Per person for a family of 3 = 45 gallons, tougher in apartments or condos but no sweat in single family homes. In the PNW we operate with 3 gal per day by (hopefully) ~9 days before a water truck might hit your neighborhood once, and we're well away from coastal areas that will be without for months from a Cascadian Subduction Zone quake. I have given up on calculating how much to store for my nuclear family of 4, instead each Christmas I give out boring 5, 15 or 30 gal containers (last 2 with simple pumps) to my family members in adjoining zip codes. Together we (45 of us) have sufficient water to avoid dying of thirst, assuming some or all of it doesn't end up under wreck and rubble. I personally store hundreds of gallons, as I anticipate helping out two aging neighbors and a handful of friends who aren't so prepared, and also anticipate friends and acquaintances caught on this side of the now broken floating bridges. They will need a place to safely bunk, and I would rather not have their inconvenience threaten my supplies.