The amount of water you need significantly varies from person to person. Size, weight, activity level, clothing, sun exposure and temperature all have an effect on how much water you need. You don't have to follow a specific number, in fact that's foolish because of personal differences as explained above, thirst is a good guide. Always drink some water when you're thirsty. You can drink too much water but most people tend to not drink enough. Don't worry about the weight. Water is heavy but there is zero calories in it so it won't make you fat, in fact having the right amount of water in your system is beneficial to your health.