This is The Correct Size of Water Heater You Should Buy

I often have people ask me “What size of water heater does a family of 4 need”? Or would a 40-gallon water heater be enough for a family of 4/5?


To know what size of tank-type water heater you need, calculate your household’s total peak hour hot water demand. Buy a water heater with a first hour rating (FHR) equal or slightly higher than that figure.

The following is a simple guide on how to size a water heater:

  • Determine Your Peak Hourly Hot Water Demand: Identify the busiest hour of the day when hot water is in highest demand. Consider all appliances and fixtures that use hot water simultaneously, such as showers, faucets, dishwashers, and washing machines.
  • Calculate Total Flow Rate: Determine the flow rate (in gallons per minute, GPM) of each hot water fixture or appliance that will be used during the peak hour. You can usually find this information on the manufacturer’s labels or in product manuals.
  • Add Up the GPM: Sum up the flow rates of all fixtures and appliances you identified in step 2. This will give you the total GPM required during the peak hour.
  • Calculate Required Capacity: To size the water heater, you’ll need to find one with a capacity that can handle the total GPM demand calculated in step 3. Multiply the total GPM by 60 (minutes in an hour) to get the required gallons of hot water storage capacity needed for the peak hour.
  • Consider Recovery Rate: Additionally, consider the recovery rate of the water heater, which indicates how quickly it can heat new water to the desired temperature. A higher recovery rate means quicker replenishment of hot water.
  • Account for Temperature Rise: Factor in the temperature rise needed to reach your desired hot water temperature. This depends on the groundwater temperature in your area. The greater the temperature difference, the more powerful the water heater you’ll need.
  • Choose the Right Type: Select the appropriate type of water heater (e.g., tankless, storage tank, heat pump) based on your capacity and space requirements, as well as your energy efficiency preferences.
  • Consult a Professional: If you’re unsure about the calculations or specific requirements, it’s advisable to consult a plumbing professional or a water heater specialist who can help you select the right size and type of water heater for your home.

For a tank-type water heater, here is a rough estimate of the size of the water heater you would need based on the size of the family:

  • 1 or 2 persons: 30-40 gallons
  • Up to 3 people: 40-50 gallons
  • Up to 4 people: 50-60 gallons
  • More than 5 people: 60-80 gallons

The above figures as mentioned above are just but rough estimates. The best thing to do is to calculate the real figures.

The Size You Need


The older the tank-type water heaters become, the less efficient they become. The hot water does not last as long as it used to and also does not get hot enough. They also tend to be less-energy efficient and noisy. Please see when to replace you water heater in this post.

The dilemma with a tank-type water heater is that if you buy a small one, you will hardly ever have enough water for everyone. The last one to take a shower in the family will have to settle for a cold one, and it is usually not out of choice.

On the other hand if you buy one which is too big, you will spend more in energy bills since the more water you heat the more energy is consumed. This is why it is important to size your water heater appropriately.

The first thing you want to check when buying a tank-type water heater is its first hour rating (FHR). A water heater’s first hour rating is the volume of hot water (in gallons) that it can produce in a single hour when it is full of hot water.

The first hour rating of a water heater is a function of its capacity, heat source (natural gas or electric), and the size of the gas burner or electrical element. Natural gas water heaters have a higher FHR than electric water heaters.

For an example, let us look at this Rheen water heater. It has a capacity of 50 gallons of water but its FHR is 78 gallons. That means that you and your household can only get 78 gallons of hot water from the heater when its full before the hot water runs out; not a gallon more.

United States Federal Trade Commission mandates tank-type water heater manufacturers to have an energy guide label on the body of the tank. In that label you will find the water heater’s first hour rating.


Estimating a Tank-Type Water Heater Peak Hour Demand

Going by the example above, how can you know if the water heater will be the right size for your household? The only way to find out is by calculating your household’s hot water consumption during peak hours.

A lot of water in the house goes down the shower drain. And that is where we need to be careful with our calculations. The tub is another culprit but folks don’t soak up daily, and especially not in peak hours.

According to the EPA, the average American uses 20 gallons of water in the shower. Of that amount, around 70-75% is usually hot water while the rest is cold water.

Since we are only interested with the hot water, we will take 75% of 20 gallons which is 15 gallons. That is the figure we will work with.

To calculate the amount of hot water used in your house hold, use the following flow chart. Of course family water usage behaviors vary so you can edit the flowchart to suit your household’s specific water usage habits.

