What size tankless water heater do I need

Our rating of the best tankless water heaters on the market is useless if you don’t know EXACTLY what size of a tankless unit you need for your house.

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 what size tankless water heater do I need?

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 endless supply of hot water. You only have to make the right choice.

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.


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


Determine the temperature rise in your region

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


Put it all together

Choose brand and model that meet all the requirements
Go to step 3

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

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
Lavatory faucet0.5-1 GPM
Kitchen faucet1.5-2.2 GPM
Showerhead1-2 GPM
Tub faucets4-7 GPM
Washing machine2 GPM
Dishwasher1.5 GPM

If we forgot something important, please let us know in the comments.

Now let’s see how you do it by looking at an example:

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

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

(2 showerheads * 2 GPM) + 2.2 GPM for a kitchen sink + 2 GPM for a washing machine = 8.2 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 isn’t very likely to happen in real life. But it’s always better to overestimate 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 units for showers or appliances. The decision totally depends 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.

Small point of use tankless water heaters solve 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 tankless water heater needs to heat the water. 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 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). If you live in the northern states where the groundwater is colder, you’ll get fewer gallons of hot water per minute 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 for your region (just hover over the map)

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 the groundwater temp from the temperature of outgoing hot water (it’s usually around 110-115°F) and you’ll get your number.

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. But it’s just a quick tip to remember. So if you haven’t considered this 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 flow rates. 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 to overestimate your average hot water demand.

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.

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.

Also, 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.

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).

Total flow rate calculator

Bathroom faucets
Kitchen faucets
Washing machine

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

To replace a 50 gallon water heater you can choose a tankless water heater of any size. Because if you sized it properly, you'll have an endless supply of hot water.

As it's been said, it all depends on your hot water demand and temperature rise. Personally I would recommend buying Rinnai RUR160iN if it fits your budget.

If you'd like to look for a cheaper alternative, consider Rinnai V-Series. It shows great performance and it can supply a two-bathroom house with endless hot water.

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're going to 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, look at Rinnai RUR160 (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 water heater do I need for a family of 5?

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