Find the right water heater tank size for your household.
Water heaters are sized on peak hour demand rather than daily use, because running out of hot water is a peak-hour problem. Add up the busiest hour: two ten-minute showers at 2.1 gpm is 42 gallons, plus roughly 8 gallons per person for basins, dishes and laundry. That total is the first hour rating you need — the gallons a heater can deliver in its first hour, which combines the stored volume with how fast it reheats. Because gas recovers about twice as fast as electric, a gas heater's first hour rating is around 1.7 times its tank and an electric one's about 1.4, which is why an electric installation needs a noticeably larger tank for the same household. For tankless, the constraint is simultaneous flow instead of volume.
Water Heater Size
First hour rating = simultaneous showers x flow x 10 min + people x 8 gal; tank = FHR / recovery factor (1.7 gas, 1.4 electric), rounded to a stock size
Water heater installation involves gas, electrical and pressure-relief requirements, and stored water above about 60 C carries a scald risk while below 60 C raises legionella concerns. Follow local code and use a licensed installer.
A 50 gallon gas or 66-75 gallon electric tank typically covers two simultaneous showers plus other use. The gas and electric answers differ because recovery rates do.
It never runs out, but it is limited by flow rather than volume — two simultaneous 2.1 gpm showers need about 5 gpm of capacity, and cold inlet water in winter reduces what a unit can deliver.