Water Heater Size Calculator
Find the right water heater size for your home: estimate your peak-hour hot water demand, the First Hour Rating (FHR) a tank needs, and the flow rate (GPM) a tankless unit needs, plus annual energy cost estimates.
Water heater size is the hot-water capacity a home needs at its busiest hour. A storage tank is sized by its First Hour Rating (FHR), the gallons of hot water it delivers in one hour starting full, which should match your peak-hour demand. A tankless heater is sized by flow rate (GPM) at the required temperature rise.
How the Water Heater Size Calculator Works
A water heater size calculator determines the optimal tank capacity or tankless flow rate your home needs by analyzing your household's peak hour demand. Peak hour demand is the maximum amount of hot water your family uses during the busiest hour of the day — typically mornings when multiple people shower, the dishwasher runs, and laundry starts. The calculator adds up the gallons used by each fixture and appliance during that peak window, then matches it to the industry-standard First Hour Rating (FHR) to recommend the correct tank size. For tankless systems, it calculates the total gallons per minute (GPM) needed for simultaneous fixtures and the temperature rise required based on your incoming water temperature.
Tank vs Tankless Water Heater Sizing
Tank water heaters store a fixed volume of hot water (typically 30 to 80 gallons) and are sized by First Hour Rating — the gallons of hot water the unit can deliver in the first hour of use. The FHR should match or exceed your peak hour demand. As a rough rule of thumb, a household of 1 to 2 people typically needs a 30 to 50 gallon tank, while families of 5 or more need 60 to 80 gallons. Tankless water heaters heat water on demand and are sized by flow rate in GPM and temperature rise. A standard shower uses approximately 2 GPM, a kitchen faucet uses 1.5 GPM, and a dishwasher uses about 1.5 GPM. To run two showers simultaneously, you need at least 4 GPM — a critical factor when choosing tankless capacity.
How to Work Out Water Heater Size: Peak-Hour Demand Example
The US Department of Energy's Sizing a New Water Heater guide uses a simple worksheet: list each hot-water use in your busiest hour, multiply by the average gallons per use, and add them up. That total is your peak-hour demand. Then pick a storage water heater whose First Hour Rating is within about 2 gallons of it. Here is the DOE's own worked example:
| Use in the peak hour | Avg. gallons per use (DOE) | Times used | Gallons |
|---|---|---|---|
| Shower | 10 | 3 | 30 |
| Shaving | 2 | 1 | 2 |
| Hand dishwashing or food prep | 4 | 1 | 4 |
| Automatic dishwasher | 6 | 0 | 0 |
| Clothes washer | 7 | 0 | 0 |
| Peak-hour demand | 36 |
With 36 gallons of peak-hour demand, DOE suggests a water heater with an FHR of roughly 34 to 38 gallons. The calculator above uses the same 10-gallon shower, 6-gallon dishwasher and 7-gallon laundry figures, adds 2 gallons of hand washing per person, then rounds the FHR up to the next 5 gallons as a safety margin.
Water Heater Size by Household (Derived from the DOE Method)
The DOE does not publish a "people to tank gallons" chart, because two families of the same size can have very different peak hours. The table below simply applies the DOE per-use figures to a common morning: everyone showers in the same hour, plus one 4-gallon food prep or hand dishwashing, with an optional dishwasher cycle. Treat it as a starting point, not a rule.
| People | Showers (10 gal each) | Peak-hour demand | Target FHR (±2 gal) | If the dishwasher also runs (+6 gal) |
|---|---|---|---|---|
| 1 | 10 gal | 14 gal | 12–16 gal | 20 gal |
| 2 | 20 gal | 24 gal | 22–26 gal | 30 gal |
| 3 | 30 gal | 34 gal | 32–36 gal | 40 gal |
| 4 | 40 gal | 44 gal | 42–46 gal | 50 gal |
| 5 | 50 gal | 54 gal | 52–56 gal | 60 gal |
| 6 | 60 gal | 64 gal | 62–66 gal | 70 gal |
Compare these numbers with the First Hour Rating in a model's specifications, not with its tank volume: tank gallons and FHR are different figures, and the FHR is what DOE recommends matching.
Water Heater Size: Tank FHR vs Tankless GPM Compared
Storage and tankless heaters are sized with different units, so a "50-gallon" tank and a "5 GPM" tankless unit cannot be compared directly. DOE's tankless example assumes a 0.75 GPM faucet and a 2.5 GPM showerhead running at once, and 50°F incoming water heated to 120°F.
| Storage tank | Tankless (demand) | |
|---|---|---|
| Size is measured by | First Hour Rating (gallons in one hour) | Flow rate (GPM) at a given temperature rise |
| What you add up | Gallons used in your busiest hour | GPM of fixtures used at the same time |
| DOE worked example | 36 gal demand → FHR about 34–38 gal | 0.75 + 2.5 GPM = at least 3.25 GPM at a 70°F rise |
| Typical capacity at a 70°F rise | Not applicable | About 5 GPM (gas) or 2 GPM (electric) |
| Efficiency vs a storage tank (DOE) | Baseline | 24–34% better at ≤41 gal/day; 8–14% at about 86 gal/day |
| What happens if undersized | Hot water runs out after the FHR is used | Water is cooler when flow exceeds the rating |
Sources: DOE Energy Saver, Sizing a New Water Heater and Tankless or Demand-Type Water Heaters.
