One month of drinking water , if at least 1 gallon a day (3.6 litres) = 3.6 x 30 = 110litres. The 1 gallon a day is for drinking / cooking and some personal hygiene uses and is a conservative estimate by any means. <br><br>The one gallon plastic bottles are good for storage of tap water etc. Most mineral water can keep for a year or so if kept out of the light. So you might want to consider that. Just don't buy 100 liters at a go with all the same expiry date, if not you'll end up with a lot of wasted water on the expiry date. Buy in several batches and cycle them. If cost is a concern, then tap water in the gallon containers will do just fine. You have to keep them out of the sun and keep cycling them. Also keep other means of water purification handy. <br>Keeping a water filter and water purification tablets at home are always a good backup.<br>And if you need to bug out, I doubt you'll be able to carry out more than a gallon of water on your person. So a water filter and water purification means are really necessary if you cache of water is left behind.