"Drink eight glasses of water a day" is one of the most repeated pieces of health advice, and one of the least evidence-based. Nobody is quite sure where it started, but it's never matched up well with actual physiology — because water needs depend heavily on body size, activity, climate, and diet, none of which the eight-glasses rule accounts for.
Where the number should come from
A more useful starting point scales with body weight — roughly 30–35 ml per kilogram of body weight for a sedentary adult in a temperate climate. That means hydration needs are genuinely different for a 55 kg adult and a 95 kg adult, which a flat "eight glasses" figure never captures.
What pushes the number up
- Exercise. An hour of moderate activity can mean an extra 500–1000 ml, more in heat or heavy sweating.
- Climate. Hot or humid conditions increase fluid loss through sweat even at rest.
- Altitude. Higher elevations increase respiratory water loss.
- Diet. High-sodium or high-protein diets typically raise fluid requirements.
- Pregnancy and breastfeeding. Both meaningfully increase fluid needs.
Food counts too
Roughly 20% of daily water intake typically comes from food, not drinks — fruits, vegetables, soups, and yoghurt all contribute. That's part of why total fluid intake and "glasses of water" were never quite the same measurement to begin with.
Reading your own signals
Thirst is a reasonably reliable guide for most healthy adults, and urine colour is a simple day-to-day check — pale yellow generally suggests adequate hydration, while dark yellow is a signal to drink more. Needing to drink to the point of discomfort or diluting electrolytes to dangerous levels is a real but rare risk, mostly seen in endurance athletes drinking far beyond thirst cues.
Get a starting number based on your own weight, activity, and climate with the water intake calculator.