What Size Hot Water Tank Do I Need?

Image of a baby playing in a bathtub used for marketing

According to the U.S. Department of Energy, water heaters consume 14%-25% of a home’s annual energy usage. So when it’s time to install a new hot water heater, it’s important to choose the right size for your home and family’s needs.

If you get one that’s too big, then you’ll have a higher water and energy bill due to the increased stand-by time it takes to maintain the hot water. On the flip side, if you get one that’s too small, you run the risk of running out of hot water and having the unit continuously run in order to keep up with your home’s water demand; not to mention, there will be a lot more wear and tear on your water heater and it could shorten the life expectancy of your unit.

With all this being said, we’ve outlined some things that should help guide you in determining what size water heater is best for you and your family.

Estimating Peak Demand – First Hour Rating (FHR)

Traditional storage tank water heaters have to hold enough water in the tank to meet your family’s peak demand. Every water heater has a First Hour Rating (FHR), which you’ll use to determine if a particular storage tank is the right size or not. This is the number of gallons of hot water the heater can supply per hour (starting with a full tank of hot water). This rating is usually based on the total number of people in your home multiplied by 12, which is the average number of gallons of hot water used for one person during their peak usage.

How to Calculate Family’s Desired FHR

You’ll want to choose a water heater that has a first hour rating within a few gallons of the number you calculate. You can use any one of the formulas below to determine the right size of tank for your family’s needs (you might need to adjust that figure a bit if you have a large bathtub, multi-head shower or other special considerations).

Typical Breakdown

  • 1-2 people – 30 gallon water heater
  • 2-3 people – 40 gallon water heater
  • 3-4 people – 40-50 gallon water heater
  • 5 or more people – 50-80 gallon water heater

Equation for Average Household:

  • Total number of people in your home x 12 gallons = FHR
  • For example: 4 people x 12 = 48 gallon water heater

More Precise Measurement:

  • Determine what time of day (morning, noon, evening) your family uses the most water. Keep in mind the number of people in your household.
  • Use the grid below to help estimate your maximum usage of hot water during this one-hour of the day. This will calculate your FHR. (This does not estimate total daily usage.)

FHR Calculator Hot Water Heater Size

FHR Calculator Hot Water Heater Size Example

** This household would need a water heater model with a FHR of 36-40 gallons

Determining the right size water heater for your family is one of the biggest factors you’ll use to decide what size water heater you should get. You might be wondering if a tankless water heater might be a good option – you can download our tankless water heater guide to get all these questions answered.