Experts say you need 1 gallon per person for 3 days; however, if you live in an earthquake zone they recommend a lot more! Try to have at least 10 days worth of water per person if you can. After the 6.7 earthquake in Northridge there were thousands of homes that didn't have access to a freshwater supply for two weeks. Larger earthquakes could damage aquaducts and pipelines. If you live in an earthquake zone it's better to have more water. ps. Avoid using old milk jugs or juice containers (they're more likely to grow bacteria).