The "8 Glasses a Day" Myth
The advice to drink eight 8-oz glasses (about 1.9 litres) of water per day has been repeated for decades, but it has no strong scientific basis. It originated from a 1945 U.S. Food and Nutrition Board recommendation that was widely misinterpreted — the original text actually noted that most of this water would come from food.
The truth is that water needs vary enormously between individuals based on body size, activity level, climate, diet and health status. A 50 kg sedentary woman in a cool climate has fundamentally different needs than an 85 kg male athlete training in summer heat.
How to Calculate Your Water Needs
The most widely used evidence-based starting point is approximately 35 ml per kg of body weight per day for sedentary adults in a temperate climate.
• 60 kg → ~2.1 litres/day
• 70 kg → ~2.45 litres/day
• 80 kg → ~2.8 litres/day
• 90 kg → ~3.15 litres/day
• 100 kg → ~3.5 litres/day
Add 500–750 ml for every hour of moderate exercise, and an additional 500 ml in hot weather.
Signs You're Not Drinking Enough
The simplest real-time indicator is urine colour:
| Urine colour | Hydration status | Action |
|---|---|---|
| Pale yellow (lemonade) | Well hydrated | Keep going |
| Yellow | Adequate | Drink a bit more |
| Dark yellow / amber | Mildly dehydrated | Drink water now |
| Brown / orange | Severely dehydrated | Seek help if persists |
Other early signs of dehydration: headache, difficulty concentrating, fatigue, dry mouth, reduced urine frequency (less than 4 times per day).
Does Coffee Count?
Yes — despite popular belief, caffeinated beverages still contribute net fluid to your daily intake. While caffeine has a mild diuretic effect, the fluid in coffee and tea more than compensates. You do not need to "subtract" coffee from your water intake.
However, alcohol is genuinely dehydrating — it suppresses ADH (antidiuretic hormone), causing increased urine output. For every alcoholic drink, add an extra 200–300 ml of water.
Water and Weight Loss
Drinking water before meals can reduce caloric intake by increasing the sensation of fullness. A 2010 study found that drinking 500 ml of water 30 minutes before meals led to 44% greater weight loss over 12 weeks compared to a control group. Additionally, being even mildly dehydrated slows metabolism by 2–3%.