Get a general recommended water heater tank size based on household size.
This uses a commonly cited rule-of-thumb table matching household size to a recommended storage tank water heater size, based on typical simultaneous hot-water demand for that many people.
The recommendation scales up with household size because more people generally means more overlapping hot-water use — showers, laundry, and dishwashing happening close together or at the same time.
This assumes a standard tank-style water heater. Tankless (on-demand) water heaters are sized completely differently, by flow rate rather than storage volume, since they don't store hot water at all.
A household of 4 people is generally recommended a 50-60 gallon storage tank water heater.
Not precisely — households with unusually heavy simultaneous hot water use (large soaking tubs, multiple showers running back to back) may want to size up from this general guideline.
Tankless units are sized by maximum flow rate (gallons per minute) they can heat on demand, not storage capacity — this calculator's guidance applies to traditional tank-style heaters only.
Indirectly — in colder climates, incoming water is colder, so a water heater has to work harder to reach the target temperature, which can effectively reduce a given tank's real-world capacity for back-to-back use.