What Size Tankless Water Heater Do I Need? (GPM Needs)

When it comes to the tankless water heater, one of the bigger mistakes is buying a unit that is too weak to satisfy all our hot water needs.

You don’t want one that is too small, nor do you want a tankless heater that’s too big and will needlessly spend energy. Tankless water heater size should be as appropriate to your household hot water needs as possible.

What size tankless water heater do I need?

Here’s the deal:

To properly answer what size tankless water heater do you need, you have to figure out two things beforehand:

  1. What are your maximum hot water needs?
  2. How much water per minute (measured in Gallons Per Minute or GPM) can a certain tankless water heater heat up, and by how many degrees?

To correctly size the tankless water heater, we need to make a rough estimation of our maximum hot water needs at any given point.

Most households have the highest hot water needs from 9 PM to 11 PM. That’s when we shower, brush our teeth using a hot faucet, and might even have a dishwasher running.

We need to tally all this hot water needs up. Here’s a useful table of how much GPMs do different water fixtures require:

Fixture Gallons Per Minute (GPM)
Shower 2.0 – 3.0 GPM
Faucet (kitchen, bathroom) 1.0 – 2.0 GPM
Dishwasher 1.5 – 2.0 GPM
Washing Machine 2.0 – 2.5 GPM

Example: If you’re taking a shower (100% flow and 110˚F hot water) and simultaneously use two faucets (100% flow and 110˚F hot water), you will need at least 5 GPM tankless water heater.

Tankless heaters can deliver anywhere from 2 GPM to 12 GPM of hot water. The 5-10 GPM ones are most appropriate for the majority of households.

Note: For low water needs up to 8 GPM, the electric tankless hot water heaters are appropriate. For bigger needs (8+ GPM), you should choose one of the best gas tankless hot water heaters here.

Difference Between Maximum Water Flow And Realistic Maximum GMPs

When looking at different tankless heaters’ specs, you will notice they note the maximum GPMs. In practice, the maximum GMP your tankless heater will realistically achieve can be much lower.

Why the discrepancy?

Because the maximum water flow in GMP is measured by warming up 77˚F water. The inlet temperature of water currently in your pipes matters quite a lot.

In south Texas, for example, inlet water temperature is 77˚F. In Minnesota, for example, the inlet water temperature can be as low as 37˚F. That is an additional 40˚F difference a tankless water heater needs to overcome.

Short calculation: Let’s say we have a tankless heater with a maximum water flow of 10 GPM. In Texas, we can actually get 10 GPM of 110˚F water because the inlet temperature is 77˚F. The heater has to heat water from 77˚F to 110˚F; that’s a 33˚F difference.

On the other hand, the inlet water temperature in Minnesota is 37˚F. To heat water to 110˚F in Minnesota, a tankless heater has overcome a temperature difference of 73˚F degrees instead of a 33˚F difference in Texas.

You’re not from Minnesota or Texas? Here’s an infographic created for Rinnai RU160iP SE+ Series 9 GPM tankless water heater that will give you an idea of what is the maximum water flow in your state (valid of USA):

different ground water temperatures in order to calculate how big of an tankless water heater you need

Here’s another example based on the infographics above: If you live in Florida (77˚F inlet temperature), the Rinnai RU160iP SE+ Series tankless heater will have a maximum water flow of 7.1 GPM. That’s enough to run several showers simultaneously.

On the other hand, if you live in New York (52˚F inlet temperature), the same tankless water heater achieves a maximum water flow of 4.5 GMP. That is the direct result of the difference in inlet temperature.

In New York, the heater has to overcome an additional 25˚F. With the same heater and the same energy expenditure, you will be able to run 2 or 3 showers simultaneously.

What Size Tankless Water Heater Do I Need For A Family Of 2, 3, 4, 5, Or 6?

One of the most questions in tankless water heater sizing is how big a unit you need for a family of several people. Obviously, a family of 3 will need a smaller tankless water heater than a family of 5. But what are the exact GPM (for gas-powered) or kW (for electric) numbers?

Given the variable water inflow temperature, it’s rather hard to pinpoint exactly how big a tankless hot water heater you need. Here is a table with estimates; the intervals are, regrettably, quite large. Households in the Northern US will need bigger units than households in the Southern US, for example:

Number Of Family Members: Gas Tankless Heater Size (GPM) Electric Tankless Heater Size (kW)
What size tankless water heater do I need for a family of 2? 6-8 GPM 10-18 kW
What size tankless water heater do I need for a family of 3? 7-9 GPM 15-23 kW
What size tankless water heater do I need for a family of 4? 8-10 GPM 20-28 kW
What size tankless water heater do I need for a family of 5? 9-11 GPM 25-34 kW
What size tankless water heater do I need for a family of 6? 11+ GPM 34+ kW

These figures are for reference only. Practically, there are several important factors like inflow water temperature and acute hot water needs that determine the exact size of tankless water heater you need.

