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What Size Should Your Tankless Water Heater Be?

What Size Should Your Tankless Water Heater Be?

Jon Hubal
4 minute read

Feel like you might have spent too much on your Water Heater? Or worse, are you taking cold showers because someone decided to run the dishwasher? Let us help you figure out what size should your tankless water heater be?

Sizing Your Tankless Hot Water Heater

A properly sized water heater will make your house more efficient while fulfilling all your hot water needs. This quick guide will help you get the right-sized heater next time you're in the market. 

Tank or Tankless

The first step is deciding whether you want to go tank or tankless. While both are viable options, they use different measures when it comes to power and efficiency.  


Tanks are the traditional option that most people have. If you'd like to replace your outdated heater with an upgraded version of the same, then a good rule of thumb is every: 

1 to 2 people = 30-40 gallon tank

2 to 3 people = 40-50 gallon tank

3 to 4 people = 50-60 gallons tank

5+ people = 60-80 gallon tank

This is an excellent tool to estimate what size heater you'll need, but it's just that – a loose estimate. There are many reasons why you may need more or less water than this graph supposes. If your house has extensive piping, or everyone likes to shower at once, you may need a bigger tank – unless you're fond of ice baths.

Here's a useful calculator to help you out.


Tankless heaters can be trickier to size than tanks, but have no fear – we explain it here. 

Propane models exist but aren't very common so we won't focus on those. Gas and electric heaters dominate the tankless market – with pros and cons we also won't get into right now. We will, however, break down the sizing for whichever you choose.  


The process for sizing an electric tankless is about the same as a gas; make sure you have enough BTU's to cover your temp rise at max flow. The process can be seen below.


Gas-fired tankless can produce higher volumes than electric ones quicker. However, even the largest, gas-fired models can't stand electric's efficiency in large households. 

Sizing Tankless

According to the U.S. Department of Energy, "Tankless Hot Water Heaters (on average) provide hot water at a rate of 2–5 gallons per minute." That includes both gas and electric.

A more accurate estimate can be found by listing the number of fixtures you expect to use at any one time (ex., dishwasher, shower, faucet).  

Once you've figured that out – this will take a bit of math – add up the flow rates of the fixtures. They don't have to be perfect, but a rough guess. Aim high if you're not sure. Average flow rates for household fixtures can be seen here. This will give you your max flow rate.  

Lastly, calculate your average temperature rise. Subtract the incoming water temp from the desired water temp. Use the coldest temperature your climate reaches to ensure you'll have winter heat. A guide to your groundwater temp can be found here.

With these numbers, you can figure out exactly how much heat you need and how fast – but it never hurts to have a plumber do the math for you!

If you're looking for a tankless water heater, feel free to check out our collection of tankless water heaters or shop by brand with Rinnai, Eccotemp, or PrecisionTemp

Want More Tankless Water Heater Info?

Want to read more about Tankless Water Heaters? Read our other related blogs below:

Need Help?

Have questions about what size should your tankless water heater be or need help deciding what tankless water heater is perfect for you? We would love to assist you in finding the best product for you! Please feel free to reach out to one of our real representatives by chatting with the chat button in the bottom right, calling us at +1 (256) 633-6553, or sending us an email at

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