June 22, 2026
How to Determine Required Water Heater Size
Why Knowing What Size Water Heater Do I Need Can Save You from Cold Showers
If you’ve ever been the last one to shower in the morning and felt that dreaded blast of cold water, you already know how much the right water heater size matters. Choosing the correct size is one of the most important decisions you’ll make when replacing or upgrading your home’s hot water system.
Quick Answer: What Size Water Heater Do You Need?
| Household Size | Tank Water Heater | Tankless (GPM) |
|---|---|---|
| 1-2 people | 30-40 gallons | 3-5 GPM |
| 2-3 people | 40-50 gallons | 5-7 GPM |
| 3-4 people | 50-60 gallons | 6-8 GPM |
| 5+ people | 65-80+ gallons | 8-10 GPM |
These are starting points. Your actual needs depend on peak usage, bathroom count, appliance use, and — especially here in central Iowa — your incoming water temperature in winter.
Heating water accounts for roughly 18% of a home’s total energy use. Getting the size right means you won’t run out of hot water during a busy morning, and you won’t waste energy keeping a tank bigger than you actually need warm around the clock.
In this guide, we’ll walk you through everything you need to know — from first-hour ratings and flow rates to how Des Moines winters affect your sizing decision.
What size water heater do I need for my household?
When our customers in Ankeny or Waukee ask us, “what size water heater do i need,” we start by looking at the heartbeat of the home: the people living in it. Household size is the most common metric for sizing, but it’s not just about the number of heads; it’s about the occupancy habits and usage patterns.
A family of four might use 100 gallons of hot water or more per day. However, that doesn’t mean you need a 100-gallon tank. Because your water heater is constantly working to recover and reheat water, the goal is to find a balance between storage and recovery. If you have teenagers who take 20-minute showers or if you tend to run the dishwasher and the washing machine at the same time, your capacity needs will be higher than a household that spreads out its chores.
To get a better idea of the different models available for your specific home layout, you can check out our guide on How to Choose the Right Water Heater.
Recommended tank sizes by occupancy
While every home is unique, we generally follow these industry standards for storage tank water heaters:
- 1–2 People: A 30 to 40-gallon tank is typically sufficient. This size works well for small apartments or couples who don’t use high-flow fixtures.
- 2–3 People: We usually recommend a 40 to 50-gallon tank. This provides enough “buffer” for back-to-back showers.
- Family of Four: A 50-gallon tank is the industry standard for a typical 4-person home.
However, if you have multiple bathrooms or high-demand appliances, a 60-gallon unit might be safer. - 5+ People: For larger families in areas like West Des Moines or Johnston, we recommend 65 to 80-gallon tanks. These larger units ensure that the fifth person in line for the shower doesn’t get a cold surprise.
Simultaneous use and peak demand
The “morning rush” is the ultimate test for any water heater. This is what we call “peak demand”—the single hour during the day when your home uses the most hot water. If two people are showering in different bathrooms while the dishwasher is starting its cycle, your peak demand is hitting its limit.
When determining what size water heater do i need, you must account for appliance efficiency as well. Modern Energy Star-rated dishwashers and front-loading washing machines use significantly less hot water than older models. If your home is equipped with these, you might be able to stick to a standard size. However, if you have a large soaking tub or a rain showerhead, your peak demand will skyrocket. For a deeper look at these variables, see our article on Top Factors to Consider When Choosing a Water Heater for Your Des Moines Home.
Understanding Tank Capacity vs. First-Hour Rating (FHR)
One of the biggest mistakes homeowners make is looking only at the “gallon size” printed on the tank. While storage volume is important, it doesn’t tell the whole story.
To truly answer “what size water heater do i need,” you have to look at the First-Hour Rating (FHR).
You can find the FHR on the top left corner of the yellow EnergyGuide label. This number represents the total number of gallons of hot water the unit can supply in a single hour, starting with a full tank. It combines the water already in the tank with how quickly the unit can heat new water as it enters. For more details on these specifications, browse our Water Heater Types in Des Moines page.
Why FHR is the key to answering “what size water heater do i need”
Think of FHR as the “real world” capacity of your heater. For example, a 50-gallon gas water heater might have an FHR of 90 gallons. This means in one hour of heavy use, you can actually get 90 gallons of hot water out of it because the burner is heating new water while you use the old water.
If your household’s peak hour demand (the sum of all showers, shaves, and appliance uses in your busiest hour) is 70 gallons, you need a water heater with an FHR of at least 70. If you choose a unit based only on the 50-gallon tank size and ignore the FHR, you might find yourself running out of water much sooner than expected.
Recovery rates for gas vs. electric
The fuel source of your water heater drastically changes its recovery rate. Natural gas models generally have a much higher BTU capacity, meaning they can heat water much faster than standard electric resistance models.
- Natural Gas: These units can often recover 40 to 50 gallons per hour.
- Electric: Standard electric units typically recover only 20 to 25 gallons per hour. Because of this slower recovery, we often recommend sizing up the tank by 10 to 15 gallons if you are using electric to ensure you have enough stored hot water to get through peak times.
- Heat Pump Water Heaters: These are two to four times more energy-efficient than conventional electric heaters.