Looking At Specifications Sheets

On specification sheets, you will notice that producers either give a maximum water flow number in GMP or maximum electric power in kW. Generally, gas-powered tankless water heaters come with a GMP number, while electric tankless hot water heaters come with a kW number.

As we’ve seen, the maximum GMP is relative. It depends where in the US you live (because that affects the inlet water temperature). Power (measured in kW), however, is absolute. We can compare how powerful different tankless heaters are by comparing their maximum wattage (as we’ve done in the table of the best tankless heaters below).

Let’s look at two examples. The first one concerns what size of a tankless water heater you need if you want to replace, let’s say, the 50-gallon water heater. The second one is based on the number of people living (and using hot water) in your household. For example, how large a tankless water heater does a family of 5 need.

What Size Tankless Water Heater Do I Need To Replace A 50 Gallon Water Heater? (Example #1)

Here is how this situation goes: You currently have a 30, 40, 50, or even 80-gallon water heater and want to replace it with a tankless water heater.

The main difference, obviously, is that with a water tank, you have, let’s say, 50 gallons of hot water, and with a tankless, you have on-demand heating of water.

During an average 10-minute shower, for example, you spend about 10 gallons of hot water. If 3 people have a shower, you run a faucet or two, a dishwasher, and so on, you can quickly spend all those 50 gallons.

However, the case with the tankless water heater is different. You don’t have hot water in storage; the powerful heating exchanger in the tankless heater heats the water when you need it with a certain maximum GMP limit.

To replace a 50-gallon water heater, you would, roughly speaking, need:

  • 10 GPM gas tankless heater or at least a 27 kW electric tankless water heater if you live in the northern part of the USA.
  • 7 GPM gas tankless heater or at least an 18 kW electric tankless water heater if you live in the southern part of the USA.

That’s why Rinnai, the world’s best gas tankless heater makes, offers a wide array of models – from 7 GPM to 11 GPM:

choosing size of tankless gas water heater with 7 gpm, 9, gpm, 10 gpm and 11 gpm
Rinnai offers its gas tankless models in several sizes. You can check them all out here.

Be mindful that this is only a rough estimate. The smart thing to do is buy a tankless heater that is a bit more powerful than the estimated needs. Better be safe than sorry.

What Size Tankless Water Heater Do I Need For A Family Of 5? (Example #2)

If 5 people live in a single household, they can use several faucets or showers simultaneously. When picking the size of a tankless water heater, we have to take this into account.

A household hot water consumer that requires hot water fastest is a shower. 5 people can also simultaneously run several hot water faucets, a dishwasher, doing laundry, and so on.

In short, a family of 5 would need a 10 GPM gas tankless heater or 27 kW electric tankless heater if you live in the northern part of the USA, where the input water has a lower temperature. The tankless heater has to work extra hard to bring the water temperature up to 110˚F or 120˚F.

However, if you live in the southern part, the capacity of the tankless water heater can be reduced by as much as 30%. So, for a family of 5 in the southern part of the USA, that would mean a 7 GPM gas tankless heater or an 18 kW tankless heater should be more than enough to satisfy all hot water needs.

53 thoughts on “What Size Tankless Water Heater Do I Need? (GPM Needs)”

      • My house has two 50 gallon tanks one on either side of our upstairs in closets. Do I need to get two tankless heaters because of the lines or just one? What sizes would I need?

        Reply
        • Hello Sherry, you don’t really need 2 tankless units because of the lines; you usually need 2 of them because you need enough hot water. To replace a 50-gallon tank, you need a 7 GPM to 10 GPM tankless heater (depending on where you live). So, the easiest solution would be to buy two 9 GPM units and install them on the existing lines. The best ones are Rinnai gas tankless heaters, you can check them here.

          Reply
  1. Hi there.
    I live in South Jersey where it looks like the ground water temp is 50F, according to the map.
    It’s a small house with just two people. We use the hot water for showers, dishes and the occasional bath. We do not yet own a washer machine but when we do we will occasionally use hot water in that as well. It will be a small machine, maybe 3cf.

    We want a tankless, electric hot water heather. Any advice?

    Reply
  2. We have 4 kids, oldest beginning to go into teenage years. So HUGE water needs for a family of 6. We have a traditional water tank now and run out of water if we have 3-4 people showering around the same time. We are in a very cold climate. I am assuming we’d need the largest unit?

