What Size Tankless Water Heater Do I Need?- Check 4 easily Step

Are you thinking about installing a water heater but afraid of a higher utility bill? Then a tankless water heater will be the best choice for you. It warms up cold water by consuming a small amount of Gas power or electricity. It provides hot water without any delay whenever you need it but doesn’t store hot water. Besides, it doesn’t need much place to fit like a traditional water heater.

Many people seem to be confused about the appropriate size of a tankless water heater. If you do not install a perfect size tankless water heater, you will not get the desired service. In this article, we will explain what size tankless hot water heater do you need step by step.

What size tankless water heater do I need calculator?

If you install a smaller tankless water heater than your need, you won’t get enough water. On the other side, if you install a bigger tankless water heater than your needy size, it will consume extra energy that causes a higher utility bill. So, you have to choose the right size for a tankless water heater. A common question always arises, “What size tankless water heater do I need?” Follow each step to know about the perfect size tankless water heater your need.

tankless water Heater

Step-1: Determine the rate of water flow in your house

As we know, tankless water heaters don’t store water. It warms up the water that your tap discharges. So, to determine the size of the tankless heater, at first, you have to figure out the water output in your house. The heater size mostly depends on this factor.

You have to estimate the measurement of hot water demand you need per minute. You can measure your maximum hot water flow rate or GPM (Gallons per minute) by adding each device’s water flow rate that needs to discharge hot water. For example, a house’s shower flow rate is 1.5 GPM, and your kitchen faucet flow rate is 2.5 GPM. By adding these two GPM, you can get the perfect measurement of the tankless water heater. So, (1.5+2.5=4) GPM will be required a tankless water heater’s GPM.

You have to be very careful when calculating the flow rate of your tap. If you determine less than the actual flow rate, the tap/devices will not deliver hot water at the usual flow rate. They may slow down when more than one tap runs.

The below chart shows the necessary household’s average GPM rate. You can estimate your house’s flow rate with the help of this chart. (Real GPM rate may vary with this chart.)

ApplianceFlow rate(GPM)
Bathroom faucet 0.5 GPM – 1.0 GPM
Shower head1.5 GPM – 2.0 GPM
Kitchen tap  2.0 GPM – 6.0 GPM
Dishwasher 1.5 GPM – 2.5 GPM
Washing machine 2.5 GPM – 3.5 GPM

Step-2: Determine required temperature rise

The next important step to measure the perfect size tankless water heater is to determine the temperature rise you need to get the required heat from your water outlet. To estimate this calculation, at first, you have to know the incoming groundwater temperature of your area.

To measure the temperature rise, you don’t need any tools. You may check your area’s average groundwater temperature from a groundwater temperature map. That may vary from season to season but, you can estimate the average temperature. Next, you have to determine the temperature rise that you want to flow out from your water outlet. The required temperature of the water is estimated by subtracting incoming water temperature from expected water temperature. (Expected water temperature – Groundwater temperature = Required temperature of the tankless water heater.)

Let’s assume our groundwater temperature is 47 degrees, and we want the tankless water heater will deliver 115 degrees. Our required tankless water heater’s temperature will be (115 – 47) = 68 degrees. If your showerhead flow rate is 1.5 GPM, your heater must heat the water by 68 degrees Fahrenheit per minute.

Note: If you can’t find your area’s groundwater temperature by groundwater map, you can estimate the temperature as 40 degrees. It will match most of the area’s ground temperature.

Step-3: Determine your household size

What if your tankless water heater can’t supply enough water in some peak hours? Sometimes, the fixture connected with the tankless water heater can’t discharge hot water at the same time. It is because you only determined the GPM of your regular water flow rate. It is step number three, which is also very important to calculate the perfect size of the tankless water heater. In this step, we will tell you what size tankless water heater you need to prevent the shortage of hot water at different times.

Your tankless water heater must have the capacity of providing the same amount of hot water from each faucet at the peak hour of a day. Peak hour is estimated when most of the faucet runs at the same time. Besides, you have to take into account that, on some occasions, you may need a massive supply of hot water from your water heater.

