This is based on the current IOM (Institute of Medicine) recommendation. It takes into account all of the fluid intake a day, including water from the food you eat.
Are you drinking enough water? See if you are dehydrated based on your urine colour. This should not replace advice of health professionals.
It is important to drink plenty of water throughout the day even when you are not thirsty.
Economy class syndrome is the formation of blood clots in veins deep within the legs (deep vein thrombosis - DVT) occurring during or just after a long haul flight.