    Reply
  3. We live in Central Florida and are building a two-story, 4200 s.f. house for a family of four (2 adults, 2 tweens). We will have 4 showers in the house but only 3 of them will be used on a daily basis. Right now, the plan is to have a Rinnai RL94E tankless gas water heater.

    Recently, we were told by one of the vendors that we may need two. Is that really necessary? Seems like an unnecessary expense.

    Thanks.

    Reply
    • Hello there, the Rinnai RL94E has a maximum how water flow of 9.8 GPM. Given that you’re located in Florida, you don’t really need 2 of those. In Boston or New York, maybe, but 9.8 GPM should be enough for a 4-person family.

      Reply
      • Thanks! We asked them about adding a recirculation system and they recommended the RUR 199E, which already has a built-in pump as well as adding a 3/4″ return line. I think having that configuration should be more than sufficient. I have no interest in maintaining 2 water heaters!

        Reply
  4. What size of electric water heater do I need for 2 persons, washing machine, dishwasher and kitchen sink. I live in the Caribbean.

    Reply
  5. Hi, I live in the norther zone of illinois. We have 5 people living in a 1800 square foot home. Would be no more than a shower, dishwasher, and efficient washing machine at same time. I assume an 8-9 gpm heater will suffice. Not sure of gas vs electric. What do you suggest?

    Reply
    • Hello Bert, you’re right, for your situation 9 GPM would be enough. Usually, gas is recommended because it saves costs in the long run (price of gas is much lower than electric) and they are more reliable in general. You can check out the best gas tankless heaters here; Rinnai unit is the best choice.

      Reply
  6. I have a family of three and use hot water for showers, dishwasher and washer as well as vanity. According to the map my inlet water is 52 degrees so what size do you thank I need. I’m in north east kentucky.

    Reply
    • Hello Aimee, if you don’t use a lot of hot water simultaneously, 7 GPM would be suffice. However, the safer option is 9 GPM. Hope this helps.

      Reply
  7. Hi Bert, what size gas tankless wh, exterior mount, Southern California. 2 people, 1 shower, 1 tub, dishwasher, washing machine. And should recommend a specific manufacturer
    Thanks
    C

    Reply
    • Hello Charles, for a 2-person household with a high initial water temperature (Southern California), you really need a small tankless heater. 7 GPM would more than suffice, for example. About the manufacturers, you really can’t go wrong with Rinnai. They produce the best gas tankless heaters; you can check more about Rinnai and other best gas units here.

      Reply
  8. Hello Charles. Building a 5200 sq ft home in central Oklahoma with a total of 6 full baths for family of 5 with guest room that is occasionally occupied by in-laws/parents. Additionally have two dishwashers and a washer that are primarily run throughout the day/overnight and don’t anticipate running either during typical shower times. That said could easily foresee 3 showers running simultaneously during morning/evening. Planning on largest tankless hot water heater with recirculation but curious if it would be better to have 2 tankless water heaters. Thanks!

    Reply
    • Hello Tarek, this is definitely a case for the biggest gas tankless water heater – 11+ GPM. It makes sense to ask about 2 tankless heaters. In general, Oklahoma has a higher-than-average inflow water temperature, and 1 really big heater might be enough. It’s really hard to estimate this. The safe version would be to install 1 unit and see if you have an adequate hot water supply. If not, you can always install 2nd unit; electric ones are much easier to install as auxiliary units. Hope this helps.

      Reply
  9. I want to replace a 40 gallon water (gas) heater with a tankless water heater. I live in Albuquerque, New Mexico (just the 2 of us). My wife has Alzheimers and I bought a walk in bath tub for her for easy access in and out. The tab capacity is 67 gallons of water. I don’t run any other water at the time she bathes. I usually take a shower after I get her bathed and dressed. What do you recommend? Thanks, Ken…

    Reply
    • Hello Ken, thank you for describing your situation so thoroughly. New Mexico has a high input water temperature and with just 2 people, you are looking for one of the smaller gas-powered tankless water heaters. A Walk-in tub does require quite a lot of hot water but with a 7-8 GPM heater that shouldn’t be a problem (if a 40-gallon heater was enough, the 7-8 GPM will certainly be enough as well). The best choice, in this case, would be the Rinnai V75IN 7.5 GPM you can get here. It has an indoor installation, it is the right size, and Rinnai is considered the best brand for gas tankless water heaters.

