When a disaster strikes, we won't want to be worrying about whether or not we have enough portable water stored. There should be enough to get a person and their family through a survival situation comfortably without threatening their health. They'll need to have more than enough water so that it'll won't be an issue.
With a gallon per day per person to plan for, this means we'll need to adequately plan ahead when storing water and consider a variety of survival situations. Think of the type of scenarios we'll likely need to use our water storage the most.
Knowing ahead of time how much water we'll need to store also comes with another problem — how to store that much water. Of course, there's the option of buying gallons and gallons of distilled water, but that could get expensive. Especially since it's important to replace water every so often so that it stays good.
A much better method is to utilize tap water, however, there's a bit of a trick to it. Learn how to use tap water as a water source to have on hand when a collapse happens after the break.