What size water heater do I need?

1. Tank size vs. First Hour Rating

A water heater has two size numbers that matter: the tank capacity in gallons (how much hot water it stores) and the First Hour Rating (FHR) in gallons per hour (how much it can deliver in a busy hour, combining storage and recovery). The DOE recommends sizing primarily by FHR, because a gas unit can reheat fast enough that a smaller tank delivers as much hot water in a heavy morning as a larger electric tank.

In practice, most people size by tank capacity and match the FHR as a cross-check. Use the First Hour Rating calculator to check whether a specific unit covers your morning peak.

2. Sizing by number of people

The table below is based on DOE and Energy Star sizing guidelines, which are sized to typical US household usage. They are guidelines, not code minimums, and assume one shower per person per day plus normal kitchen use.

Source: U.S. Department of Energy sizing guidelines and Energy Star. FHR (First Hour Rating) is printed on the unit's EnergyGuide label.

3. Why gas and electric need different sizes

Gas burners heat water two to three times faster than electric heating elements. A standard gas heater with a 40,000 BTU/hr burner at 80% efficiency recovers roughly 55 gallons per hour. A 4.5 kW electric element recovers only about 25 gallons per hour. That is why the same household needs a larger tank with an electric unit: the tank compensates for the slower reheat.

Heat pump water heaters are an exception — they move heat rather than generate it, so a heat-pump unit can reach efficiencies of 2–4×. They reheat more slowly than gas but more efficiently than a standard electric element, so sizing follows the electric column but operating costs are much lower.

4. How to size yours in three steps

1. Count the people and note the fuel type. Use the table in Section 2 to find the recommended range for your household. Gas units can use a smaller tank; electric units need more storage to cover the same morning rush.
2. Check the First Hour Rating on the label (printed on the EnergyGuide label required by US law on every new water heater). FHR = usable storage + recovery in the first hour. If the FHR matches your busiest-hour demand, you won't run cold.
3. Add a usage cushion if your habits are heavy. Back-to-back long showers, a large bath, a household that runs the dishwasher during the morning peak — any of these push you toward the high end of the range. When in doubt, choose the larger size; the cost difference between a 50-gallon and a 60-gallon tank is modest, and running out of hot water is genuinely unpleasant.

Or just use the calculator: Water heater size calculator → Enter your household size and fuel type and it returns the recommended tank size and FHR target directly.

5. Small water heaters: point-of-use and under-sink

Point-of-use (POU) water heaters — those compact 6–20 gallon tanks often installed under a kitchen sink — exist to eliminate the long wait for hot water at a remote tap, not to supply a bathroom. They store far too little for a shower. A typical shower draws 15–20 gallons (55–75 L) of hot water, which is several times the capacity of an under-sink unit. These small heaters are useful extras but cannot replace the main whole-house unit.

For tankless (on-demand) water heaters, sizing is by flow rate in gallons per minute (GPM) or litres per minute, not by tank volume. Combi boilers and inline instantaneous heaters are sized by simultaneous outlet demand, not storage.

6. Common sizing mistakes

• Replacing like-for-like without checking usage. If the old unit never kept up, replacing it with the same size guarantees the same shortfall. Take the opportunity to size correctly.
• Ignoring fuel type. Gas and electric units of the same nominal capacity deliver very different amounts of hot water per hour. A 40-gallon gas unit often beats a 50-gallon electric unit over the morning rush.
• Going too big. An oversized tank keeps a large volume of hot water warm around the clock, wasting energy on standby losses. Match the size to your actual demand — not to your maximum imaginable demand.
• Confusing nominal capacity with delivered hot water. A 50-gallon tank holds 50 gallons but you can only use about 70–80% before it runs cold (the rest is mixed with cool incoming water near the bottom). The FHR number on the label accounts for this; the nominal capacity figure does not.

Common questions

Is a 50 gallon water heater enough for a family of four?

Yes, for most families of four a 50-gallon gas water heater is sufficient. Electric units recover more slowly, so a 50-60 gallon electric tank suits the same household. If your family runs multiple showers back-to-back, a 60-gallon electric or a unit with a high First Hour Rating is the safer choice.

What size water heater do most homes use?

The 50-gallon (190 L) tank is the most common size in US and Canadian homes, sized for a household of 3–4. It comfortably meets typical morning demand for a family using a gas burner; electric households of the same size often step up to 50–60 gallons.

Is a 7 gallon water heater enough for a shower?

No. A typical shower uses 15–20 gallons (55–75 L), so a 7-gallon tank runs out in the first minute. These small point-of-use or under-sink heaters are designed for a single sink only — to cut the wait for hot water at a remote tap — not for bathing.

Should I buy a 50 gallon or 75 gallon water heater?

A 50-gallon tank is right for most families of 3–4. Choose 75 gallons for a household of 5 or more, for an electric unit with a family of 4 who uses lots of hot water, or if you run a large jetted tub regularly. The extra capacity costs more to heat but prevents cold-water surprises.