How is the Capacity of a Water Heater Measured?

Last Updated On June 11, 2024

Updated on November 13, 2021



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how is the capacity of a water heater measured

Wondering what size water heater you need for your home and family or household? 

The right-sized water heater will suit all of your house’s hot water needs while also avoiding excessive utility bills, but how do you know how to pick the right size?

In this PlumbingNav guide, you will learn how to determine what water heater capacity will work best for your family. 

What's In This Guide?

      How Do I Choose the Right Water Heater Size for Our Household Needs? 

      If you choose a water heater that is too small, you may run out of hot water when you need it most. However, an oversized water heater may use more energy than necessary, adding to your utility bills. This guide will help you pick the right water heater size for your home. 

      First, let’s set some standard metrics to help us determine the answer to this question. Use the following to estimate the amount of water used for each activity:

      • 8 minute shower- 20 gallons
      • Dishwasher (one load) – 7 gallons
      • Laundry – 20 gallons
      • Sinks (per minute) – 2 gallons

      Keep in mind the above numbers are averages based on standard appliances. Inefficient appliances will use more water, while efficient appliances will use less. 

      To determine the right size water heater for your home, estimate how many gallons will be used at one time during peak usage times. For most people, peak usage time is in the morning while getting ready for the day, but it varies from household to household.


      What size water heater do I need for a family of 2?

      For 2 people, you need at least a 30 or 40 gallon water heater. 

      If one person takes a shower while another person runs the dishwasher, a 30 gallon tank should cover it. However, if both people want to shower at the same time, a 40 gallon tank will work best. 

      What size water heater do I need for a family of 3?

      For 3 people, you need at least a 40 or 50 gallon water heater. 

      If two people take a shower at the same time, it will use the 40 gallon tank up quickly with no room to do anything else. A 50 gallon tank will work best in this situation. However, one person showering and one person prepping for dinner for ten minutes will have adequate water from the 40 gallon tank. 

      What size water heater do I need for a family of 4?

      For 4 people, you need at least a 50 or 60 gallon water heater, but you may require a larger one. This decision will have more to do with your lifestyle, daily schedules, and water usage routines. 

      For example, if three people take a shower at the same time in the morning it uses approximately 60 gallons of water right there. Some homes opt for an 80 gallon tank to allow everyone to have a hot shower in the morning. 

      Is there a big difference between a 40-gallon and 50 gallon water heater?

      Yes. Generally speaking, every 10 gallons will accommodate one additional person in the home. If your 40 gallon water heater appears to be working perfectly with 3 people, that does not mean it will work for 4 people. 

      The ten gallons make a significant difference and may be necessary if new people are moving into the household. 

      Is a 50 gallon hot water heater enough for a family of 4?

      If you have a family of four and you want to know what size tank will accommodate your family, consider your household activities. Things to consider include:

      • How many people can shower at one time?
      • How long do people take in the shower?
      • When do you do laundry?
      • Do you have efficient show heads and appliances?

      While a 50 gallon water heater may provide sufficient hot water for a family of 4, this falls on the low end of the estimate. You probably want to upgrade the tank size to a 60 gallon or even an 80 gallon based on your usage. 

      See this post to read our reviews on the best 50-gallon electric water heaters.

      How many people does a 60 gallon water heater support?

      A 60 gallon water heater will generally support 4 to 5 people. 

      Three people taking a shower will use up 60 gallons quickly. However, one person taking a shower in the morning while one person does the laundry will leave plenty of water for two people to take a shower later in the day. 

      How many people per household does a 75-80 gallon water heater support?

      A 75 – 80 gallon water heater will generally support 5 – 6 people. 

      If all 6 people take a shower in the morning, the hot water will be gone. However, most people don’t shower at the same time every morning. Consider each person’s schedule. 

      If an 80-gallon water heater doesn’t suit your needs, you may need to increase it to a 100 gallon water tank when it’s time to replace it. That said, most people find that buying a quality, 75-gallon water heater does the job.

      How many showers can I expect from a 50 gallon water heater?

      Using our metric above, a 50 gallon water heater will cover 2.5 8-minute showers. The third person will run out of hot water. 

      Efficient showerheads can use as little as 12 gallons of water in an 8-minute shower. This means 4 people can shower comfortably without a problem. 

      Since showers take up a lot of the hot water in the morning, buying efficient showerheads can fix the problem in a cost-effective way as opposed to purchasing a brand new water heater. 

      If the water only runs out in the morning when people are getting ready for the day, consider showering at different times, too. 

      Hot Water Recovery Speed

      When considering water heater size, it’s not only about the size of the water heater tank but how quickly it recovers. Every conventional water heater comes with a first-hour rating (FHR). 

      The first hour rating indicates how quickly the tank fills up with hot water after the water is used. 

      A high FHR can make up for a small tank. Even if 2 people use all of the hot water in a 30 gallon tank, new hot water will replace it quickly if the tank has a high FHR. 

      For example, a 30 gallon tank with a FHR of 60 gallons will heat up in 30 minutes. This can accommodate most needs. However, a 30 gallon tank with a FHR of 15 gallons will take 2 hours to heat up. 

      How Many People Can A Tankless Water Heater Support Per Household?

      Previously, we have been discussing water heaters with a tank. Tankless systems on the market take up less space and work more efficiently, saving money on your utility bills. However, can a tankless system support your household? 

      A tankless system provides hot water “on-demand” as opposed to a tank that holds the hot water for when you need it. 

      Tankless water heaters measure capacity through flow rate (gallons per minute). To calculate the flow rate you will need, you need to calculate how many items may run at once and add up their flow rates.  

      See the following for estimated flow rates of standard appliances in gallons per minute (GPM):

      • Faucet – 1.5 GPM
      • Dishwasher – 3.5 GPM
      • Showerhead – 2.5 GPM
      • Washing machine – 4 GPM

      You need a tankless water heater with a flow rate that accommodates all of your needs during the busiest time of day. 

      Tankless water heaters can only produce 200,000 BTUs (roughly 11 GPM). If your needs exceed this, you should stick with a conventional water heater or look into hybrid options.

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      About Plumbing Navigator

      We write about "all things plumbing," helping you navigate common questions, repairs, and the best plumbing products on the market.

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      We write about “all things plumbing,” helping you navigate common questions, repairs, and the best plumbing products on the market.

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