Help please! is drinking 5 litres of water a day too much for a 16 year old?

Sounds like a lot: That sounds like a lot of fluid to take in, but are they super active and sweating a lot? People need increased fluids if very physically active. If that's not the case, and this is a new thing, you may to have the child seen by their doctor to make sure there isn't something else going on, like new onset of diabetes. http://voices.yahoo.com/water-intoxication-much-water-too-much-201296.html.

Just an FYI....: To be uber precise, maintenance hydration for a non-infant child, and adults with a reasonable lean body mass is 1500 mls/m2/day (m2 is short for meters squared of body surface area). You can determine bsa by multiplying height in centimeters by weight in kilograms, dividing by 3600 and then taking the square root of that result. The average adult male is about 1.73 m2. That is abou 2.5 liters/day.