Typically, residential water heater sizes range from 40 to 100 gallon tanks.
Usually, most homeowners find that either a 40 or 50 gallon hot water tank is the best fit for their needs. Consequently, these are the two most frequently installed sizes.
However, every family has different water demand requirements. This depends on the number of people living in the house, on the frequency of water use, as well as peak hours usage.
If you don’t calculate the tank size properly, you will end up getting a water heater that is too small, and it will simply not be enough to service your household. In practical terms, this may mean running out of hot water when you are in the middle of the shower 🙁
Lets see how you can avoid this scenario and get the correct tank size.
How to determine the size of a water heater tank
Before buying a tank water heater, most homeowners want to get a rough idea of the water heater size their family needs. This is because water heater size directly impacts its cost. The bigger the water heater tank, the more expensive it will be.
However, you don’t want to try to save money on a water heater by cutting down on size, because you will be stuck without enough hot water. Here is a complete guide on how much it costs to install a hot water heater.
Moreover, a water heater that is too small will also break more frequently, because it will be overworked beyond its normal capacity. This way the money you were trying to save and then some will go towards frequent repairs.
To figure out the heater tank capacity that will actually work for your household, you need to consider 2 things:
1. Tank’s FHR (1st Hour Rating)
The FHR rating indicates how many gallons of water a heater can produce at any given hour. You will find this number on the Yellow Energy Guide Label. (see below)
After the first hour, the water will continue to cool down, depending on how quickly you’re using it up. It will then take a certain amount of time for the heater to return to its full FHR. This time varies on different devices, and should also be a consideration when choosing your hot water heater.
Water Heater Repair
$250  $350
40 Ga. Water Heater
$850  $1100
Tankless Water Heater
$1200  $3500

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2. Your peak hour demand
This is the amount of hot water your household uses during the busiest time of day for your family. For example: taking 23 showers at the same time, while doing laundry, and running a dishwasher. (see below how to calculate peak hour demand).
Keep in mind that taking a shower uses up the most water. This means that your peak hour will be during the time when most people in your household are taking a shower, especially if a few of them are doing it at the same time.
Also, keep in mind the length of time your family members like to spend in the shower. For example, while I am out in under 5 minutes, my teenage daughter and wife, each take a 3040 minute shower every day!!!!
Once you have these two numbers, compare them, and pick a hot water tank that is within 13 gallons of your Peak Hour Demand.
Keep in mind that the size of the tank will directly impact its cost (bigger tank = more money). So if you are on a strict budget, you have all the more reasons to spend the extra time to determine what tank capacity will work best.
How to estimate your peak hour demand
Here are two ways ( one easy, the other more involved) you can estimate your households Peak Hour Demand.
1. Easy Calculation
Step 1: tally the number of people living in your home. If you have a family member that is there frequently, or on a regular basis (but not full time), like a step child for example, count them in as well.
Step 2: multiply the number of people in your household by 12. This is the FHR. For example, if you have 5 people in your house, your FHR will be 60 (5×12).
2. More Involved Calculation
If you want to figure out how much water you actually use during Peak Hour Demand and get an accurate FHR number, follow these steps. This method is used by professional plumbers, when they do all the work for you:)
Step 1: Determine what time of day (morning, noon, evening) people in your household use the most hot water during a 1 hour window. Keep in mind the number of people living in your home.
Step 2: Use the chart bellow to determine which of the activities listed your household members do during this peak usage time. Be sure to multiply each activity by the number of times it was done in that peak hour. Add up all the activities to get the number for Total Peak Hour Demand.
Activity  Average Gallons of Hot Water Per Usage  (x)  Times Used During 1 Hour  (=)  Gallons Used in 1 Hour  

Shower  10  x  =  
Shave (0.5 gallons / min)  2  x  =  
Hands/Face Washing (2 gallons / minute)  4  x  =  
Hair Shampoo (2 gallons / minute)  4  x  =  
Hand Dishwashing (2 gallons / minute)  4  x  =  
Food Prep (2 gallons / minute)  4  x  =  
Automatic Dishwasher  6  x  =  
Clothing Washing Machine  7  x  =  
Total Peak Hour Demand 
Contact your local plumbers to get free quotes and advice on water heater installation.
Size a water heater based on number of people in a household
If you don’t want to bother with any calculations, here is an easy reference chart you can use to figure out the right tank capacity for your home.
Family Size  Water Heater Capacity 

12  2336 
23  3646 
34  4656 
56  5680 
67  80100 
7 or more  100 or more 
Its evident from this chart that a family of 5, which is very common in the US, requires a 5680 gallon size water heater.
However, keep in mind, that this is a very loose estimate and it may not be accurate for your household’s water usage. This is because this calculation does not take into account any unusually high water usage demands, which you may have.
How would you know if your hot water usage demand is very high?
Its easy to tell: if you use more 23 appliances that run hot water at the same time, you will require a bigger tank than one indicated for your household size. If this is the case, then consider installing the next size up from your household size.
On the other hand, if your family members have the habit of typically running only 1 hot water appliance at a time, than an average size tank indicated for the number of people in your home, would be more than enough to do the job.
Additionally, if you have an oversized whirlpool/jacuzzi tub or another deep tub, your tank capacity should be 100% or more of the tub’s capacity (for example: 75gallon tub/80gallon water heater tank).
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I’ve been boiling water inside a kettle so I’ll have some hot water to use in my bathtub when I’m bathing, but I find that it’s timeconsuming to do that. I think I’ll have a hot water tank installed in my home instead so I’ll be able to save more time in bathing in the morning. Thanks for mentioning that I’ll be able to determine what size I need by considering the amount of hot water one can produce at an hour, so I’ll be sure to do that.
Your advice to figure out how much water you use during your family’s busiest time is a great idea. This could help you make sure you have a tank that will work for your home. Once you’ve figured out how much you need, you’d probably want to consult with a professional to make sure you choose the proper size and that the tank is properly installed.