What size water heater do I need?

For a storage tank, size by household: about 30 gallons for 1 person, 40 for 2, 50 for 3–4, and 65–80 for 5 or more. Gas tanks recover faster than electric, so a smaller gas tank can serve the same household. For tankless, size by flow rate instead.

Add up the flow of the hot-water fixtures you’d run at once (about 2.5 GPM each), then check the incoming water temperature — the colder it is, the bigger the temperature rise to 120°F and the lower a unit’s usable flow. Pick a unit rated for your GPM at that rise.

Tankless heats on demand, never runs out, and saves space and standby energy, but costs more up front and has a flow-rate ceiling. Tank is cheaper to install and delivers a big burst easily, but can run out and loses standby heat. Household size and simultaneous demand drive the choice.