How to Calculate the Right Water Heater Size for Your Home

Choosing the right water heater isn’t just about picking the most powerful or the cheapest unit — it’s about selecting a size that matches your household’s needs. If the water heater is too small, you’ll constantly run out of hot water. If it's too large, you'll waste energy and money heating water you don't use. That’s why proper water heater calculation is essential.

In this article, we’ll guide you through the basics of water heater sizing and help you calculate the perfect capacity for your home.

Why Sizing Matters

An incorrectly sized water heater can lead to:

• Insufficient hot water during peak times (e.g., morning showers).

• Higher energy bills due to over-sized systems.

• Unnecessary wear on the unit, reducing its lifespan.

  
  

Step-by-Step: How to Calculate Water Heater Size

Step 1: Estimate Peak Hour Demand

The first step is determining your household’s peak hour demand — the maximum amount of hot water used during the busiest hour of the day.

Here's an example of typical hot water usage during peak times:

Activity	Hot Water Used (gallons)
Shower (per person)	10
Shaving	2
Hand washing	1
Dishwashing (by hand)	4
Dishwasher	6
Laundry (one load)	7

Example:

A family of four might use hot water like this during a busy morning:

• 3 showers = 30 gallons

• 1 dishwasher load = 6 gallons

• 1 hand wash and shaving = 3 gallons

  Total = 39 gallons

  
  

In this case, a unit with a First Hour Rating (FHR) of at least 40 gallons is recommended.

Step 2: Choose the Type of Water Heater

There are different types of water heaters, and the sizing approach varies slightly:

• Tank Water Heaters: Look for the First Hour Rating (FHR) on the label. This tells you how many gallons it can supply during peak hour usage.

• Tankless Water Heaters: Rated by flow rate (GPM) and temperature rise. These are sized based on the number of simultaneous hot water uses, not total volume.

  
  

  

  
  

Tankless Water Heater Calculation Example

Let’s say your household often runs:

• 1 shower (2.5 GPM)

• 1 dishwasher (1.5 GPM)

Total flow rate = 4 GPM

Now, calculate temperature rise:

• If your incoming cold water is 50°F and you want 120°F hot water, you need a 70°F rise.

Choose a unit that can provide 4 GPM at a 70°F rise.

Other Factors to Consider

• Climate: Colder regions require higher temperature rise.

• Energy Source: Gas heaters usually heat faster than electric.

• Efficiency Ratings: Look for ENERGY STAR® certified models to save on utility bills.

  
  

Conclusion

Properly calculating your water heater size is the key to comfort, efficiency, and long-term savings. Whether you're replacing an old unit or installing a new one in a renovated home, always start with your household's specific hot water needs. If you're unsure, it's best to consult with a professional plumber or HVAC expert who can help guide you through the process.

Need help calculating your home's water heater size? Contact our team for a free consultation and let us take the guesswork out of your hot water needs.