The "8 glasses a day" rule has no scientific foundation. It originated from a 1945 US dietary recommendation that was widely misquoted for decades. Your actual water needs depend on your body weight, activity level, climate, and diet — and they vary significantly from person to person.
What the Research Actually Says
The National Academies of Sciences recommends a total daily water intake of approximately 3.7 litres for men and 2.7 litres for women — but this includes water from all sources, including food. Around 20% of daily water intake typically comes from food. So the drinking target is closer to 3.0L for men and 2.2L for women under average conditions.
💡 Key insight: These are averages for sedentary adults in a temperate climate. If you exercise, live somewhere hot, or sweat heavily, your needs can be 50–100% higher.
Weight-Based Formula
A practical starting point used by sports dietitians: drink 30–35ml of water per kilogram of bodyweight per day. For a 70kg person, that is 2.1–2.45 litres from drinking. For an 85kg person, 2.55–2.975 litres.
| Body Weight | Minimum (30ml/kg) | Active (40ml/kg) |
|---|---|---|
| 50 kg | 1.5 L | 2.0 L |
| 60 kg | 1.8 L | 2.4 L |
| 70 kg | 2.1 L | 2.8 L |
| 80 kg | 2.4 L | 3.2 L |
| 90 kg | 2.7 L | 3.6 L |
| 100 kg | 3.0 L | 4.0 L |
Factors That Increase Your Needs
Several conditions significantly increase daily water requirements:
| Factor | Extra Water Needed |
|---|---|
| Exercise (60 min moderate) | +500–750 ml |
| Hot climate (above 30°C) | +500–1000 ml |
| High altitude | +500 ml |
| Pregnancy | +300 ml |
| Breastfeeding | +700 ml |
| High protein or high fibre diet | +200–400 ml |
The Simplest Hydration Check
Urine colour is the most practical real-time hydration indicator. Aim for pale yellow — like lemonade, not water. Dark yellow or amber means drink more. Completely clear urine can indicate over-hydration, which dilutes electrolytes and is not beneficial.
⚠️ Over-hydration warning: Drinking excessive water, particularly during endurance events, can cause hyponatremia — dangerously low sodium levels. Drink to thirst during exercise, not to a fixed schedule.
When to Drink
Spreading water intake throughout the day is more effective than drinking large amounts at once. The kidneys can process approximately 0.8–1.0 litres per hour. A practical schedule: a glass on waking, a glass before each meal, and steady sipping throughout the day. Drinking 500ml 30 minutes before a meal has also been shown to modestly reduce calorie intake at that meal.