The easiest way to determine this calculation is to add all the faucet’s GPM that may have to run simultaneously. For example, you have three bathroom faucets (1*3=3 GPM), two showers (2*2=4 GPM), and one washing machine (3*1=3 GPM) that you may have to run simultaneously sometimes. Tankless heater size is estimated on the highest possible usage of a house’s faucet. So, the perfect size of your tankless water heater is (3+4+3) = 10 GPM. If you buy a tankless water heater of 10 GPM, you will get enough hot water both in usual and peak hours.

Step-4: Determine Power source (Gas vs. Electric tankless water heater)

The next step to measure the perfect size of a tankless water heater is to determine the power source of your house. At first, you have to select, either you will buy a gas-powered tankless water heater or an electric tankless heater. We cannot say which water heater will be better for you because it varies about your preferences of which will be the best for you. We will explain the advantages and disadvantages of both gas and electric tankless water heater below. You can choose according to your preferences.

The electric tankless water heater is more affordable than a gas water heater because you don’t have to install fuel piping or venting and its installation process is too easy and cheap. (The cost of gas and electric supply may vary from area to area.) If you need a little amount of GPM in less heat, you should defiantly go for the electric tankless water heater.

A gas tankless water heater can provide a larger amount of hot water in lesser time compared with an electric water heater. If your house has a large amount of hot water demand, you should go for gas water heaters. It is safer than an electric water heater.

At first, you have to determine which power source will cost less in your house and which will be more preferable for you. If your home has enough supply of gas power, you should go for a gas tankless water heater. On the other hand, if your home is not eligible to supply proper gas power, you can go for the electric tankless water heater.

The bellow chart indicates the size of tankless water heater depending on the number of a family member. You can select your required tankless water heater size by considering your family’s demand.

Family Size

Demand GPM Capacity
Electric Gas
1-2 High 40-50 40-50
Regular/low 30 30
2-3 High 50 40-50
Regular/low 40 40
3-4 High 80 50-75
Regular/low 50 40
5+ High 80+ 75
Regular/low 80 50
Credit: PEX Universe

Frequently Asked questions

What size tankless water heater do I need for a family of 5?

The exact size can’t estimate without knowing the number of water faucets of a home. But, an estimation can be done that a family of five need approximately 10 GPM tankless water heater. This size of water heater may not be applicable for all the house. The best option is to calculate the GPM of individual faucets of your house that you need to run simultaneously.

How many gallons per minute does a tankless water heater use?

The amount of hot water releases from a tankless water heater connected tap depends on the heat rise per minute. It varies from different models. As the heater can raise the temperate of the water, the tap discharges hot water. Most of the tankless water heater releases 4 to 8 gallons of hot water per minute.

What is the downside of a tankless water heater?

A tankless water heater is more preferable to a traditional water heater. All things have downsides besides numerous benefits. One of the most common downsides of a tankless water heater is, it is too expensive to afford. In the outage of power, there is no way to get hot water.

Can a tankless water heater fill a tub?

Of course, a tankless water heater can fill a tub and also without taking too much time. A tankless water heater supplies hot water until its maximum ability exceeds. If you estimate your water heater size accurately, it will not slow down of supplying hot water when there is running more than one faucets.  

How long does a tankless water heater last?

If you maintain the heater properly, a tankless water heater will last more than 20 years. Its maintenance process is too easy. On the other hand, a traditional water heater lasts for 10-15 years.


A tankless water heater is the best option for getting hot water for household needs. It has no tank that doesn’t take much space to fit. You don’t have to wait for getting hot water. But there is a common question that always becomes the problem of choosing a tankless water heater, “What size tankless water heater do I need?”

You can get the best features of the tankless water heater when you will able to find the proper size of the heater you need. If you follow the four steps properly, you can get the calculation of a perfect size water heater as your family need.

3 thoughts on “What Size Tankless Water Heater Do I Need?- Check 4 easily Step”

  1. I’m looking for the right size water heater to save on my electricity bill. This guide is really useful 🙂 thank you for writing this


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