Plumbing, Water Heaters

Choose What Size Water Heater You Need Like a Pro

Water Heater Size

When you’re shopping for a new water heater, one of the first things to think about is the water heater’s capacity (i.e., the number of gallons the tank holds). Conventional wisdom would tell you to get the largest heater possible. However, it depends more on the number of people in your household and the recovery ability of the water heater.

It’s recommended you have 10-15 gallons of hot water per person in your household. For a family of four, a 50-gallon water heater should be sufficient. Look at the water heater’s capacity, the First Hour Rating (FHR), and your personal Peak Hour Demand calculation. A general estimated water heater size you will need is:

  • A family of 2 will need a 30-gallon water heater.
  • A family of 3 will need a 30-40 gallon water heater.
  • A family of 4 will need a 40-50 gallon water heater.
  • A family of 5 will need a 50-60 gallon water heater.
  • A family of 6 or more needs a 60-80 gallon water heater.

These numbers are only estimates and vary greatly based on how much hot water you use within your busiest hour.

The Peak Hour Demand calculation is the amount of hot water your home is likely to use within a busy one-hour period.

The First Hour Rating (FHR) is the amount of hot water the heater can produce in one hour. It would help if you also thought about the fuel source and its physical size.

Water Heater Needs

Determining Tank Size Based on Family Size

Tank-style water heaters have a storage capacity of 30 to 80 gallons. For most homeowners, 40-60 gallons is sufficient. But again, it depends on your overall hot water demand. To get the right size for your household, you need to do the math. Here’s a rough estimate to get you started:

  • Households with one to two persons will likely only need a 30-gallon water heater.
  • Two and three-person families need at least a 40-gallon water heater.
  • For a four-person family, a 50-gallon water heater is sufficient. 50-gallons is good if you’re using electricity, and 40 gallons will work for natural gas or propane.
  • Families with more than five persons may benefit from an 80-gallon electric water heater or 50-gallon gas water heater.

Remember that the list above is just a guideline; hence, you cannot draw solid conclusions. Hot water needs vary from one household to the next.

A three-person family, for example, might find a 40-gallon water heater isn’t enough to meet their needs, while the same tank capacity might do the job for a 5-person family.

Some people take longer showers than others, or some households have too many appliances and fixtures drawing hot water from the water heater. So, it’s a matter of how much water a family uses per day.

To calculate your typical usage, you can start with these pointers:

  • The number of people showering and the time at which they are showering. 
  • If the major appliances are used simultaneously with people showering.
  • The fill capacity of the major appliances used.
  • How often the bathtub is used. Do you fill the tub halfway or all the way?
  • Are you planning to remodel your bathroom or kitchen any time soon? If so, will you be getting a larger bathtub? 
  • Since a water heater will last up to 15 years, are you planning on having more children or getting married soon?
  • Consider installing one water heater for large homes for every two bathrooms or one water heater per floor.
Fhr Lg

Evaluating the Peak Hour Demand and First Hour Rating

Now that you’ve completed the lifestyle audit above figuring out the Peak Hour Demand (PHD) and the First Hour Rating (FHR). This information will help you get a water heater that keeps up with your hot water needs.

What is the Peak Hour Demand? The peak hour demand is the amount of hot water used during rush hour use. It determines the time of day that you use the most hot water in your home. 

Your peak hour may be 8pm before getting everyone ready for bed or at 6 am when everyone is hurrying to get ready for school or work. Simply, it’s the hour of the day the most hot water is consumed.

What is the First Hour Rating? The First Hour Rating (FHR) is the amount of hot water the heater can produce in one hour. Keep in mind this is not the same thing as the amount of water the tank can hold. 

The FHR depends on the fuel source, tank size, and the size of the burner. You can find the FHR information on the Energy Guide Label of the water heater.

Below is a checklist to help you estimate your peak hour usage. Keep in mind the peak hour demand should be a little lower than your heater’s first-hour rating.

  • Hair shampooing (per household member): 4 gallons
  • Dishwashing by hand: 4 gallons
  • Face/hand washing (per household member): 4 gallons
  • Shaving: 2 gallons
  • Showering (per household member): 10-15 gallons
  • Automatic dishwasher: 14 gallons
  • Automatic washer: 10-30 gallons (Older clothes washers can use up to 45 gallons of water while modern energy-efficient washers use as little as 5 gallons)

For example, for a family of five, here’s how you can calculate your peak hour demand. If every morning, three people shower, two face wash, and one shaves and washes the dishes by hand, 74 gallons are the average amount you will use.

