How to Size a Water Heater

If your family is picking names on who gets the last shower because your water heater can't keep up with the number of people in your household, then it might be time to choose a different water heater size. But knowing what water heater size you need isn't always a straightforward process. Follow this guide to help you discover the right water heater size for your household.

How to Determine the Size of Water Heater Required

To determine the correct-sized water heater for your home, you’ll need to take into consideration the size of your household. Here is a breakdown of water heater sizes based on the number of people in your home:

A general rule of thumb is to increase the size of your water heater by 10 gallons per additional person after a household increases to over five members.

Other Water Heater Considerations

The number of people in your household isn't the only factor for determining what size water heater you need. You also need to consider the type of water heater, the water heater’s first-hour rating, and your water consumption. 

Type of Water Heater

Not all water heaters have tanks. If you choose a tankless water heater, then determining the correct water heater size will look different from choosing the right-sized conventional, solar, or heat pump water heater.

First-Hour Rating

The first-hour rating (FHR) is the amount of hot water your fully heated water heater can produce in an hour. This rating tells you if your water heater is efficient enough to heat your water. You can find the FHR in the manufacturer’s description of prospective water heaters.

Peak-Hour Demand

The FHR is not the only part of this equation. To determine if the FHR is adequate for your home, you then have to look at the peak-hour demand in your household. The peak hour demand is how much hot water your home uses during the peak hour of hot water consumption in a given day. 

Using some simple math, go around your home and calculate how many gallons of water your home uses during the peak hour. If your home uses 50 gallons of water during your peak hour, then it's a good idea to choose a water heater with an FHR that is higher than that amount to ensure you don't run out of hot water.

Do You Need a Tankless Water Heater?

Homes with limited space may find that the size of the water heater needed to supply hot water to the household is too large for the home. An alternative to traditional water heaters with a tank is a tankless water heater. These water heaters are typically installed on the wall and take up far less room than a conventional water heater because they don’t have a tank. 

If you decide on a tankless water heater, then you'll need to calculate the right size unit for your home using a different method than traditional water heaters. Let’s look at how you can select the right-sized tankless water heater.

How to Determine Tankless Water Heater Sizing

There are two main components to keep in mind when calculating what size tankless water heater you need for your home: the flow rate and the incoming cold water temperature. You need to know the maximum flow rate in gallons per minute (GPM) needed for your household and how many British Thermal Units (BTUs) your tankless water heater needs to effectively heat water on demand. 

Homes with very low groundwater temperatures will need a more powerful tankless water heater to increase the temperature of that groundwater, while homes in regions with a higher groundwater temperature can get by with a tankless water heater with a lower BTU. 

Follow these steps to calculate the right tankless water heater size for your house:

1. Calculate the number of fixtures you plan on using during peak-hour demand.

2. Find the flow rate of every fixture to calculate the total flow volume.

3. Look up your region’s inlet water temperature.

4. Subtract the inlet water temperature from the maximum hot water heater temperature for your water heater.

5. Compare the flow rate and temperature rise you need in a tankless water heater to the specs of any given tankless water heater to see if they meet the demand for your home.

Heater Sizing by GPM

While you can calculate the size of your water heater by the number of people in your home, it's helpful to know what size water heater you need by GPM—especially tankless water heaters, as they don't have a number of gallons that they need to hold for your family. Here's a closer look at tankless water heater sizing by the number of GPM. 

Keep in mind that these are estimates. The water inflow temperature determines which end of the range your household will need. If you live in a southern, warm state, then you can get away with choosing a tankless water heater on the lower end of the range. Homes in the north that have lower water inflow temperatures need a water heater with a higher GPM.

Do You Need a Pro to Install a Water Heater?

Installing a water heater is not an easy project. For example, you might need a local plumber and an electrician to install a water heater that doesn’t retroactively fit where the old water heater was stored. 

While some savvy DIYers may not need a professional to complete the installation, we strongly suggest that you contact a water heater contractor near you to get the job done correctly.