...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 was NOT a scientific recommendation. If you're not getting sample tested, most doctors would first direct you to urine color and skin snappiness
What the source says:
Studies have produced varying recommendations over the years. But your individual water needs depend on many factors, including your health, how active you are and where you live.
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No single formula fits everyone.
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Most healthy people can stay hydrated by drinking water and other fluids whenever they feel thirsty.
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Your fluid intake is probably adequate if:
-You rarely feel thirsty
-Your urine is colorless or light yellow