Water Heater Sizing Calculator: What Size Water Heater Do I Need?


Adequately sizing a water heater is not that easy. When choosing a water heater capacity for a family of 2, 3, 4, 5, or 6, there are two common mistakes most people make:

  1. getting too small water heater (undersizing), If your water heater’s capacity is too low, you are likely to run out of hot water while you shower. Example of a very small water heater: Installing a 30-gallon water heater for a family of 5. This is over 50% of the water heater capacity you really need.
  2. getting too big water heater (oversizing), If the water heater capacity exceeds your hot water needs, you have unnecessarily high water heater costs. Example of a very large water heater: Installing a 60-gallon water heater for a family of 2. That’s about 20-gallon more than you need, and you’ve unnecessarily spent $150 extra dollars on the heater, and wasted space.

Both of these mistakes can be avoided if you know how to properly choose the water heater capacity for your hot water needs.

here’s the thing:

you only have At a time On sizing your water heater. You don’t want to blow it up and buy a unit that is either too big or too small.

this is what we will see How to choose the right size of water heater for your home. This includes factors that increase/decrease water heater capacity requirements (the size of your family, hot water needs, etc.). we include general rule of thumb Any water heater sized to fit (it’s easy enough to use, but you need to estimate your peak hour hot water demand properly).

On top of that, we set the size of the water heater to be a . made simple with ‘Water Heater Sizing Calculator’, You simply input the size of your family, hot water needs, and the calculator can give you an estimate of how large a water heater you need based on average hot water consumption. Here’s how the results look (screenshot):

hot water heater capacity calculator
You can check out this ‘Water Heater Sizing Calculator’ further. Simply input family size and relative hot water needs, and you’ll get a rough estimate of how large a hot water heater you need.

Let us first familiarize ourselves with water heater sizing concepts like ‘First hour rating’, ‘Water Heater Capacity’, And ‘Peak hour hot water demand’, With this knowledge, you will easily understand the finished table with the sizes of water heaters for a family of 1, 2, 3, 4, 5, and 6:

How to estimate the size of a water heater you need?

The main tip that helps us to adequately size a water heater is the water heater capacity rating.

We talk about 30-gallon, 40-gallon, 50-gallon, 60-gallon water heaters. That number of gallons is the ‘water heater capacity rating’. it is also known as ‘First hour rating’, This is a significant amount of hot water that a heater can provide within the first hour, starting with a tank full of hot water.

How to find out what the rating of the first hour is?

Easy. FTC (Federal Trade Commission) rules mandate that every conventional storage water heater manufacturer must include this rating on the EnergyGuide label. Just check the label, and you’ll find out how big your water heater really is.

Here’s a rule of thumb for water heater size:

first hour rating (heater capacity) peak hour demand (usually 1 hour when we shower)

This means that your water heater must be at least large enough to meet your home’s most demanding 1-hour hot water demand.

Example: Three people take a bath from 10 pm to 11 pm. In 1 shower we use 20 gallons of hot water (according to . US Energy SaverThis means you’ll need a 60-gallon water heater or larger.

As we’ll see next, the exact size of a water heater depends on your hot water needs. This roughly translates to how big your family is. Example: A family of 5 will spend less hot water than a family of 3.

After that first hour (we started with a tank full of hot water), the water heater’s output of hot water depends on three primary factors. these:

  1. Water heater tank capacity.
  2. heat source (Gas vs Electricity), It can be either a burner (gas water heater with tank) or element (electric water heater with tank). Let’s talk about electric water heaters
  3. size of the said heat source, Namely, how big the burner or how big of an element you have.

Obviously, any water heater will be able to produce less hot water in the second, third, etc. hour than after the first hour.

Example: If you have a 50-gallon water heater (the first hour rating is 50 gallons), you’ll get a 50-gallon water heater for 1 hour. However, in the second hour, you’re likely to get less than 50 gallons of hot water (about 16 gallons on average in this case).

If we want to follow the above rule of thumb for sizing a water heater, we need to assess how much hot water we need during the busy times of our shower. Here’s a little help to do that:

Peak Hour Demand Estimation: This is an important step

In principle (using a rule of thumb) sizing a water heater is fairly simple. Add just enough hot water to what you need in the most intense hot water hour. This is usually the time when a large part of the entire house receives rain.

The size of the water heater should match that capacity, and you can add a few gallons on top of that.

Now, the critical step is sufficiently Estimate Peak Hour Hot Water Demand, This means that you have to add up all the hot water you use in that hour, including showering, brushing teeth, washing hands, shaving, dishwasher, etc.

To help you out, we’re here to go over some of the most common activities that require hot water with a hot shower:

  • shower: 20-gallon per shower.
  • Teeth Washing: 0.5 gallon.
  • wash hands: 3 gallons.
  • Shaving: 2 gallons.
  • Top-loader clothes washer: 25 gallons.
  • H-axis clothes washer: 15 gallons.

Here is an example of how you can estimate peak warm hour needs in practice. Let’s say you have a family of 4 with two bathrooms. From 10 pm to 11 pm, 3 people take a bath, 1 person shaves, and all 4 family members wash their teeth.

Accumulated hot water needs are:

3×20 gallons (3 rain) + 1×2 gallons (shaving) + 4×0.5 gallons (tooth wash) , 64 gallons

This means you need a water heater with at least a 64-gallon capacity. You can easily get a 70-gallon water heater in this case but a 60-gallon water heater will be too small (undersized). An 80 gallon water heater is usually too big for a family of 4 people.

If you determine your hot water needs using this ‘peak hour demand’ rule of thumb, you have a little space to head upwards. You can add about 10% to 15% to the calculated water heater capacity. That means if you calculate peak hour hot water demand to be 64 gallons you can get a 70-gallon water heater.

However, you should not opt ​​for a smaller water heater capacity. In practice, if you have calculated that you have 64 gallons of peak hour hot water needs, you should choose a 64-gallon water heater or larger. never small,

pay attention: Undersizing a water heater will be more problematic than reducing the size of the water heater.

If this all sounds a bit complicated, don’t worry.

There is an easy way to size a water heater. It is based on 3 principles:

  1. household size.
  2. Average use of hot water per hour per person. According to engineering toolbox, an average person consumes 10 gallons of hot water per hour, We certainly talk about peak hour demand.
  3. Baseline of 15-20 gallons of water per family. Every household has some basic need for hot water, no matter how many people live in the house.

Based on these 3 principles, we can roughly estimate what size water heater you’ll need, given how big your family is:

Water Heater Sizing Calculator (Based on Family Size)

this is a ‘What size water heater do I need’ Calculator that will give you a rough estimate of how big a water heater you need. You simply input your family size and hot water requirements (below average, average, or above average), and the water heater size calculator will dynamically determine the water heater capacity in gallons:

Based on this water heater sizing method, you can roughly see what size water heater you need for your family:

household size Water Heater Size (Gallon)
1. family of 25 gallons
2 . family of 35 gallons
3. family of 45 gallons
4. family of 55 gallons
5. family of 65 gallons
6. family of 75 gallons

Note when buying a water heater: Check the Energy Star label as well. Energy efficient water heaters pay off in most cases. You can read more about Requirements for Energy Star Labels Here,

We hope this was all helpful.

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