Few health recommendations are as widely repeated — and as poorly sourced — as the advice to drink eight 8-ounce glasses of water per day. The truth about hydration is more nuanced and individualized.

Where the 8-Glass Myth Came From

The origin is likely a 1945 Food and Nutrition Board report that mentioned 2.5 liters of daily water intake — but noted that most of this comes from food. The second half of the recommendation was lost, and a myth was born. There is no scientific study supporting eight glasses as a universal target.

How Much Do You Actually Need?

Water needs vary enormously based on body size, activity level, climate, diet, and health status. A sedentary person in a cool climate needs far less than an athlete training in heat. The best evidence suggests letting thirst guide intake for most healthy adults. Your kidneys are remarkably efficient at maintaining fluid balance.

Signs of Dehydration

Urine color is a reliable indicator — pale yellow means well-hydrated, dark amber suggests you need more fluids. Other signs include headache, fatigue, dry mouth, decreased urine output, and dizziness. Severe dehydration can cause rapid heartbeat, confusion, and requires medical attention.

Can You Drink Too Much?

Yes. Hyponatremia (dangerously low sodium from overhydration) is a real risk, particularly during endurance exercise. Marathon runners and endurance athletes are most at risk. Drinking to thirst, rather than forcing fluids, is the safest approach during exercise.

Beyond Plain Water

All beverages contribute to hydration, including coffee and tea (the diuretic effect is modest and offset by fluid intake). Water-rich foods like cucumbers, watermelon, oranges, and soups also count significantly toward daily intake. You likely get 20-30% of your water from food.