So, if your peak hour demand is 74 gallons, you should look for a water heater with an FHR of 76-80 gallons. 1

Sample Worksheet for Estimating Peak Hour Demand/First Hour Rating *

Shower10× = 
Shaving (.05 gallon per minute)2× = 
Hand dishwashing or food prep (2 gallons per minute)4× = 
Automatic dishwasher14× = 
Clothes washer10× = 
   Total Peak Hour Demand= 

My Families Personal Example

3 showers10x3=30
2 shave2x2=4
1 Automatic dishwashing6x1=6
1 Clothes washing10x1=10
Peak Hour Demand   =50

*The above worksheet is based on my family’s usage. Your family’s peak-hour use will likely vary.

Sizing a Tankless Water Heater

Tankless water heaters aren’t as common as tank-style units. However, they are slowly gaining popularity simply because they take up less space and reduce energy costs by 25%.

However, if you opt for a tankless water heater, a different set of factors come into play. These units don’t store water; hence, the tank’s capacity is the least of your worries. What you should pay close attention to, however, is the flow rate and temperature rise.

To find the perfect tankless water heater for your family, you need to estimate the water flow rate inside your home and the temperature change. Flow rate is usually measured in gallons per minute(GPM).

Use the following information to get an estimate of the flow rate in your household.

  • Dishwasher: 1.5 GPM
  • Washer: 2 GPM
  • Shower: 2.5 GPM
  • Kitchen/Bathroom Faucet: 1.5 GPM
  • Running bathtub: 4 GPM

It would help if you implemented the same calculations used in the estimation of peak hour demand. Add up all the flow rates of faucets and appliances used at the same time. 

If, for example, you’re running three showers and a dishwasher simultaneously, you’ll need a tankless tank with a minimum flow rate of 9 gallons per minute.

The next step is determining the required water temperature rise. To do so, you need to subtract the incoming water temperature from the set output temperature of the unit, which is usually 100-120 degrees Fahrenheit.

Here are some incoming temperature estimations based on location:

  • 40 degrees Fahrenheit for the northern parts of the US
  • 50 degrees Fahrenheit for the southern parts of the US
  • 60 degrees Fahrenheit in the southwest, south California, and gulf states

If you still don’t know the incoming water temperature in your area, you can use the 50 degrees Fahrenheit estimation. 

If you want to be more accurate, you can turn on the cold water in your kitchen faucet and let it run for a few minutes. Take a thermometer, stick the metal end into running water, and measure its temperature to get an accurate measurement of incoming temperature.

Now let’s say the incoming/cold water temperature is 40 degrees Fahrenheit and the household hot water temperature is 120 degrees Fahrenheit. 

The required temperature rise would be 80 degrees Fahrenheit. This means you’ll need a water heater that produces a temperature rise of 80 degrees Fahrenheit at a flow rate of 9 gallons per minute. 

It’s a good idea to round up the numbers a bit to ensure your new water heater can meet your hot water demands. Fortunately, most manufacturers provide a flow chart on their sites.

Other Factors That Affect Water Heater Size

  • Inlet water temperatures vary by region and season. Summer doesn’t require as much input as in the winter. Hence you need to ensure there is sufficient hot water flow on the cold winter days.
  • The temperature rise of tankless gas water heaters is usually larger than that of electric units. Gas-fired tankless heaters can produce a temperature rise of 70 degrees Fahrenheit at a flow rate of 5 GPM and the same temperature at 1.5 to 2 GPM through an electric model.
  • Thermostats control some tankless water heaters. These units provide flexibility because their output temperature varies according to the incoming water temperature and flow rate.

Final Thoughts on Water Heater Sizing

Suppose you are still unsure what size water heater you need for your family and consult with a licensed plumber for help. Another option is to look at the on-demand tankless water heater options. Check out our Buyers Guide: What Tankless Water Heater Do I Need.


Hubert Miles | Licensed Home Inspector, CMI, CPI

Hubert Miles is a licensed home inspector (RBI# 2556) with more than two decades of experience in inspection and construction. Since 2008, he has been serving South Carolina through his company, Patriot Home Inspections LLC. As a Certified Master Inspector, Hubert is dedicated to providing his expertise in home inspections, repairs, maintenance, and DIY projects.