A Guide To Determine The Size Of A Tank Water Heater

//A Guide To Determine The Size Of A Tank Water Heater

A Guide To Determine The Size Of A Tank Water Heater

After weighing up the pros and cons of tank and tankless water heater, the next part is settling on the size of the tank you need. This water heater size guide presents you general guidelines for size calculation. For accurate size, you can rope in your 24 hour plumber to determine your hot water requirements, especially during the busiest hours. 

Check the table below to roughly quantify your requirements for hot water. 

Number of users Amount  (gallons) 
1 to 2 30 to 40 
2 to 3 40 to 50
3 to 4 50 to 60
5+60 to 80 

The above table shows loose estimates only. You want accurate estimation so that all family members can get sufficient hot water. The heater of the right size efficiently delivers optimal performance without skyrocketing power bills. So, keep reading and see how you can arrive at exact water heater size calculations. 

If You Are Looking For A Loose Estimate

As cost factor is linked to the performance and capacity of the heater, many people want to start with a rough idea of the amount they have to pay. The cost of the bigger water heater that heats up water fast is higher. However, the price tag also depends on the type of heater and fuel type. Your 24 hour plumber can educate you more on fuel type, performance and price tag. So, depart from this cost factor for a while and home in on the capacity of the heater.

To make a size estimation, count the number of people going to use the heater. If you are a household of 3, a 40 gallon hot water heater might be enough. However, you can also find a family of 5 living comfortably on a 30 gallon water heater. This reflects that the water heater capacity also depends on your user habits.

For example, if you are using more than two hot water appliances within the same hour, don’t count on the size estimations illustrated in the table. You need a bigger one. If you are hardly using more than one appliance in the same hour, you need a smaller one. In case you cannot determine the water heater capacity you need, let a 24 hour plumber help you. Keep in mind that your tank size requirements must be accurate because of the following reasons:   

  • You might end up buying a tank water heater that is too small. You will never get enough water. The heater will overwork and this will lead to frequent repairs or even premature breakdown.
  • You might buy a heater that is too big. Even when you will not overuse the equipment, you will be paying higher-than-necessary energy bills.

What Is The Exact Water Heater Capacity You Need? 

Start with calculating your “peak hour demand”. This demand refers to the maximum gallons of hot water you need during the busiest hours. Bear in mind that most of the hot water is used in showers. All members of the family shower in “peak hour” in the morning. You have to determine that “peak hour”.  

Once you have determined your peak hour, make a list of all the hot water activities fitting into that peak hour. Check the below worksheet:

First Hour rating/Peak Hour Demand  

UseGallons per usage
Times Used During 1 Hour 
Gallons Used in 1 Hour 
Shower 10X
Shaving (.05 gallon/minute)2X
Food prep or hand dishwashing (2 gallons/minute)4X
Automatic Dishwasher  6X
Clothes Washer 7X
Total Peak Hour Demand 


Let’s see an example: 

3 Showers 10X3=30
1 Shave 2X1=2
1 Hand Dishwashing 4X1=4
Peak Hour Demand


Finding A Water Heater Having “First Hour Rating”

Before you buy a tank water heater, check its first hour rating (FHR). First hour rating is a number specifying the total number of hot water gallons the unit can offer. So, the tank water heater will address your needs if your peak hour demands and first hour rating match. At the time of making a purchase, see under the features, specifications or performance section. There you will find the First Hour Rating of the water heater.


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