Water Heater Sizing Calculator: Tank vs Tankless Sizing (2026)

A water heater that's too small runs out halfway through the second shower. One that's too big heats and stores water nobody uses. The right size depends less on the number of bathrooms than on how much hot water your household draws in its busiest hour.

A family of four with typical routines needs about a 50-gallon gas tank, or a tankless heater that delivers about 5 gallons per minute. At the calculator's defaults, that household uses about 112 gallons of hot water a day and 34 gallons in its busiest hour. Enter your own household below.

How the calculator estimates hot water use

The calculator adds up your daily hot water from these assumptions:

The shower figure matches EPA WaterSense research, which puts the average shower at about 8.2 minutes; at roughly 2.1 gallons per minute that's about 17 gallons. The usage setting scales the total: low 0.75, average 1.0, high 1.25, very high 1.5.

The calculator then assumes 30% of the day's hot water is used in the busiest hour. That peak hour, not the daily total, is what a tank has to cover.

Sizing a tank: first-hour rating

A tank's first-hour rating (FHR) is how many gallons of hot water it can deliver in an hour, starting full. It's printed on the EnergyGuide label, and it matters more than the tank's volume. A gas tank reheats faster than an electric one, so a gas tank with the same volume has a higher FHR.

The calculator multiplies your peak hour by 1.2 for a safety margin, then works out the tank size that meets it. It assumes about 70% of a tank is usable hot water, and reheating at about 40 gallons per hour for gas and 20 for electric. It also checks a household-size table and recommends the larger of the two answers:

Sizing a tankless heater: flow and temperature rise

A tankless heater has no stored water, so what matters is how much flow it can heat at once. The calculator assumes 2.5 gallons per minute for each fixture running at the same time. That's the federal maximum for a showerhead, so it sizes on the safe side; WaterSense showerheads use 2.0 or less.

Heating that flow takes power. The output needed is gallons per minute × temperature rise × 500, in BTU per hour. The calculator assumes water arrives at 55°F and leaves at 120°F, a 65°F rise.

For the default household's 5 gallons per minute, that's 162,500 BTU per hour of heat. A gas unit at an assumed 88% efficiency needs about 184,700 BTU per hour of input, just under the 200,000 BTU per hour federal limit for residential gas models. ENERGY STAR-certified condensing gas models are rated 0.95 UEF or higher, and non-condensing models rate lower.

An electric unit would need 47.6 kW, about 198 amps at 240 volts, which is why whole-house electric tankless heaters often fall short in cold-water regions. Our tankless sizing guide covers this in detail.

Three households

Tank sizes are for gas. For an electric, heat pump or solar tank, the household table's larger sizes apply.

Running cost by type

Annual energy cost for the default household at $0.18 per kWh and an assumed $1.35 per therm, using the calculator's assumed efficiencies:

A heat pump water heater uses about a quarter of the electricity of a resistance tank for the same hot water. Efficiencies vary by model, so compare the Uniform Energy Factor (UEF) on each unit's label. Solar systems aren't estimated here, because their cost depends on the collector and the climate.

Frequently asked questions

What size water heater does a family of four need?

About a 50-gallon gas tank or a 65-gallon electric or heat pump tank, or a tankless heater delivering about 5 gallons per minute. Big bathtubs or long showers push that up.

What size water heater for two people?

A 30-gallon gas tank or a 40-gallon electric tank covers most two-person households. A tankless heater needs about 3 gallons per minute for one shower at a time.

Is first-hour rating more important than tank size?

Yes. Two 50-gallon tanks can deliver quite different amounts of hot water in an hour, depending on how fast they reheat. The first-hour rating on the EnergyGuide label captures both.

What size tankless water heater for a whole house?

Add up the flow of every fixture you might run at once, then check that the heater can deliver it at your coldest incoming water temperature. See our tankless sizing guide.