      Reply
  10. I have family of 6 in Chicago area with 3 bathrooms. I have the opportunity of getting an 8.4 tankless and an 6.4 tankless. Can I add the two different sizes together for one system?

    Reply
    • Hello Mike, you do have high hot water needs and it makes sense to use 2 units. You can add the two of them together for one system, yes.

      Reply
  11. Hi! I have a Southern California 2918 sq foot house with three bathrooms. 2 adults and 4 kids. We just purchased our home and are remodeling it, and i am thinking of replacing a traditional water heater with a natural gas Rinnai 199eN. We don’t currently have a dedicated return line, but I was wondering if the 11gpm and the use of the built-in circ pump and thermal bypass that is included would be enough for our family?

    Reply
    • Hello Elaine, as a family of 6, you do have high hot water needs 6 but Southern California has a very high ground water temperature (up to 77°F, actually). 11 GPM tankless water heater would be more than sufficient. We’re talking about Rinnai 11 GPM RU199en-Natural Gas/11 (this one), right? That’s the perfect choice in your situation. Hope this helps.

      Reply
  12. Hi, I live alone in Austin Texas. I have an 800 sq/ft home with one shower, one bathtub, one dishwasher, one washing machine, and two hot water sinks. Can you tell me what I need in an electric tankless water heater?
    Thanks so much,

    Reply
    • Hello Helen, given you have very low hot water needs, you need one of the smaller electric tankless heaters. 10-12 kW should more than suffice. You can check some examples of great electric tankless heaters here; Stiebel Eltron is pretty much the best brand. Check out their 12 kW Stiebel Eltron – Tempra 12 Plus model, that would be a good choice in your situation.

      Reply
  13. Thank you, this is the most helpful site I have visited concerning this topic. I am building a 3200 sq ft home in southern Virginia with inlet of 52F. Family of 4 using typically dishwasher, washing machine and maybe shower or 2 and one faucet between 6pm and 9pm Builder offers a 7.2 or 9.5 Rinnai but not sure if its worth the expense for the 9.5. Would that be sufficient or should I get the 7.2 and change to 11 gpm later.

    Reply
    • Hello Alexander, for a family of 4, 9.5 GPM would be a safe choice. The 52˚F inlet water temperature is right in the middle; at 60˚F, for example, 7.2 GPM would be a viable choice. However, given your water requirements (2 showers + 1 faucet max.) you can get away with 7.2 GPM. For 3 simultaneous showers, for example, 7.2 GPM would not be enough. Hope this helps a bit.

      Reply
  14. Hello l live in far west Texas and need to replace a tank water heater with a tankless water heater. I have a family of three and use dishwasher daily and 2 and half bathrooms. Would a Rinnai V75IN 7.5 GPM be sufficient ??

    Reply
  15. Hi! I live in Southern California and I have 1 shower ,1 tub (that doesn’t get used) and a smaller bathroom with sink as well as the kitchen sink, no dishwasher. 3 adults live here and take daily showers. What size tankless electric outdoor water heater would I need?
    I would really like to have 2, one by the kitchen and the other on the other side of the house for the shower.
    Thanks Don

    Reply
    • Hello Don, electric units are not as powerful as gas units, and you’ll probably be better off with 2 tankless electric heaters if you find that more convenient. For 1 shower, 1 rarely used tub and so forth, you would require no more than 20 kW (given that Southern California has a high input water temperature). For specific recommendations, you can check our article about the best electric tankless hot water heaters here; #1 Stiebel Eltron is arguable the best unit, and offers 12kW to 36kW units. The optimum choice would be 1 20kW unit or 2 12kW units (for the kitchen and other side of the house). Hope this helps a bit.

      Reply
  16. Hello!
    Thank you very much for this article! Definitely the most helpful resource I could find on that topic so far.
    We live in San Diego, a family of 3, 2 bathrooms, dishwasher, situations with 2 showers running at the same time are very rare. We’re eyeing Rinnai RU130iN vs Rinnai RU160iN. What would be your recommendation between the 2? Or even maybe you can suggest a totally different model…
    Again, thank you for this great resource!

    Reply
    • Hello Oleg, for your situation the 13 GPM will be more than enough. Rinnai does produce the best units, and natural gas is pretty much the most economic way of heating up water. Rinnai RU130iN would be an adequate choice. Thanks for the compliment, we do try to help everybody out as best we can.

      Reply
      • Sorry, I think there is a typo there, Rinnai RU130iN has 7 GPM rating.
        Another interesting point is most individual contractors and water heater installation companies recommend Navien and Norits over Rinnai, at least in our area. I suppose, it’s because of simpler installation, maybe?

