You’ve probably seen how much money manufacturers put into advertising how efficient tankless water heaters can be.

And they’re not wrong; tankless water heaters are way more efficient than the average tank.

For example, the main benefit is that you have close to never-ending hot water at home.

Even if somebody takes a long shower, you’ll probably still have hot water by the time you get in the shower.

That limitless hot water is by far the most significant advantage you get from using one of these units.

On the other hand, conventional tank water heaters take forever to accomplish the same results.

If somebody spends too much time in the shower, by the time they get off the shower, you won’t have any hot water left.


Performance is an overall benefit. But have you ever thought about the time your tankless water heater will be by your side?

An on-demand water heater can generally last anywhere from 15 to 20 years.

You might stretch it out, but those numbers are where these units tend to face some technical issues.

To put it into perspective, a regular tank’s average lifespan is about 7-12 years.


Is a tankless water heater worth it? There are various benefits to installing a tankless water heater, and one of them is the space.

They take a lot less space than other traditional tanks.

If you’ve ever looked at the size of a standard tank, you know how big those things can be.

On the other hand, tankless water heats are relatively small, meaning you can hang them in the wall or anywhere without taking up unnecessary space.

Better Value For Money

If you use a lot of hot water, a tankless water heater will save you lots of money in the long.

While it is true that it’s a lot more expensive upfront than a standard tank, it’s worth the investment over time.

Furthermore, if you ever want to upgrade your model, the installation process will be a lot quicker.

You can take off the old one and put in the new one without spending unnecessary time, which can save you big bucks.

No Water Leaks

Any homeowner with a standard tank water heater knows how these units get corroded over time.

Now, there are plenty of reasons why your tank is getting corroded.

It could be due to a poor soldering job, two dissimilar metals contacting each other, or a water leak.

Nonetheless, this is a corning problem.

This issue can lead to leaks and flooding over time.

Luckily, tankless water heaters don’t face this as often as they do not have a tank.

So, there’s a high chance you’ll never experience flooding.


Ever read on the news how somebody died after the tank went off? This is pretty rare, but it definitely happens.

The water heater can go off if you disable the emergency pressure valve—the pressure will build-up, which will blow up the water heater.

This mere thought is enough to turn most people off from getting one at all.

The safest solution to this problem is a tankless water heater.

As the name suggests, these units do not have a tank, meaning the odds of it going off are close to zero.

Cons of Using a Tankless Water Heater

On-demand tankless water heats are pretty useful, but it’s not all flying colors. Let’s take a look to find more about these significant drawbacks:

Upfront Cost

Perhaps one of the biggest reasons on-demand water heaters are so popular is that they’ve been advertised as an affordable and straightforward way to save up money.

Let’s put it into context: the average yearly cost for a tankless water heater is about $225, whereas a standard tank can cost $300 a year.

If we do the math for both water heats, we would have to pay $18.75 a month for the tankless water heater.

On the other hand, the standard tank costs about $25 a month. So, you’d be saving $6,25 a month.

Overall, that does not sound like a life-changing reason to switch from a traditional tank to a tankless water heater.

They are super expensive, and the installation process is quite tricky.

Maintenance Cost

Another con of getting a tankless water heater is that they are relatively expensive to maintain.

Unfortunately, there’s this myth about tankless water heaters being maintenance-free.

The average cost to flush a water heater can cost $300. Depending on your model, you might have to flush annually or twice a year.

You Still Need To Wait

Here is one of the most common tankless water heater pros and cons: they take some time to heat up.

They do heat up the water instantly. However, you won’t have instant hot water at the sink.

There’s a waiting process for this to happen. The cold water has to come out before any water fixture can have hot water.

If you plan on getting a tankless water heater, you will be pretty disappointed when you find this out.

If you want instant hot water, use a circulation pump.

How To Determine The Right Size For My Tankless Water Heater

To get the most out of any tankless water heater, you need to determine the model’s right size.

Otherwise, you won’t have any hot water at home. That said, consider the following factors before getting one:

Different Models And Sizes

There are plenty of sizes of tankless water heaters available. For example, the small ones are meant to be used for warm hand-washing.

