Six people x 3 days = 18 people-days = 18 gallons/72 liters.

I have one active 50-lb (110kg) dog that drinks 1.5 quarts/liters a day, probably more in hot weather.

I have six cats that probably drink a total of one quart/liter per day (a generous estimation, and they eat dry food).

So, you could probably add one full gallon per day (total) for all your animals.

18 gallons for 6 people for 3 days + 1 gallon per day for pets = 21 gallons.

In your climate, I would call those 21 gallons 'drinking water'. You would need more for washing people and clothes.

Look more closely around your home -- there are often unused or awkward spaces that could be used for water storage. Water stored in several places would be safer than putting them all in one place. And it would spread out the weight.

A fifty-five gallon rain barrel at each corner of your house could easily be the answer to washing water, if not drinking water. How much value do you think those 4 barrels would have if all drinking water stopped flowing from the usual sources?

Sue