Q: Our water heater is on its last legs. I’ve increased the temperature on the tank to its highest setting, but the water coming out just isn’t hot anymore. So, it’s time to buy a new water heater! Our existing tank has a 30-gallon capacity, which was fine when we bought the house 10 years ago, but our family has grown, and we’re now using more water for bathing and laundry. What size water heater do I need?
A: It sounds like you will need a larger water heater this time around. And you’re right: More family members mean more water usage. In fact, “the number of users is key for tank-style water heaters because the output is more or less fixed,” explains Daniel O’Brian, the technical expert for online plumbing retailer SupplyHouse.com. Whether you’re interested in replacing your existing unit with a tank or tankless model, O’Brian has suggestions for determining what size water heater will best meet your needs.
When I faced a similar situation a couple of years ago, I was overwhelmed by the choices. My family had also outgrown our old 30-gallon tank, and I learned that sizing a water heater isn’t just about picking a number—it’s about matching your household’s lifestyle. For instance, if you’ve got teenagers taking long showers or you’re running the dishwasher and laundry at the same time, you’ll need to plan for those peak moments. According to the U.S. Department of Energy, water heating accounts for about 18% of a home’s energy use, so choosing the right size can also save you on utility bills. I found that taking a hard look at our daily routines—like when everyone showers or does chores—helped me make a smarter choice. You might want to jot down your household’s hot water habits to get a clearer picture before shopping.
For a Tank-Style Heater, Household Size Is a Simple Indicator of Hot Water Needs
You can estimate the necessary tank capacity (in gallons) based on the number of people in the house:
- 1 or 2 people—23 to 36 gallons
- 2 to 4 people—36 to 46 gallons
- 3 to 5 people—46 to 56 gallons
- 5 or more people—over 56 gallons (add 10 gallons per additional person)
For the average family of four, a 40-gallon water heater like A. O. Smith’s ProLine Power Vent Gas Water Heater (available from SupplyHouse) should get the job done. But it shouldn’t be your only consideration. You can get a more accurate sense of your family’s hot water needs by digging a little deeper.
I’ve found that these general guidelines are a great starting point, but they don’t tell the whole story. For example, if your family includes someone who loves long, hot baths or you have a high-efficiency washing machine that uses less water, you might need to adjust your estimate. A friend of mine with a family of five went with a 50-gallon tank instead of the recommended 56 gallons because their home had low-flow fixtures, which reduced their hot water demand. Also, consider the energy efficiency of the unit—newer models often have better insulation, which can stretch your hot water supply further. The U.S. Environmental Protection Agency notes that Energy Star-certified water heaters can save you up to $350 annually, so it’s worth checking the energy rating when picking a tank size.
In Addition to Tank Capacity, Consider a Water Heater’s First Hour Rating
The first hour rating (FHR) describes how much hot water a given tank can produce in a single hour when it’s fully heated. This figure reflects the appliance’s efficiency (how quickly the water heater can reheat the water) and gives an idea of what it can handle at a peak hour of usage. For example, the A. O. Smith 50-gallon ProLine Power Vent Water Heater (available from SupplyHouse) has an FHR of 90 gallons, so it will provide up to 90 gallons of hot water in an hour.
You want to look for an FHR that meets or exceeds the number of gallons of hot water needed for all the activities that could be performed simultaneously at the busiest point of the day. Use the numbers below to help you estimate peak hour usage and determine the ideal FHR.
- Bathing or showering (per person)—20 gallons
- Washing hair (per person)—6 gallons
- Washing hands (per person)—2 gallons
- Washing dishes by hand—6 gallons
- Shaving—3 gallons
- Running dishwasher—14 gallons
- Running clothes washer—30 gallons
When I was sizing my water heater, the FHR was a game-changer. I added up the hot water needs for our busiest time—mornings when two people shower, someone washes dishes, and the washing machine is running. That came to about 76 gallons, so I knew I needed a tank with an FHR above that. It’s worth noting that the FHR can vary based on the heater’s power source—gas heaters often recover faster than electric ones, which can boost the FHR. If you’re in a rush, you can also check with a plumber to confirm your calculations. They’ll often use tools like the ones from the Plumbing-Heating-Cooling Contractors Association to fine-tune the estimate based on your home’s specific setup.
