The amount of water you need to store is directly proportional to the length of time you want to live. After air to breathe, water should be the main concern of for your survival. Remember the rule of three: you can go about 3 minutes without air, 3 days without water and 3 weeks without food. But simply understanding that water is important is only the first step. The key is to have enough water on hand to survive. At a bare minimum, you will need one gallon of water per person per day. This amount is for consumption purposes only. It does not allow for the use of water in cleaning or personal hygiene. A better bet is to plan for three gallons of water per person per day. This means a family of four will need to store 84 gallons of water to last a week and about 350 gallons to last a month. As you can see, the volume of water needed is pretty substantial. If you're stockpiling bottled water, that family of four will need 2,800 16-ounce bottles for the month. If you have the space, a more viable option to consider is 50-gallon plastic barrels. For survival beyond a month, storage becomes decreasingly less preferable from a logistical standpoint. In this type of scenario, you'll need to look for natural source collection (rain, streams, rivers, lakes, etc.) and a dependable filtration system to go beyond the initial amount you have stored.