Hydration Calculator

The famous "8 cups of water a day" rule is too simplistic. Real water needs depend on body weight, physical activity, climate, diet, and individual physiology. This calculator uses the most accepted reference — 35 ml per kg of body weight as baseline + 350 ml extra per 30 min of exercise — and shows the result in liters, cups (250 ml), and bottles (500 ml) for easy tracking.

Formula:

Daily water (ml) = Weight (kg) × 35 + (Exercise min / 30) × 350
Cups (250 ml)   = Total / 250
Bottles (500 ml) = Total / 500

A 70 kg sedentary person: 70 × 35 = 2,450 ml ≈ 2.5 L. Same person with 1 hour of exercise: 2,450 + 700 = 3,150 ml ≈ 3.2 L. On hot or humid days, add 500–1,000 ml. Note that about 20% of daily water comes from food (fruits, soups, vegetables), so the amount you actually need to DRINK is less than the total shown.

Signs of adequate hydration: pale yellow urine (not dark, not fully clear), no thirst between meals, stable energy throughout the day. Avoid overdoing it: more than 4-5 L in a sedentary person can cause hyponatremia (sodium drop) — rare but dangerous. Distribute water across the day, not all at once. Caffeine and alcohol are diuretic, so each cup of coffee or beer requires about half a cup of extra water.