        Reply
        • Hello Oleg, you’re completely right; 130iN has a 7 GPM rating. Typing too quickly, sorry about that. There are many factors that come in why an installation company recommends a certain product; Noritz units, for example, are easier to install, that’s true, and that may be the main factor.

          Reply
  17. This has been a really informative article! I just want to make sure we’re not going overkill on our tankless water heater though… We’re in Canada (eastern Ontario), two people, but would likely end up using washing machine and either bathtub or shower at the same time, possibly also running the tap in the kitchen sink. Or substitute dishwasher for washing machine. I was looking at ordering a Rinnai RU199iP. Is this recommendable, or would you suggest stepping down to the RU180iP?

    Reply
    • Hello Cindi, thank you. In Canada, you’ll like have the ground water temperature of 35°F or even lower. That means you’ll have to have a powerful tankless water heater to increase the water temperature adequately; gas-powered units are the right choice and you can’t go wrong with Rinnai. It’s very likely that RU180iP will be sufficient for two people.

      Reply
  18. Hello, we live in north Texas in an old farm house. We are in the process of adding an addition onto the house ,which is a large bathroom. Is it possible to get a tankless system to to supply that room only? There are only 2 of us , but we have a large bathtub that holds 50 gallons of water.

    Reply
    • Hello Kim, it’s possible for a tankless water heater to only service one room. You have to talk with the professional installer to get the plumbing correct, and that can be quite costly.

      Reply
  19. Hello I am looking to upgrade to an electric tankless water heater. I own a mobile home. One bathroom with one faucet and one shower. Kitchen has one faucet and a dishwasher. And an washer. Just me and my wife. Now all signs point to low volume but we use a lot of water . We do a lot of laundry and dishes and long showers. Basically what i am tryin to figure out how much 2 people could possibly use. Thank you do or any input you could offer.

    Reply
    • Hello Robert, the key to sizing a tankless heater is to figure out how much hot water you need simultaneously. So, even if you use a lot of water, but you use it throughout the day, you can get a very small electric tankless heater. The major component is also the input water temperature. Considering all the available data here, the 20 kW electric tankless heater should be enough. You can check our post about tankless electric heaters here; #1 Stiebel Eltron is available in 20kW capacity, for example.

      Reply
  20. Very useful site, thanks! I’m considering a seasonal outdoor 80 gallon whirlpool bath installation. 55°F water temperature. I already have a Rennai propane tank for the house. Do you think I can use some of the “camp” propane models just for the tub? What would be the lowest gpm I should consider?

    Reply
    • Hello Roger, well, a lot depends on the GPM of the faucet that will be used to fill the whirlpool bath. For smaller hot water needs, it’s advisable to install an electric tankless heater (cheap but can spike electricity costs). If you opt for a gas tankless heater, the smallest Rinnai propane unit has is a 7 GPM model. In most cases that should suffice, but again, it does depend on what kind of faucet you have (in terms of GPM) for filling the bath. Hope this helps.

      Reply
  21. Hello. I live in upstate NY (42F water intake zone). My house has one bath, dishwasher, washing machine and my personal sanitation needs. My house could accomodate 3 people for the same usages. My tub faucet outflow is 4 GPM.What size tankless gas unit do I need for BTU’s and GPM to satisfy that potential occupancy?. Thank you.

    Reply
    • Hello Mike, the intake temperature (42F) is quite low. The tub faucet outflow of 4 GPM is quite high. If you want to use some of these appliances simultaneously, you would require quite a sizable gas tankless heater. Something in the neighbourhood of 9-11 GPM. You can check which are the best gas tankless heaters here; Rinnai is universally considered the best gas tankless heater brand.

      Reply
  22. Hi,
    I have a 2 bed/2Bath short term vacation rental in Key West. Sleeps 6 max. It has a washing machine and dishwasher. It currently has a 80 gal. electric water heater and we have never had a complaint regarding compacity. It is very important that a tankless unit handle everything running at the same time as it is quite likely that will happen regularly. Based in the article above and using some approximate percentages, I calculated that a 30kW electric unit would be effective. Do you agree or have a recommendation?

    Reply
    • Hello Kevin, you have quite high hot water needs. The real advantage you have is that you’re located in Key West, Florida, where the ground water temperature is relatively high. 30 kW might be enough. A yet better would be a 10+ GPM gas tankless water heater (you can check some of the best gas tankless heaters here; Rinnai is the #1 brand). Gas units might cost a lot more than electric units but the low cost of gas compared to electricity will save you a lot more on electricity costs in the next 10-20 years. Hope this helps.

      Reply

Leave a comment