You've probably heard "drink 8 glasses of water a day." It's simple, memorable, and mostly wrong. Water needs vary enormously between individuals based on body size, activity, climate, and diet. There is no universal prescription — but there is a formula.
Why the 8-Glasses Rule Is Oversimplified
The "8×8" rule (eight 8-ounce glasses = about 1.9 litres) has no strong scientific backing. The National Academies of Sciences recommends total water intake of 3.7 litres per day for men and 2.7 litres for women — but this includes water from food, which accounts for around 20% of daily intake.
A 50kg sedentary woman in a cool climate needs far less water than a 95kg male construction worker in a tropical climate. One number cannot serve both.
A Science-Based Formula
A practical evidence-based starting point is 35ml of water per kilogram of body weight per day at rest in a temperate climate. From there, adjust for:
| Factor | Adjustment |
|---|---|
| Lightly active (walking, light exercise) | +350ml |
| Moderately active (regular gym training) | +700ml |
| Very active (intense daily training) | +1,000ml |
| Hot climate | +600ml |
| Temperate climate | +300ml |
Signs of Dehydration
The simplest indicator of hydration status is urine colour. Pale straw-yellow indicates good hydration. Dark yellow or amber means drink more. Clear urine suggests you may be over-hydrating (which can dilute electrolytes).
Early dehydration — losing just 1–2% of body weight in fluid — can impair cognitive performance, reduce exercise capacity, cause headaches, and increase fatigue. Thirst is a lagging indicator; by the time you feel thirsty, you're already mildly dehydrated.
Does Coffee and Tea Count?
Yes. Despite the myth, moderate caffeine consumption (up to 400mg/day — about 4 cups of coffee) does not cause net dehydration. The water in caffeinated beverages more than offsets any mild diuretic effect. Tea, coffee, and even milk all contribute to your daily fluid intake.
Water-Rich Foods
Roughly 20% of daily water intake comes from food. Foods with high water content include cucumber (96% water), lettuce (95%), tomatoes (94%), watermelon (92%), and oranges (87%). A diet rich in fruits and vegetables meaningfully reduces your drinking requirement.
When to Drink More
- Before, during and after exercise — aim for 500ml before a workout
- In hot or humid weather — increase by at least 500–1,000ml
- During illness involving fever, vomiting or diarrhoea
- During pregnancy or breastfeeding (add 300–700ml above baseline)
- After alcohol consumption — alcohol suppresses ADH, increasing urine output