What Size Tankless Water Heater Do I Need? Ultimate Guide

what size tankless water heater do i need

A tankless water heater is an energy-efficient and space-saving alternative to traditional water heaters. They are becoming increasingly popular due to their ability to provide hot water on demand. Tankless water heaters are highly efficient and can save you money by using less energy than traditional tank-type heaters. They are also small and can be used for various applications, such as in a shower, sink, or hot tub. But what size tankless water heater do I need?

The tankless water heater you need depends on your home size and the number of bathroom fixtures. Knowing what size tankless water heater is right for your home can help you save money and enjoy a more efficient hot water system.

In this article, we’ll explore what size tankless water heater do I need, how to identify the right size, and how to get the most of the tankless water heater.

What Size Tankless Water Heater Do I Need: Factors To Consider


You need to consider some factors when sizing a tankless water heater: 

Flow Rate (GPM)

Flow rate is the hot water you will require given time. First, figure out which fixtures you wish to operate simultaneously & how much hot water each will utilize. This is known as flow rate and it is measured in gallons per minute (GPM).

In addition, you can decide the total rate at which the water heater will offer hot water by adding the flow rates for fixtures.

Temperature Rise (ΔT)

Temperature rise is the distinction between the incoming cold water temperature & the desired temperature.

You need to calculate the temperature distinction between the water entering the heater (groundwater temperature) & the temperature desired at the faucets. This is known as temperature rise (professionals call it ΔT), or the temperature by which a water heater must increase the water.

Usually, the temperature of a faucet is between 110° F and 120° F. On average, the US groundwater temperature is 57° F, but it can differ depending on the area. In the southern parts of Florida, it’s usually around 70° F, while it can be close to the freezing point in the northern states.

The wider the gap between the underground water and the water required for fixtures, the fewer fixtures a water heater can provide.

Maximum Hot Water Needs

To pick the ideal tankless water heater; you need to calculate approximately the highest possible requirement for hot water at any moment.

The demand for hot water in most households is most intense between the hours of 9 in the evening and 11 at night. This is when people take showers, brush their teeth and possibly even use water for their dishwasher.

You need to tally all these hot water needs. Here’s a table of how much GPMs different water fixtures need:

FixtureGallons Per Minute (GPM)
Faucet (kitchen, bathroom)1.0 – 2.0 GPM
Washing Machine2.0 – 2.5 GPM
Shower2.0 – 3.0 GPM
Dishwasher1.5 – 2.0 GPM

For instance, if you are taking a shower with 100% flow & 110˚F hot water and at the same time you are using faucets with 100% flow & 110˚F hot water, you will require at least a 5 GPM tankless water heater.

GPM A Tankless Water Heater Heat Up

Tankless heaters offer 2 GPM to 12 GPM hot water. 5-10 GPM tankless water heaters are the best option for most homes. However, it is crucial to note that the price of a tankless water heater rises with its capacity.

Please Note: Electric tankless water heaters are perfect for low water needs of 8 GPM. And, for high water needs of 8+ GPM, select one of the best gas tankless water heaters.

Size Chart For Residential Tankless Water Heater

What Size Tankless Water Heater Do I Need For My Family?


When deciding which tankless water heater to buy, consider the size appropriate for a household with multiple individuals. Home of 3 people will require a smaller tankless water heater than one with 5 family members. 

However, what is the precise GPM (gas-powered) or kW (electric) measurements needed?

It is tricky to precisely determine the size of a tankless water heater you require due to the changing temperature of the incoming water. 

Number of Family Members(What size tankless water heater do I need)

Gas Tankless Heater Size (GPM)

Electric Tankless Heater Size (kW)
For a family of 26-8 GPM10-18 kW
For a family of 37-9 GPM15-23 kW
For a family of 48-10 GPM20-28 kW
For a family of 59-11 GPM25-34 kW
For a family of 611+ GPM34+ kW

The numbers presented are just estimates. In reality, several considerations, such as the temperature of the incoming water and the amount of hot water needed, will state the size of the tankless water heater that is necessary.

Looking At Specifications Sheets

When viewing manufacturer data sheets, you may observe that they provide either the highest water rate in GMP or the most significant electric power in kW. Generally, tankless water heaters powered by gas have an assigned GMP rating, whereas those powered by electricity have a kW rating.

