Water Heater Size Calculator
Choosing the right water heater size avoids two frustrating outcomes: running out of hot water mid-shower with an undersized tank, or wasting energy keeping a large volume of water hot all day with an oversized one. The US Department of Energy recommends using the First Hour Rating (FHR) method to match a storage tank water heater to your household's peak demand. FHR represents the total hot water a heater can deliver in one busy hour starting from a full, hot tank. You estimate FHR by adding 12 gallons per person in the household to the gallons associated with your simultaneous peak uses (for example, two people showering and running the dishwasher at the same time). The calculator then recommends the smallest standard residential tank size (30, 40, 50, 65, or 80 gallons) whose published FHR meets or exceeds your calculated need. For tankless sizing, a simpler rule of 2.5 GPM per person gives a starting point for selecting a demand water heater. Always check the EnergyGuide label FHR figure when comparing specific models, as FHR varies between gas and electric units of the same tank capacity.
How the First Hour Rating formula works
The US DOE's FHR method estimates peak hot-water demand by combining a per-person base use with the gallons consumed by appliances running simultaneously during the busiest hour. The recommended tank is then the smallest standard size that exceeds the FHR.
FHR (gal) = (number of people x 12) + peak simultaneous use gallons
Tankless GPM = number of people x 2.5
Tank sizes: 30 / 40 / 50 / 65 / 80 gal (round up to next standard size)
Worked example
A household of 3 people with peak use of 2 showers plus dishwasher (25 gal):
- FHR = (3 x 12) + 25 = 36 + 25 = 61 gallons
- Next standard tank at or above 61 gal = 65-gallon tank
- Tankless sizing = 3 x 2.5 = 7.5 GPM
On the EnergyGuide label, look for a gas 65-gallon model with FHR at or above 61 gallons, or an electric model with the same or higher FHR (electric recovery is slower, so some 65-gallon electric units may have FHR below 61 gallons).
Water heater size calculator: frequently asked questions
What is First Hour Rating and how does it determine tank size?
First Hour Rating (FHR) is the number of gallons of hot water a storage tank water heater can supply in one hour starting with a full tank at temperature. It accounts for both the stored hot water in the tank and the recovery (reheating) capacity of the heating element or burner during that hour. The US DOE recommends matching FHR to the peak hour demand of the household: estimate 12 gallons per person in the home plus the gallons associated with simultaneous peak uses such as showers, dishwasher, and laundry. Look for the FHR figure on the EnergyGuide label attached to every tank water heater sold in the US.
How do tank and tankless water heaters compare in sizing?
Tank water heaters store preheated water and are sized by FHR in gallons. Tankless (demand or instantaneous) water heaters heat water on demand and are sized by flow rate in gallons per minute (GPM) at a given temperature rise. A tankless unit must handle the simultaneous flow from all hot water fixtures running at once. A rough guide is 2.5 GPM per person in the household. Tankless units are generally more energy-efficient because there is no standby heat loss, but they have a higher purchase and installation cost, and may require upgrading the gas line or electrical panel.
What standard tank sizes are available?
Residential storage water heaters are manufactured in standard sizes of 30, 40, 50, 65, and 80 gallons for most gas and electric models. A 30-gallon tank suits one or two people with light hot-water use. A 40-gallon tank is common for two to three people. A 50-gallon tank covers three to four people with moderate use. Larger 65- and 80-gallon tanks suit four or more people or households with high simultaneous demand. Always confirm the FHR on the EnergyGuide label rather than relying on tank capacity alone, since recovery rate varies by fuel type and heating element power.
What is a good recovery rate for a water heater?
Recovery rate is the number of gallons per hour the heater can raise by 90 degrees Fahrenheit (the standard recovery temperature rise used on EnergyGuide labels). A typical 40-gallon gas water heater recovers around 40 to 50 gallons per hour, giving an FHR of 70 to 80 gallons. A same-size electric water heater typically recovers only 18 to 25 gallons per hour because electric resistance elements have lower power than gas burners, giving a lower FHR of 55 to 65 gallons. If your household has high morning demand, a gas model or a heat pump water heater (which has very high efficiency and good recovery) may be more suitable.
What is the Energy Factor or Uniform Energy Factor for water heaters?
The Uniform Energy Factor (UEF) replaced the older Energy Factor (EF) as the US DOE's standard efficiency metric for residential water heaters as of 2017. UEF measures the ratio of useful energy delivered to total energy consumed under a standard test cycle. Higher UEF means lower operating costs. Standard electric resistance water heaters have UEF around 0.90 to 0.95. Heat pump water heaters have UEF of 2.0 to 4.0, meaning they deliver two to four times the heat energy for each unit of electricity consumed. ENERGY STAR certifies water heaters meeting minimum UEF thresholds by product category.
Official sources
- US DOE water heating: Energy Saver: Water Heating.
- ENERGY STAR water heaters: Certified Water Heaters.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology. Always verify FHR on the EnergyGuide label of the specific model.