…because the shape of the water bottle isn’t uniform either. Part of my new year’s resolution is to drink 4 liters of water per day, so I spend a lot of time looking at those lines. Edit: 4 liters is a lot, but appropriate for my size and activity level. Without conscious consideration I don’t drink enough water. It looks blue because the bottle is blue.
That doesn’t sound right at all, as obviously it would depend on the size of your body. The recommendation I’ve heard is 30-40 millilitres per kilogram of body weight, so by that to need 4 litres per day you’d have to weigh 115 kilograms.
I’m not entirely convinced by body weight either though, because e.g. perspiration is affected more by the body’s surface area than mass, and surface area does not grow linearly with body mass. Water loss via respiration is probably the same regardless of your size, because your lungs are still the same size. Cellular metabolism, I imagine, doesn’t scale linearly either because as you get bigger you don’t get more cells, the existing ones just get bigger, but I know very little about this.