How To Calculate The Size Of The Water Heater I Need?

If you are building or restoring your house or office, you may be in search of the perfect water heater for it.

One of the main thing is to consider when it comes to getting a new water heater is its size. Size is important not only because you want to optimize the costs but also to avoid issues.

As we’re about to discover, picking the wrong size for your water heater can have annoying consequences.

For this reason, we thought of providing you with an article dealing with how to calculate the size of the water heater you need.


Consequences of Installing a Wrong Sized Water Heater

Is picking the water heater of the right size so important? Well, it is. Because if you mess up with this choice, you need to deal with the consequences for years and years (you don’t replace your water heater every other month!). In particular:

  • When you have a small (too small) water heater, you’ll often run out of hot water. Possible scenario? After the first couple of showers in the morning, the third member would have to have a cold shower because there is no more hot water available.
  • When you have a big (too big) water heater, you have unnecessary additional costs.


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How To Calculate The Right Size For Your Water Heater?

Not to put too much pressure on this moment, but you choose your water heater size once, and you need to handle the consequences of your choice for years, so how can you make the right choice?

Well, you need to consider various aspects and then decide to provide the ideal hot water supply for your place.

The aspects you need to consider are:

1. The size of the house


Check the cubic capacity of your apartment, house, or office, and then provide this information to the water heater provider or seller.

Every water heater comes with a suitable cubic capacity, and the comparison of the data should be easy.

The fact is that, however, you shouldn’t limit yourself to this parameter but move on to consider the following.

2. Number of family members

Number Of Family Members

The next parameter you want to consider is how many members will leave the house or attend the office every day.

This is because, if the place is small, but there are many of you living there, you may want to purchase a bigger water heater than the ones that are recommended according to the cubic capacity.

Subsequently, if you have a big house that is all for you, then you may save money by purchasing a water heater that serves your needs without waste.

Here’s how you can calculate the size of the water heater based on the number of family members:

Number of MembersWater Heater Size
1-2 people23-36 gallons
2-4 people36-46 gallons
3-5 people46-56 gallons
5 or more peoplemore than 56 gallons

3. Number of plumbing fixtures

Number of plumbing fixtures

Other than the number of people, you want to count the plumbing fixtures that are in the place: how many showers, sinks, dishwashers, washing machines, and such…

4. The incoming water temperature

The incoming water temperature

Lastly, you may want to consider the incoming water temperature. It will depend on the location, climate, and elevation of your place.

5. First Hour Rating 

First Hour Rating 

The FHR or First Hour Rating is the amount of hot water a tank can produce in an hour. FHR tells the appliance efficiency.

Here are some number that will help you to determine FHR as per the peak hour usage:

  • Bathing or showering (per person) – 20 gallons
  • Washing hands (per person) – 2 gallons
  • Washing hair (per person) – 6 gallons
  • Shaving – 3 gallons
  • Running dishwasher – 14 gallons
  • Washing dishes by hand – 6 gallons
  • Running clothes washer – 30 gallons


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Here’s What Reddit Users Suggest

Water heater size for a couple living in a 2 story house.
by u/killercamine in homeowners

How to Find Your Peak Hour Demand?

Here’s an easy way using which you can calculate the peak hour demand of your household:

  1. Add the total number of people in your house. If you have a family member that does not stay in the house longer, count them still.
  2.  Now multiply the total number of people in your house by 12; the result you get is your FHR. For example: if there are four people in your house, then your FHR will be (4×12) 48.


How to Set The Temperature of a Water Heater?

Wrap Up

As you can see, there are a lot of factors that you need to consider while choosing the ideal size of water heater for your family. FHR, house size, budget, etc., are all factors one must consider.

We hope that this guide on how to calculate the size of the water heater I need was helpful to you.

What happens if you buy a water heater that's too big?

A water heater that’s large for your family would heat up more water which you will not need. As a result, you will get increased energy bills.

Will a bigger water heater use more electricity?

Kilowatts are used to measure electricity, so the bigger the water heater, more will be the more watt it consumes. On average, a water heater runs at 1125 watts of energy.

Will turning off the water heater during night help me in saving money?

Modern day water heaters are designed in such a way that they are very well insulated and so there is minimum heat loss.

So if you turn off the water heater at night, you will only save about $2 a month. So it is upto you wether you want to turn it off at night or not. 3

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