Peak hour demand, not tank size, is the real number
Water heaters are not sized by how much hot water you use in a day. They are sized by your peak hour demand: the single busiest hour, usually the weekday morning when showers, a shave and the dishwasher stack up. A workable rule of thumb is about 12 gallons per person, plus a few gallons for every bathroom past the first to allow for overlapping use. A household of four with two bathrooms lands somewhere near 50 gallons in that busy hour.
For a storage tank, match that number against the first-hour rating (FHR) on the yellow energy label, not the tank's nominal capacity. FHR is what the heater can actually deliver in one hour: the hot water already in the tank plus whatever it can reheat while you draw. Because of that recovery, a tank typically delivers noticeably more than its rated gallons in the first hour, so a 40 gallon gas tank can often cover a 50 to 60 gallon peak. Gas units recover faster than electric ones, so an all-electric home usually needs a physically larger tank for the same household.
A tankless unit is sized on flow instead: how many gallons per minute it must heat at once, and by how many degrees. That second half is temperature rise — your target temperature minus the incoming ground water temperature. The same unit produces far fewer gallons per minute in a cold northern winter than in a warm climate, so always read the manufacturer's flow chart at your rise rather than the headline GPM on the box. Two showers running together is the usual worst case to design for.
Estimates for planning only. Sizing, venting, gas line capacity, electrical supply and temperature limits should be confirmed by a licensed plumber, and scald risk means the setpoint should follow local code and the manufacturer's instructions.