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From the above example, let us imagine a family of 4 members where each has to shower, they have to prepare a meal, wash hands before eating, then run the dishwasher. One family member also needs to shave.

The amount of water needed is (15 gallons x 4 family members) + 4 gallons during food preparation +15 gallons washing dishes + (1 gallon hand washing x 4 family members) + 2 gallons for shaving which is a total 85 gallons of hot water.

In this example, the water heater above falls short by 7 gallons. You will therefore need to buy a bigger water heater or compromise on some of the hot water demands.

The rule of thumb is to look for a water heater model that exceeds your household’s peak hour hot water demand by about 2 gallons. That should give you enough cushion.

What Size of Tankless Water Heater Do I Need?


Tankless or demand-type water heaters are very popular in Europe and Asia and are only start making their way into the United States. The main advantage of these heaters compared to tank-type water heaters is that they produce hot water any time you want them to.

They are also very energy efficient whereby unlike tank-type water heaters which heat water throughout, they only heat water when it is in demand, and hence their name. Tankless water heaters are also very space-efficient as they are usually mounted on the wall so if you need to free up some space in your basement then you should consider them.

In order to fully enjoy the benefits of a tankless water heater, you need to make sure that you have the correct size. If you have a smaller size, the temperature of the outflowing water will be low while if they are too big you will pay more in energy bills.

So, how do you know what size of a tankless/demand-type water heater to buy? Just like in tank-type water heaters, you will need to calculate your household’s water demand during peak hours and then look for a matching tankless water heater.

When choosing a tankless water heater, there are 2 terms that you need to be aware of. These are flow rate and the other one is temperature rise. This is how demand-type water heaters are rated.

The temperature rise of a tankless water heater refers to the difference in temperature between the water flowing in the water heater and that of the water flowing out.. The temperature rise can be adjusted but bear in mind that the higher it is the lower the output water flow rate.

The Flowrate of a tankless water heater is the amount of hot water that the heater is producing, or being sent to your fixtures. This is usually measured in gallons per minute.

Estimating the Size of a Tankless Water Heater


To determine what size of tankless water heater you need, calculate the total household hot water demand during peak hours (in gallons per minute). Use that figure to purchase tankless water heater model with similar ratings.

To do this, you need to be aware of the flow rates of all of your fixtures. Let us look at an example:

  • Shower head: 2.5 gallons per minute
  • Sink faucet: 2.2 gallons per minute
  • Washing machine: 2 gallons per minute
  • Dish washer: 1.5 gallons per minute

If for instance at your peak hour you will most likely have 2 family members taking a shower (different bathrooms) and running the dish washer, you would need a tankless water heater with a rating of 6.5 gallons per minute.

But as a said, the water heater’s flow rate will also depend on the temperature rise as well. If you need a higher flow rate you should lower the temperature rise. Usually the water heater will come with a chart indicating the flow rates at different temperature rises.

If you live in a warm area, you will need a tankless water heater with a lower rating than folks who live in cold areas. Let us look at how your location/weather affects the water heater.

  • Let us assume that your desired temperature output from the water heater is 120 degrees Fahrenheit (49 degrees Celsius). To know the temperature rise to set on your water heater, get the difference between your desired hot water temperature and the temperature of the cold water entering the water heater.
  • To do this, plug off your kitchen faucet and turn on the cold water for about 2 minutes. Use a thermometer to measure its temperature.

Assuming your cold water temperature is 60 degrees Fahrenheit (15.5 degrees Celsius), you need a tankless water heater that can produce a temperature rise of 60 degrees Fahrenheit (15.5 degrees Celsius).

Just like tank-type water heaters, there are gas-type and electric-type tankless toilet heaters. Gas-types tankless water heaters have a higher flow rate that electric-type tankless water heaters.

In conclusion, the temperature of the incoming cold water and high fixtures flow rate will lower the overall hot water temperature in your house. If for instance you need to fill a bathtub while running other fixtures, you need to open the faucet partially to get the right water temperature.

What Size of Solar Water Heater Do I Need?


Estimating what size of solar water heater you need for your house is not always easy. It will depend on your location within the United States as well as your own individual household hot water usage habits.

The 2 things you need to determine are the solar collector size and the volume of the storage tank. A bigger solar collector size will need a big storage tank and vice versa.

According to the Solar Tribune, the following are the rough estimates for the solar collector sizes and storage tanks for different household’s sizes:

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