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What Size Water Heater Do I Need?

Note: This post may contain affiliate links. This means that at no cost to you, we may receive a small commission for made purchases.

Tankless water heaters continue to gain in popularity (especially in larger households) while traditional tank models are still much more common and won’t be going away anytime soon. So how do you know which one is the best fit? It’s simple. Just start with the basics.

What is the Capacity of a Water Heater Measured In?

First things first. What exactly is water heater tank capacity measured in?

In the United States where the imperial system is used, water heater capacity is measured in Gallons. The most common capacities being between 40 and 60 gallons.

In countries that instead use the metric system, the holding capacity of a water heater is measured in Liters.

Storage Tank Water Heater Sizes

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There are two things you want to consider if you’re going with a traditional storage tank water heater.

    1. Peak Hour Demand – The amount of hot water your household uses in a busy 1-hour period
    2. Water Heater’s First Hour Rating (FHR) – The amount of hot water the water heater can produce in a 1-hour period

To be certain you have a water heater that’s the right size for your home you’ll have to do some math. Add up the hot water demand of all the hot water devices in your home that you assume you’d be using in a period of one hour.

Using the chart above, add the average gallons of hot water per usage of these devices using the. For example, say two showers are taken and in between, the washing machine is started. Together that equals 72 gallons of hot water needed.

In this instance, you would need a water heater with a “first hour rating” of at least 72 gallons before you run out of hot water during that 1-hour period. In most cases, you would need a 50-gallon tank water heater in this situation considering most 40-gallon models have a lower FHR.

An average family of four would use somewhere around 200-400 gallons of water per day for dishwashing, showering, washing laundry, etc. A couple living in an apartment might use half of that whereas a family of six might use up to 600 gallons per day or more.

The size of the tank really depends on how much water you use per day not so much on family size (some individuals use more water than others). Distance from the water heater to the outlet also plays a role.

Someone with a large house who’s master bath is on the opposite end of the house as the water heater, will need a larger tank than the same size family that lives in a home where the most commonly used water outlets are close to the water heater.

Tankless Water Heaters Sizes

Tankless water heaters are great for saving money on water usage and are perfect for larger families, when you need to draw hot water from two or more sources at a time, or have a large Jacuzzi tub in your bathroom. Their compact size also means having to hide them isn’t an issue like it can be with some tank models.

Tankless models are rated in gallons-per-minute. When all you want to know is what size tankless water heater do I need, calculating the right size for your home can be accomplished in three easy steps.

  • Add up all of the hot water you’ll be using simultaneously. Say you want to jump in the shower and you need to run the dishwasher at the same time. Average usage rates for each would be: 1.5 – 3.0 gallons per minute for a shower and 1.0 – 3.0 for a dishwasher. On the high end that’s 6 gallons per minute.
  • Depending on what side of the US you live in will determine the average groundwater temperature. In general, if you live in the northern areas, you will need a stronger tankless model than someone in a warmer climate as it takes more work to heat up cooler groundwater.
  • Once you have figured out the gallons per minute and the appropriate groundwater temperature you can determine which tankless water heater is the right size for your home. Most tankless water heater manufacturers will have a water heater size calculator on their site along with their model recommendations once you decide on a brand.

Whether you go tank or tankless, be sure to think longer term and possibly get the next size up in case your family grows unexpectedly (or if single you decide to get married).

Choosing the right size water heater and type doesn’t have to be a long and drawn out process. With a bit of math and personal choice you’ll have the perfect size water heater for your home in no time.

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Anthony Barnes
Anthony Barnes is the founder of Water Heater Hub and a second-generation plumber by profession. Before developing Water Heater Hub, Anthony Barnes was a full-time plumber, and he has undertaken a wide variety of projects over the decades. As a second-generation plumber, it was easy for Anthony to get used to the technicalities of all from a tender age


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