Water Heater Size Calculator: What Size Do You Need

First Hour Rating: The Right Metric for Tanks

The first hour rating measures how many gallons of hot water a tank water heater can deliver in the first hour of heavy use, starting with a full tank at operating temperature. This metric accounts for both the tank capacity and the recovery rate (how fast the unit reheats incoming cold water). A 40-gallon gas tank with a high-output burner may have an FHR of 70 gallons, while a 50-gallon electric tank with standard elements may have an FHR of only 58 gallons. The gas 40-gallon actually delivers more hot water in the peak hour despite holding 10 fewer gallons.

The FHR is printed on the yellow EnergyGuide label on every new water heater. When comparing models, compare FHR to FHR rather than gallon capacity to gallon capacity for an accurate picture of real-world performance.

Calculating Your Peak Hour Demand

To determine the right FHR for your household, estimate your hot water usage during the busiest hour of the day (typically the morning rush). Add up the approximate gallons used by each activity happening during that peak hour.

Typical hot water usage per activity: a shower uses 10 to 15 gallons (with a low-flow showerhead, closer to 10; with a standard head, closer to 15). Shaving with running water uses 2 to 3 gallons. A hand and face washing uses 2 gallons. A dishwasher cycle uses 6 to 10 gallons. A load of laundry in warm or hot water uses 7 to 12 gallons. General food preparation and cleanup uses 3 to 5 gallons.

For a four-person household where two people shower (25 gallons), one person shaves (3 gallons), and the dishwasher runs (8 gallons) during the morning rush, the peak hour demand is approximately 36 gallons. A water heater with an FHR of 40 or higher would meet this demand comfortably, which means a 40-gallon gas tank or a 50-gallon electric tank would be adequate. If a third person also showers during that same hour, the demand jumps to 48 gallons, requiring a unit with an FHR of 50 or higher.

Sizing for Tankless Units

For tankless water heaters, the sizing metric is flow rate in gallons per minute (GPM) combined with temperature rise. A tankless unit must heat the incoming cold water to the desired output temperature at whatever flow rate the household demands simultaneously.

The temperature rise is the difference between the incoming cold water temperature and your desired hot water temperature (typically 120 degrees Fahrenheit). In warm climates with incoming water at 65 degrees, the required rise is 55 degrees. In cold climates with incoming water at 40 degrees, the required rise is 80 degrees. The higher the required temperature rise, the fewer GPM the unit can deliver.

Typical fixture flow rates: a shower uses 1.5 to 2.5 GPM, a bathroom faucet uses 0.5 to 1.5 GPM, a kitchen faucet uses 1.0 to 2.0 GPM, and a dishwasher uses 1.0 to 1.5 GPM. To size a tankless unit, add up the flow rates of all fixtures that might run simultaneously during peak demand. If two showers and a kitchen faucet might run at the same time, you need 4.5 to 7 GPM. Choose a unit that can deliver that flow rate at your local temperature rise.

Quick Sizing Guide by Household

One to two people: a 30 to 40-gallon tank with an FHR of 40 or higher, or a tankless unit rated for 3 to 5 GPM. Two to three people: a 40 to 50-gallon tank with an FHR of 50 to 60, or a tankless rated for 5 to 7 GPM. Four to five people: a 50 to 65-gallon tank with an FHR of 60 to 80, or a tankless rated for 7 to 9 GPM. Six or more people: a 65 to 80-gallon tank with an FHR of 80+, or a tankless rated for 9 to 11 GPM (gas models only at this capacity).

These are general guidelines. Your actual needs depend on your specific usage patterns, fixture flow rates, and whether demand is staggered or simultaneous. A household of five where showers are staggered over two hours has lower peak demand than a household of four where everyone showers within the same 45-minute window.

Oversizing vs Undersizing

An undersized water heater runs out of hot water during peak demand, which is the most common complaint from homeowners. Running out of hot water during a morning shower is uncomfortable and frustrating. If you are between sizes, go with the larger option. The cost difference between a 40 and 50-gallon unit is only $50 to $200, a small price for reliable hot water.

An oversized unit wastes energy by keeping more water hot than you need. A single person with an 80-gallon tank is paying to keep 80 gallons at 120 degrees around the clock when they only use 20 to 30 gallons per day. The standby heat loss on the extra capacity is wasted money. The energy cost of oversizing a tank by one size (40 to 50 gallons) is approximately $20 to $40 per year, which is a reasonable insurance premium against running out of hot water. Oversizing by two or more sizes wastes significantly more.

For heat pump water heaters, oversizing has a smaller energy penalty because the unit's high efficiency means standby losses cost less per gallon. Some installers recommend choosing a slightly larger heat pump model to ensure the heat pump mode can handle demand without frequently falling back to the less efficient electric resistance backup mode.