How Much Water Do You Really Need?
The popular "8 glasses a day" is a simplification — fluid needs scale with body size and activity. A common evidence-informed baseline is 30–35 ml per kilogram of body weight per day; this calculator uses 33 ml/kg and adjusts for your activity level.
Worked Example
A 70 kg person with moderate activity: 70 × 0.033 ≈ 2.3 liters per day — about 9 glasses of 250 ml. An athlete of the same weight lands closer to 3.2 liters.
What Raises Your Needs
- Exercise: add roughly 0.5–1 liter per hour of sweaty activity.
- Heat and altitude: hot climates and elevations above ~2,500 m increase losses.
- Pregnancy and breastfeeding: guidelines add ~0.3 and ~0.7 liters respectively.
- Illness: fever, vomiting and diarrhea deplete fluids quickly.
Water from Food Counts
Roughly 20 % of fluid intake comes from food — fruit, vegetables, soups. Coffee and tea count toward hydration too; the diuretic effect of normal caffeine amounts is smaller than the fluid they deliver. The simplest self-check remains urine color: pale yellow signals good hydration.
People with kidney or heart conditions should follow their doctor's fluid recommendations.