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

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

  1. Getting a too small water heater (undersizing). If your water heater capacity is too low, you are likely to run out of hot water while showering. Example of a too small water heater: Installing a 30-gallon water heater for a family of 5. That’s more than 50% of the water heater capacity that you actually need.
  2. Getting a too big water heater (oversizing). If the water heater capacity surpasses your hot water needs, you have unnecessarily higher water heater costs. Example of a too big water heater: Installing a 60-gallon water heater for a family of 2. That’s at about 20-gallons more than you need, and you have needlessly spent $150 additional dollars on the heater, and wasted space.

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

Here’s the thing:

You only have one shot at sizing your water heater. You don’t want to blow it and buy a unit that’s either too big or too small.

That’s we will look into how to pick the right water heater size for your home. That includes factors that increase/decrease the water heater capacity requirements (size of your family, hot water needs, etc.). We include the general rule of thumb for sizing any water heater (it’s fairly easy to use, but you need to estimate your peak hour hot water demand properly).

On top of that, we simplified water heater sizing with a ‘Water Heater Sizing Calculator’. You just input the size of your family, hot water needs, and the calculator will give you a rough estimate of how big a water heater you need, based on average hot water consumption. Here’s how the results look like (screenshot):

hot water heater capacity calculator
You can find this ‘Water Heater Sizing Calculator’ further on. Just input family size and relative hot water needs, and you will get a rough estimate of how big a hot water heater you need.

Let’s 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 prepared table with water heater sizes for a family of 1, 2, 3, 4, 5, and 6:

How To Estimate The Size Of A Water Heater You Need?

The key spec that helps us 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’. This is also known as ‘first hour rating’. It is a volumetric amount of hot water a heater can provide within the first hour, starting with a tank full of hot water.

How to find what the first hour rating is?

Simple. The FTC (Federal Trade Commission) rule is that every conventional storage water heater producer has to include this rating on the EnergyGuide label. Just check the label, and you will find out how big your water heater actually is.

Here is the rule of thumb for water heater sizing:

First Hour Rating (Heater Capacity) ≤ Peak Hour Demand (Usually 1 hour when we shower)

That means that your water heater should be at least big enough to meet the most demanding 1-hour hot water demands your household has.

Example: From 10 PM to 11 PM three people have a shower. In 1 shower we use 20 gallons of hot water (according to US Energy Saver). That means that you will need a 60-gallon water heater or bigger.

As we will see further on, the exact size of a water heater depends on your hot water needs. That roughly translates into how big a family you have. 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 hot water output of a water heater depends on three primary factors. These are:

  1. Water heater tank capacity.
  2. Heat source (gas vs electricity); this can be either burner (gas water heater with tank) or element (electric water heater with tank). We talk about electric water heaters
  3. Size of the said heat source; namely, how big a burner or how big an element you have.

Obviously, any water heater will be able to produce less hot water in the 2nd, 3rd, etc., hour after the 1st hour.

Example: If you have a 50-gallon water heater (first hour rating is 50 gallons), you will get 50-gallons of water heater for the 1st hour. However, in the 2nd hour, you will likely get much 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 our showering peak hour. Here’s a little help to do just that:

Estimating Peak Hour Demand: This Is The Key Step

It’s fairly simple to size a water heater in theory (using the rule of thumb). Just add up all the hot water you require in the most intense hot water hour. That’s usually the hour when a large part of the whole household showers.

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

Now, the key step is to adequately estimate the peak hour hot water demand. That means that you have to add up all the hot water you use in that hour, including showering, washing teeth, hand washing, shaving, dishwashers, and so on.

To help you out, we specific some of the most common activities that require hot water with the hot water needs here:

  • Showering: 20-gallons per shower.
  • Washing teeth: 0.5 gallons.
  • Hand washing: 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 the peak hot hour needs in practice. Let’s say that you have a 4 family household with two bathrooms. From 10 PM to 11 PM 3 people have a shower, 1 person is shaving, and all 4 family members wash their teeth.

The accumulated hot water needs are:

3×20 gallons (3 showers) + 1×2 gallons (shaving) + 4×0.5 gallons (washing teeth) = 64 gallons

This means that 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 likely be too small (undersized). An 80-gallon water heater for a family of 4 is generally too big.

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

However, you should not choose a smaller water heater capacity. In practice, if you calculated you have 64 gallons peak hour hot water needs, you should choose a 64-gallon water heater or bigger. Never smaller.

Note: Undersizing a water heater will be more problematic than oversizing a water heater.

If all this seems a bit complex, don’t worry.

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

  1. Size of the family.
  2. Average use of hot water per person per hour. According to Engineering Toolbox, an average person consumes 10 gallons of hot water per hour. We talk about the peak hour demand, of course.
  3. Baseline of 15-to-20 gallons of water per household. Every household has some basic hot water needs, independent of how many people live in the house.

Based on these 3 principles, we can roughly estimate what size water heater you need given how big a family you have:

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:

0.00 Gallon Water Heater

Estimate Of What Size Hot Water Heater You Need


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

Family Size Water Heater Size (Gallons)
Family of 1 25 Gallons
Family of 2 35 Gallons
Family of 3 45 Gallons
Family of 4 55 Gallons
Family of 5 65 Gallons
Family of 6 75 Gallons

Note of buying a water heater: Check for the Energy Star label as well. Energy-efficient water heaters in most cases pay off. You can read more about the requirements for an Energy Star label here.

We hope all of this was helpful.

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