Incoming Water Temperature and Energy Costs
Your incoming water temperature directly affects water heater performance and energy costs. In northern US states, groundwater temperatures can be as low as 40°F in winter, requiring a 70°F temperature rise to reach the standard 120°F delivery temperature. Southern states may have incoming water at 65 to 70°F, needing only a 40 to 45°F rise. This difference significantly impacts both the size of tankless heater needed and annual energy consumption. As a rough estimate, a standard 50-gallon electric tank water heater costs approximately $550 to $700 per year to operate, while a gas tank heater costs $350 to $500. Tankless gas units typically cost $200 to $350 annually due to their higher efficiency, though the upfront installation cost is $1,500 to $3,500 compared to $800 to $1,500 for tank systems.
Tips for Choosing the Right Water Heater
Consider future household changes when sizing your water heater — if your family is growing, size up by one tier. Energy Star-certified models use 8 to 15 percent less energy than standard units and qualify for federal tax credits under the Inflation Reduction Act of 2022. For homes with natural gas, a condensing tankless unit offers the best long-term savings. Heat pump water heaters are 2 to 3 times more efficient than conventional electric tanks and work well in warm climates where they also provide free dehumidification. Always verify that your electrical panel or gas line can support the unit before purchasing, especially for tankless installations that may require upgraded gas lines or dedicated electrical circuits.
Last updated: October 2026. Sizing method and per-use gallons: US Department of Energy, Energy Saver.
Frequently Asked Questions
What size water heater do I need for a family of 4?
A family of 4 typically needs a 50 to 60 gallon tank water heater with a First Hour Rating (FHR) of roughly 45 to 60 gallons, depending on how many showers fall in the same hour. This accounts for peak morning usage including 2 to 3 showers, dishwasher, and hand washing. If your family takes baths instead of showers, consider a 60 to 80 gallon tank since a single bath uses approximately 20 gallons compared to 10 gallons for a shower. For tankless systems, a flow rate of 5 to 7 GPM handles most family-of-4 scenarios comfortably.
What is First Hour Rating and why does it matter?
First Hour Rating (FHR) is the number of gallons of hot water a tank water heater can deliver in the first hour of use starting with a fully heated tank. It combines the tank capacity with the burner or element recovery rate. FHR is the most important specification when choosing a tank water heater because it measures real-world delivery capacity during your peak usage hour. The US Department of Energy requires all tank water heaters to display the FHR on the EnergyGuide label. Your peak hour demand should not exceed the FHR of the unit you choose.
How do I determine my peak hour hot water demand?
To calculate peak hour demand, list every hot water activity that occurs during your busiest hour of the day. Typical usage rates are: shower (10 gallons), bath (20 gallons), shaving or hand washing (2 gallons per person), dishwasher cycle (6 gallons), and washing machine load (7 gallons). Add up the gallons for all activities happening in that peak hour. For example, if 3 people shower and you run the dishwasher during the morning rush, your peak demand is 3 times 10 plus 6 equals 36 gallons. Choose a water heater with an FHR that meets or exceeds this total.
What size tankless water heater do I need?
Tankless water heaters are sized by flow rate in gallons per minute (GPM) and required temperature rise. First determine how many fixtures you want to run simultaneously — a shower uses about 2 GPM, a kitchen faucet 1.5 GPM, and a dishwasher 1.5 GPM. Add the GPM for all simultaneous fixtures. Then calculate the temperature rise needed by subtracting your incoming water temperature from 120 degrees Fahrenheit. A typical home needs 5 to 7 GPM with a 50 to 60 degree rise. Gas tankless units handle higher flow rates than electric models and are recommended for homes needing more than 4 GPM.
Is a tankless water heater more efficient than a tank heater?
Yes, tankless water heaters are 24 to 34 percent more efficient than conventional tank heaters for homes using 41 gallons or less of hot water daily, according to the US Department of Energy. They eliminate standby heat loss — the energy wasted keeping a tank of water hot 24 hours a day. However, for homes using over 86 gallons per day, the efficiency advantage drops to 8 to 14 percent. The annual energy cost for a tankless gas unit averages $200 to $350 compared to $350 to $500 for a gas tank. The higher upfront cost of $1,500 to $3,500 versus $800 to $1,500 for tank systems means the payback period is typically 6 to 12 years.
How does incoming water temperature affect water heater sizing?
Incoming water temperature varies by geographic region from about 40 degrees Fahrenheit in northern states during winter to 70 degrees in southern states. Lower incoming temperatures require a greater temperature rise to reach the standard 110 to 120 degree delivery temperature, which directly affects heater sizing. For tankless heaters, a unit rated at 7 GPM with a 35-degree rise may only deliver 4 GPM with a 70-degree rise. For tank heaters, colder inlet water means slower recovery time between uses. This calculator factors in your inlet temperature to give accurate recommendations for your specific location.
What size water heater do I need?
Size a storage water heater by peak-hour demand, not by tank volume. Using the US Department of Energy method, add up the hot water used in your busiest hour: about 10 gallons per shower, 2 for shaving, 4 for hand dishwashing or food prep, 6 for an automatic dishwasher and 7 for a clothes washer. Then choose a model whose First Hour Rating is within about 2 gallons of that total. For a tankless heater, add the flow rates (GPM) of fixtures used at the same time and check the unit's GPM at your required temperature rise.
Is a 40-gallon water heater big enough for a family of 4?
It depends on the heater's First Hour Rating, not its 40-gallon tank volume. If all four people shower in the same hour, DOE per-use figures give 40 gallons for showers plus about 4 gallons for food prep, or 44 gallons of peak-hour demand, so you would look for an FHR of roughly 42 to 46 gallons. Running the dishwasher in that same hour adds about 6 gallons. A 40-gallon model is enough only if its listed FHR covers your peak-hour total.