Mark asked me about water while boondocking.
I carry four 1 gallon screw top water jugs. They are easy to refill anyplace you stop for gas, or the refill station in Walmart, or a house. (The pop top water jugs pop their tops on bumpy roads.) I also carry a 5 gallon container. Several portable plastic hiking containers. A 2 quart plastic container for the “sun tea”.
Please realize that my water needs are not anyone else’s. #1. I hate drinking water. #2. I am not very concerned about washing anything with water. Even my face or hair. Much less the dishes. #3. I started out with 7 more gallons, but felt the weight/gas wasn’t worth it. #4. The truck is very well maintained. I check the hoses and fluids frequently.
I can get by* for 14 days, with a couple of gallons left. I try to follow the rules and stay no more than the 14 days in any BLM or NF. I drink club soda, teas, juices, coffee, ensure, pediolite drinks, Gatorade from powder. Water is the last voluntary choice. I double the use of most water … boiling soft boil eggs in the coffee water. Etc.
So …. I have no idea how much you should carry. You will need to over-supply your water needs until you feel confident in your ability to judge. It also depends on your style/intent of traveling. Sitting by the river reading a scholarly book doesn’t require extra water. Bouldering or kayaking require lots more. It also depends on where you are traveling. Desert and high altitudes affect your need for water. It may depend on your personal hygiene level. It will depend on how much attention you pay to your water consumption.
Advice: Take lots more than you think you might need. Take less next time.
*”getting by” just means if there is enough water for coffee.