General Size Guidelines per household size shown below.
Add 3-1/2 gallon tank capacity for each additional bathroom
In the home. If there is an automatic dishwasher add
5 gallons to the water heater capacity.

1-2 people:        2-3 people:      3-4 people:      5+ people:
30-40 gallons     
40-50 gallons     50-60 gallons  
   60-80+ gallons