Proper hydration is essential for virtually every bodily function. But "8 glasses a day" is a myth — your ideal water intake depends on body weight, activity, climate, and health status.
Formula-Based Approach
The most common evidence-based recommendation:
Daily water intake = Body weight (kg) × 35 ml
Example: 70 kg person:
70 × 35 = 2,450 ml = 2.45 litres per day
Alternative: use 30–40 ml per kg, adjusting for activity level.
Reference Table by Body Weight
| Weight | Low activity | Moderate | High activity |
|---|---|---|---|
| 50 kg | 1.5 L | 1.75 L | 2.5 L |
| 60 kg | 1.8 L | 2.1 L | 3.0 L |
| 70 kg | 2.1 L | 2.45 L | 3.5 L |
| 80 kg | 2.4 L | 2.8 L | 4.0 L |
| 90 kg | 2.7 L | 3.15 L | 4.5 L |
Adjusting for Exercise
Add 500–750 ml for every hour of moderate exercise. For intense exercise or hot conditions, add up to 1 litre per hour.
Sweat rate varies greatly — check urine colour as a guide (see below).
Adjusting for Climate
- Hot weather: Add 500 ml–1 litre per day
- Humid conditions: Add 200–500 ml
- Air-conditioned offices: May increase loss slightly
The Urine Colour Test
The simplest hydration check — no maths required:
| Colour | Hydration status |
|---|---|
| Pale yellow (straw) | Well hydrated ✓ |
| Dark yellow | Mildly dehydrated |
| Amber/orange | Dehydrated — drink now |
| Brown | Severely dehydrated or medical issue |
| Colourless | Overhydrated — reduce intake |
Does Coffee and Tea Count?
Yes, with caveats:
- Moderate caffeine intake (up to 400 mg/day = 4 coffees) contributes to hydration
- The mild diuretic effect of caffeine does not outweigh the fluid intake
- Alcohol is dehydrating — for each alcoholic drink, add 250 ml of water
Signs of Dehydration
- Headache
- Fatigue or difficulty concentrating
- Dark urine
- Dry mouth
- Muscle cramps
Overhydration Warning
Drinking excessive water (hyponatraemia) can dilute blood sodium to dangerous levels. This is rare in healthy people but a risk during endurance sports. Do not drink beyond thirst if you are exercising for less than one hour at moderate intensity.