Medium-sized models can provide enough hot water to a single sink.

On the other hand, the large ones can supply water to an entire family.

All manufactures have a sizing chart, which you can use to learn more about this.

The main point to take from here is that different models serve different purposes.

Even though the small one might seem like a great option, you might be limited to how many fixtures can run hot water.

Figure Out How Many Fixtures Must Run At The Same Time

You need to know that it does not matter how many fixtures you have at home.

What really matters is how many fixtures will run hot water simultaneously, though most households don’t need to run hot water simultaneously.

In other words, think about how many family members need to use hot water simultaneously.

For example, let’s say that you wake up early in the morning to take a hot shower, but somebody is doing the dishes downstairs and needs to run hot water.

You would need to find the average gallon per minute, also know as GPM, for both the shower head and the sink fixture or any other water fixture that requires hot water.

Flow Rate

To determine the GPM, check your manufacturer’s manual for the shower head, dishwashers, and any other fixtures you want to run hot water, and then add all those numbers up.

For example, if you expect to run a hot water faucet with a flow rate of 1GPM, a cloth washer with a flow rate of 2GPM, and three shower heads with a flow rate of 2.5GPM, the heater needs to be at least 11GPM.

If you can’t figure this out, you could fill up a one-gallon bucket to calculate how much time it takes to fill it up.

When you’ve figured that out, divide the amount of time it takes to fill up the bucket by 60.

Once you’ve determined the GPM for all of those fixtures, look up any sizing chart to determine if there’s any model that can support your flow rate.

Determine The Temperature Where The Heater Will Be Installed

How big of a tankless water heater do I need?

After you have done the math for every fixture flow rate that will run hot water, you now have to determine the temperature rise for your model.

Tankless water heaters can supply endless hot water.

However, it can be overloaded if there’s too much demand.

So, to get around that, find out the ground temperature where the unit will be installed.

Meaning, if the water temperature is 62°F, you need to subtract that number from the desired water temperature.

In most cases, the desired water temperature can go anywhere from 110°F to 120°F.

So, let’s go with the latter. In this case, you would need to subtract 120 degrees from 62°F.

Therefore, your on-demand water heater needs to produce at least 58°F.

Now, keep in mind that temperature differs by location and can change throughout the year.

For example, in a northern area, average temperatures can go anywhere from 37°F all the way up to 52°F.

For those who are in the central zone, you should expect anywhere from 52°F to 62°F. And those in the southern zone should expect 63°F to 77°F.

If needed, you can go by the lowest temperature rise in your area to determine if the on-demand water heater will be undersized during winter.

For more information on this broad topic, check this video out:

Frequently Asked Questions (FAQs)

If there’s anything else you’d like to learn about on-demand water heats, check this section out:

How big tankless water heater for family of 4?

A family of 4 would need anywhere from 50 gallons all the way up to 75 gallons.

What is the downside of a tankless water heater?

Most tankless water heaters are advertised as instant hot water heaters, but that’s far from the truth.

They are faster than the standard tank, but it still takes some time to get hot water.

For example, you still need to turn on the tap and then wait for all that cold water to come out.

Is a tankless water heater suitable for a large family?

Yes, a large water heater is good enough for a large family. However, it depends.

This is determined by the number of fixtures that will run simultaneously and the flow rate.

While a 50-gallon water heater might be too much for one person living in the house, this might be enough for a large family.

Can a tankless water heater be too big?

There’s no need to get a big water heater if you’re living alone.

Instead, determine the number of gallons you actually need and then add that number to the final flow rate.

This step is essential as you don’t want to get an expensive water heater you’ll never use to its fullest.

Final Words

How big of a tankless water heater do I need? As long as you figure out the flow rate and the temperature rise, you should be good to go.

Most manufacturers offer a sizing chart, which you can use to find a tankless water heater that meets your needs.

There’s also no need to get a bigger tankless water heater than you need.

You’d only be wasting your money, and chances are you’ll never come close to using its full potential.

With that being said, we hope this sizing guide helped you choose the right water heater for your house.