Quote Originally Posted by garlic08 View Post
I carried one 1-liter repurposed soda bottle and one 2-liter Platypus bladder as a reserve and for dry camps. Total empty weight 2 oz, plenty of water for me on the AT the year I hiked.

I often hiked with no water in my pack at all, especially on those days in the South and in New England when my feet were wet all day from walking in fresh Appalachian spring water. I agree with MW above, that may not be too smart, but we all choose our risks, and sometimes have to deal with our fears.
I tend to be be in garlic sauce camp when it comes to water on the AT.

To the OP. Are you planning a thru and if so where and when will you start. The time of year drives how much if any water you will need to carry. Let's assume you will start on a thru in the spring. Generally the temperatures are cooler and the water sources are plentiful. In this case, if you have the databook then you can can easy learn water management with little if any risk. Garlic's 3 liter suggestion is probably more than sufficient to allow you to dry camp anywhere on the trail. I believe you will find that you quickly learn water management if you pay attention to what you are doing and are willing to optimize your approach. Do you hit a marked water source and still have a liter of water? Adjust. You don't need to have everything figured out when you hit the trail.

if you are planning a hike in PA in the late summer then a different approach will be needed. Reliability of sources and distances between sources may require a higher carry.