There is no “one size fits all” when buying water heaters. You may come across dozens of articles claiming that “they got the right size.” Still, the exact size of tankless water heater you need to buy depends on your needs, number of bathrooms, taps, and desired water temperature.

In general, a family of 2 needs a 6-8 GPM gas tankless water heating unit. The perfect tankless water heater size depends on several factors, including the size of the home, Water Flow rate, and temperature of underground water in your area.

This article will provide detailed information on tankless water heater sizes. So, you can buy a suitable heating unit according to your needs.

Tankless Water Heater Sizing – A Science Lesson Overview.

Two factors determine the “size .”Normally, every water heating unit follows 

ΔL = αLΔT equation. 

We have broken it down to save you from a complex physics equation. 

  1. Flow Rate (Gallon Per Minute / GPM) – Required Quantity of Hot Water
  2. Temperature Rise (ΔT) – Difference of Temperature From Desired & Supplied Water.

The typical faucets temperature is around 110°F to 120°F. 

The average underground water temperature in the U.S is 57°F. 

It can rise to 70°F in southern parts while freezing in the northern regions.

Determining The Tankless Water Heater Size – Finding Out What You Need.

There is no hard rule. All you need to do is figure out the amount of hot water you’re going to consume. 

Here, have a look at how much water daily home appliances use.

  • Faucets – 0.75GPM to 2.5GPM
  • Low Flow Shower Heads – 2.2GPM
  • Standard Shower Heads – 3GPM to 3.5GPM
  • Washing Machine – 2 GMP
  • Dishwasher – 1GPM to 1.5GPM

The determining formula is simple and basic. Let’s suppose that, On a moment, you need to run a faucet, running washing machine, and crockery is being washed in the dishwasher. 

Then you need;


The quick math can be done in different situations and numbers of fixtures. 

Have a quick brainstorming on the following tip:

Consider buying the tankless water heating unit assuming that you need to run all the taps at once if you have a family of 5. For a family of 4, 8GPM to 10GPM is the ideal size.

What size of s tankless water heater do you need to replace a 50-gallon traditional water heating unit?

The short answer to this specific question is, “It depends on your demographics.” 

If you live in the southern part of the U.S.A, then a 7 GPM gas tankless water heater or 18kW electric tankless water heater will work just fine.

On the contrary, you’ll need to buy a 10 GPM tankless gas heating unit or 27 kW electric water heater for the replacement if you’re living in the northern part.

Our Assumed Size – Depending On Your Family Members.


For your convenience, we have done a lot of research and found out the “ideal size of gas and electric water heaters” depending on the size of your family.

The Number Of MembersGas Tankless Water Heater (GPM) Electric Tankless Water heater Size (kW)
What size tankless water heater do you need to purchase for a couple?6-8 GPM10-18 kW
What is the ideal size of Tankless water heating unit I need to buy for a family of 3?7-9 GPM15-23 kW
What would be the perfect tankless water heater size for a family of 4?8-10 GPM20-28 kW
What is a suitable size water heating unit for a family of 5+?9-11 GPM25-34 kW

Caution: The perfect size may vary depending on the underground water temperature and quantity + required temperature.

The Silent Factor – Underground Water Temperature.


This image has been taken from Renni Tankless Water Heater underground water map.

Underground Water Temperature.

The temperature of underground water also determines the size of the heating unit. 

If you live in the northmost part of the U.S, the water will be colder, and it might be impossible for a 5 GPM gas water heater to instant supply the hot water.

On the contrary, In Florida, the temperature of underground water is relatively high. There will be an instant supply of hot water and less energy consumption.

How to pick the perfect size tankless water heater for your home?

While looking for a gas tankless water heater, it is advised to consider that such heating units consume more energy. The primary difference between electric and gas heating units is the former has less life span while the latter consumes more power.

Tankless Water Heater Buying Guide – Know Your Needs

Here is a list of a few things you should consider before buying a tankless water heater.

1. Daily Usage.

The size will depend on the usage. If you need to run more taps simultaneously, you need to buy a large heating unit.

2. Location / Demography.

The north v/s south debates look absolute right here. The southern states have more hot underground water, while the northern part of the U.S has cooler water.

3. Size of Home.

If you live in a big home, you may need to invest in an ample water tankless heater for a constant supply. 

On the other hand, if you are called a cozy place home, a small size water heater will work wonders for you.


A water heating unit is an essential household item. Regardless of all the scientific equations, there is no one-size-fits-all. Just right size depends on several aspects, including your needs, supply line, and area of living. 

While in the meantime, a 6 GPM gas tankless water heater is an ideal assumption for a couple. We would recommend a 10GPM gas heating unit for the family of 4.


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