The amount of GMP (Gallons per Minute) will vary depending on the region of the US since it is contingent on the water inlet temperature. On the other hand, the power output is a fixed measurement expressed in kW (kilowatts). You can compare how powerful tankless heaters are by comparing their maximum wattage. Check if a tankless water heater is worth it.

What Size Tankless Water Heater Do I Need To Replace A 50 Gallon Water Heater?

You have a 30, 40, 50, or even 80-gallon water heater & wish to replace it with a tankless heater.

The primary distinction between a water tank and a tankless system is that the former has a set amount of hot water, for instance, 50 gallons, while the latter provides hot water as required.

Generally, when someone takes a 10-minute shower, it uses up to 10 gallons of hot water. If there are 3 people taking showers, along with running other fixtures such as faucets and a dishwasher, it’s possible to use up 50 gallons of water within a brief period.

In contrast, a tankless water heater does not store a hot liquid reservoir. Instead, the powerful heating element of the tankless heater will warm the water when you require it up to a specific maximum flow rate.

To replace a 50-gallon water heater, you require the following: 

  • 10 GPM gas tankless heater/27 kW electric tankless water heater for people in the northern USA.
  • 7 GPM gas tankless heater/18 kW electric tankless water heater for people in the southern USA.

The world’s best tankless gas heater, Rinnai, offers a wide array of models from 7 GPM to 11 GPM.

It is essential to remember that this is merely a rough guess. The prudent thing to do is to purchase a tankless water heater that is more powerful than the estimated requirements. It is better to be safe than sorry.

What Size Tankless Water Heater Do I Need For A Family Of 5?

If 5 people live together in one residence, they may need multiple taps or showers running simultaneously. This can be considered when deciding on the size of a tankless water heater.

A shower is the quickest way for a domestic hot water user to get it. Additionally, up to five individuals can operate multiple hot water taps and a dishwasher and do laundry simultaneously.

If you are in a family of 5 people living in the northern US, where the input water temperature is low, you will require a 10 GPM gas tankless heater/27 kW electric tankless heater. Such a heater needs to work extra hard to increase the water temperature to 110˚F or 120˚F.

People who live in the southern part of the USA should be aware that the capacity of the tankless water heater might be diminished by up to 30%. Consequently, a 7 GPM gas tankless heater/18 kW tankless heater should be sufficient to meet the hot water needs of a 5 people family.

If you own a larger tankless water heater, you can save large money on hot water by getting a circulation pump. This pump is ideal if you want to experience immediate hot water from the faucet or shower. You will no longer have to wait for the hot water to come and consequently waste cold water.

FAQs (Frequently Asked Questions) On What Size Tankless Water Heater Do I Need

1. How do I choose the right size tankless water heater?

When selecting a tankless water heater, the most crucial consideration is the flow rate per minute. A 3.2 GPM heater can heat 3.2 gallons of water each minute and should be sufficient to offer hot water for a shower and a sink. Calculate the combined flow from all fixtures you anticipate using simultaneously to determine the necessary capacity.

2. What size tankless water heater do I need for a family of 4?

For a gas tankless heater, you need 8-10 GPM size & for the electric tankless heater, you need 20-28 kW size.

3. What size tankless water heater do I need to replace a 40-gallon water heater?

Comparing a 3.2 GPM tankless water heater to a 40-gallon tank-type heater is possible. To illustrate, if the tankless unit supplies hot water at 3.2 gallons per minute, it will cause the 40-gallon tank-type heater to be emptied of water in only ten minutes. Unlike tankless heaters, tank-type heaters keep a reserve of water.

4. What happens if you oversize a tankless water heater?

The size of tankless water heaters is of utmost importance. If you select one that is too small, it won’t be able to provide enough hot water. And, if you go for a huge unit, its purchase and operational costs will be higher than needed, thus eliminating the main reason for using tankless water heaters, which is their energy efficiency.

Summary: What Size Tankless Water Heater Do I Need


A tankless heater’s size depends on factors, including the size of your home and the number of bathroom fixtures you have. Knowing what size tankless water heater is right for your home can help you save money and enjoy a more efficient hot water system.

When deciding the size of a tankless heater, you need to consider some factors: flow rate(GPM), temperature rise (ΔT), maximum hot water needs, and GPM, a tankless water heater that heats up.

I hope this article on what size tankless water heater do I need sounds helpful!

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