If a Larger Tank Won’t Fit in the Existing Space, Consider a Tankless Heater
Thanks to their lack of a hefty storage tank, “tankless heaters, which are normally wall-hung, can fit in tighter areas,” O’Brian explains. For example, a 50-gallon tank water heater can be as much as six feet high and 22 inches in diameter. But a tankless option with comparable output—for instance, the Takagi Tankless Natural Gas Water Heater (available from SupplyHouse)—is just 20 inches high, 14 inches wide, and less than 10 inches deep. If space is limited, a tankless water heater may be the way to go.
I’ll be honest—when I first looked at tankless heaters, I was skeptical about whether they could keep up with our family’s demands. But their compact size is a huge plus, especially if your utility closet is as cramped as mine was. Tankless units also tend to last longer—sometimes up to 20 years compared to 10-15 for tank models, according to the Department of Energy. The catch? They’re pricier upfront, often costing $1,000-$3,000 installed, versus $500-$1,500 for a tank model. But if you’re tight on space or want to save on energy in the long run (they can be up to 34% more efficient for homes using 41 gallons or less daily), it’s worth considering. Just make sure your home’s gas line or electrical system can handle the load—check with a pro if you’re unsure.
To Select the Right Tankless Water Heater, Estimate the Necessary Flow Rate Based on Hot Water Usage and the Required Change in Temperature
Tankless water heaters do not store hot water. Instead, they heat it as it’s needed. “Flow rate is the key spec when sizing a tankless water heater,” O’Brian says. The following information will help you estimate your family’s flow rate needs.
First, Calculate Peak Hot Water Usage
Peak usage is that point in time when you’re consuming the greatest amount of hot water. Use this list of typical flow rates (per fixture) in gallons-per-minute (gpm) to calculate the maximum amount of hot water you would use at once:
- Sink faucet—1 gpm
- Bathtub—3 gpm
- Shower—2.5 gpm
- Dishwasher—3 gpm
- Clothes washer—3 gpm
For example, if your peak usage occurs after dinner when you’re running the dishwasher and taking a shower at the same time (3 gpm + 2.5 gpm), you’ll need a tankless water heater with a minimum 5.5 gpm flow rate.
Next, Determine the Temperature of the Water Entering Your House
Simply turn on a cold water faucet—let it run for a couple of minutes—and then use a thermometer to take the temperature of the cold water. Subtract the cold water temperature from 110 degrees Fahrenheit (the average household hot water temperature) to determine the required temperature rise. For instance, if the cold water temperature is 65 degrees Fahrenheit, the water heater will need to heat the water 45 degrees Fahrenheit in order to reach 110 degrees Fahrenheit, so 45 degrees Fahrenheit is the required temperature rise.
Adjust the Flow Rate by the Required Temperature Rise
“All units should have a chart that indicates the flow of hot water at various temperature rises,” O’Brian says. For example, the Takagi T-D2-IN Natural Gas Tankless Water Heater (available from SupplyHouse) offers a maximum flow rate of 10 gpm—the operative word being “maximum.” In a warm climate where a temperature rise of only 20 degrees is necessary, the Takagi has an efficient flow rate of 10 gpm. If you need the water heater to raise the water’s temperature by 45 degrees Fahrenheit, however, the flow rate falls to about 7 gpm, as indicated on the manufacturer-provided flow rate chart.
For a more visual explanation, check out this SupplyHouse video on sizing tankless water heaters.
Sizing a tankless heater felt like a bit of a science experiment when I went through it. I grabbed a cheap digital thermometer from the hardware store to measure the cold water—turns out, ours was about 60°F, so we needed a 50°F rise to hit that comfy 110°F. One thing I learned is that if you live in a colder climate, like the Northeast, your incoming water might be closer to 50°F, which means your heater has to work harder, lowering the flow rate. A plumber I talked to suggested oversizing the flow rate by about 10-20% to avoid running out of hot water during peak times, especially in winter. Also, if you’re eco-conscious, tankless heaters pair well with low-flow fixtures to maximize efficiency—something I wish I’d known before upgrading my showerheads.