When SHTF, you don't want to have to be worrying about whether or not you have enough potable water stored for you and your family to get through the situation comfortably or even with your full health intact. You'll want to have more than enough water so that it's not even a real concern.
With a gallon a day per person to shoot for, this means you'll have to plan ahead with what sorts of situations you're more likely to see in which you'll need to use your water stores.
Knowing now how much water you'll need to store, now comes the problem of how you'll store that much water. Of course, you could purchase gallons and gallons of distilled water, but that'll quickly run your bill up to numbers you didn't know existed. Especially if you replace them as often as you're supposed to.
A better method is to use tap water. But there's a bit of a trick to it. Learn how on the next page.