They pull heat from the surrounding air to warm the water. While they are incredibly efficient, they can have slower recovery rates in “efficiency mode,” though most have a “high demand” setting for busy mornings.
If you’re considering switching fuel types during a replacement, check out our Water Heater Replacement Guide for more information.
Sizing for Tankless Water Heaters
Tankless water heaters, or “demand-type” heaters, work differently. They don’t have a tank at all. Instead, they heat water as it flows through the unit. This provides a virtually endless supply of hot water, but there is a limit to how many fixtures it can serve at once.
When sizing a tankless unit, we don’t talk about gallons; we talk about Flow Rate, measured in Gallons Per Minute (GPM.) To learn more about the mechanics, visit our Tankless Water Heaters: Complete Guide.
Calculating GPM to decide what size water heater do I need
To size a tankless unit, you must calculate your “maximum simultaneous demand.” This means adding up the flow rates of every fixture you might use at the exact same time.
- Showerhead: 2.0 to 2.5 GPM
- Kitchen Faucet: 1.0 to 1.5 GPM
- Dishwasher: 1.0 to 2.0 GPM
- Washing Machine: 1.5 to 2.5 GPM
If you want to be able to run two showers (5.0 GPM) and the dishwasher (2.0 GPM) simultaneously, you need a tankless unit capable of delivering at least 7.0 GPM. Most homes in the Des Moines area require a system capable of 6 to 10 GPM to handle a typical family’s needs.
Temperature rise and Iowa winters
This is where sizing gets tricky in Iowa. A tankless heater is rated by how many gallons it can heat at a specific “Temperature Rise” (also called Delta T).
In May 2026, or any other year, the groundwater in central Iowa can get very cold in the winter—sometimes dropping to 40°F. If you want your shower at 110°F, your water heater has to raise the temperature by 70 degrees. As the required temperature rise goes up, the flow rate (GPM) the unit can provide goes down.
A unit that provides 9 GPM in Florida might only provide 5 GPM during a Des Moines January. We always size tankless units based on our local winter conditions to ensure you have hot water year-round.
Key Factors and Signs Your System is Undersized
Sometimes, the standard charts don’t tell the whole story. If your home has “luxury” plumbing, the rules of thumb for what size water heater do i need change quickly.
High-demand fixtures and appliances
If you have a large jetted soaking tub, it might hold 60 to 80 gallons of water. A standard 40 or 50-gallon tank literally cannot fill that tub with hot water in one go. Similarly, high-flow “rain” showerheads or multi-head shower systems can use 5 GPM or more on their own, which will drain a standard tank in minutes.
If you are planning a bathroom remodel in Urbandale or Clive, it is vital to size your water heater for the future fixtures, not just what you have now. If you’re seeing signs of trouble, read our guide on How to Know When to Replace Your Water Heater.
Warning signs of an undersized unit
How do you know if your current system is the wrong size? Look for these red flags:
- The “Last in Line” Problem: If the third person to shower always gets cold water, your tank capacity or FHR is too low.
- Frequent Recovery Lags: If you have to wait 45 minutes between showers for the water to get hot again, your recovery rate is insufficient.
- System Strain: An undersized unit has to work much harder to keep up, which can lead to a shorter lifespan. While tank heaters usually last 8–12 years, an overworked one might fail in 6 or 7.
- Noises and Discoloration: While often signs of sediment buildup, “popping” noises or rusty water can also indicate that a unit is being pushed past its limits.
Before you buy, make sure your home is ready by following our steps on Preparing Your Home for Water Heater Installation.
Frequently Asked Questions about Water Heater Sizing
Is a 40-gallon or 50-gallon tank better for a family of four?
For most families of four, a 50-gallon tank is the better choice. While a 40-gallon tank can work if you have highly efficient fixtures and staggered shower schedules, a 50-gallon tank provides a much-needed margin of error. If you have two bathrooms and everyone gets ready at the same time, the 50-gallon tank (especially one with an FHR of 80+) will prevent those morning arguments over hot water.
How does the Des Moines climate affect my water heater size?
Our climate means our water heaters work harder.
Because our winter groundwater is so cold, your heater uses more energy and takes longer to reach the target temperature than a heater in a southern state. This is why we often recommend units with higher BTU ratings or slightly larger storage capacities for our neighbors in places like Grimes and Indianola.
Can I switch from a tank to a tankless water heater?
Yes, but sizing is critical. You can’t just swap a “50-gallon tank” for a “standard tankless.” We have to calculate your peak GPM and account for the 70°F+ temperature rise needed during Iowa winters. You also need to ensure your gas lines and venting can handle the higher BTU requirements of a tankless system.
Conclusion
Choosing the right water heater size isn’t just about avoiding a cold shower; it’s about energy efficiency, system longevity, and the overall comfort of your home. Whether you are in Altoona, Ankeny, or anywhere else in the Greater Des Moines area, getting a professional assessment is the best way to ensure you don’t overspend on a unit that’s too big or suffer with one that’s too small.
At True Comfort Heating, Cooling and Plumbing, we pride ourselves on providing honest assessments and dependable service. We treat our customers like family, offering flexible maintenance plans and ensuring your home stays comfortable through every Iowa season. If you’re still wondering, “what size water heater do i need,” give us a call or visit our Water Heaters service page. We’ll help you find the perfect fit for your household’s unique needs.