Drinking Water Purification and Storage - from LDS.org
You need 1/2 ounce for each pound of body weight (non-active person) and 2/3 ounce for each pound of body weight (active person).
Weight 160 pounds = 80 ounces per day
1 gallon is 128 ounces.
Store 1 gallon water per person per day for 14 days minimum**