Water Intake
Daily hydration needs
About This Calculator
Adequate daily hydration is essential for nearly every bodily function — temperature regulation, digestion, nutrient transport, joint lubrication, and cognitive performance. Daily water needs depend on body weight, activity level, climate, and diet. Foods contribute about 20% of daily fluid intake.
Formula
Basic: Daily water (L) = Weight (kg) × 0.033
Active: add 500-750 ml per hour of exercise
Standard guideline: 8 cups = 2 liters/day (general baseline)
Urine color test: pale yellow = well hydrated; dark yellow = drink more
Example Calculation
70 kg person who exercises 1 hour per day
- Base = 70 × 0.033 = 2.31 liters
- Exercise addition = 0.5 liters
- Total = 2.31 + 0.5 = 2.81 liters/day ≈ 11.7 cups
Recommended: ~2.8 liters (about 12 cups) per day
Daily Water Needs by Weight
| Weight | Sedentary | Lightly Active | Very Active |
|---|---|---|---|
| 50 kg | 1.65 L | 2.1 L | 2.7+ L |
| 60 kg | 2.0 L | 2.5 L | 3.2+ L |
| 70 kg | 2.3 L | 2.8 L | 3.6+ L |
| 80 kg | 2.6 L | 3.2 L | 4.0+ L |
| 90 kg | 3.0 L | 3.6 L | 4.5+ L |
| 100 kg | 3.3 L | 4.0 L | 5.0+ L |
Frequently Asked Questions
Is 8 glasses a day really necessary?
The '8×8' rule (eight 8-oz glasses = ~2 liters) is a useful starting point but not universal. Needs vary by body size, activity, climate, and diet. Thirst is a reliable guide for healthy adults — drink when thirsty and check urine color to fine-tune.
Can you drink too much water?
Yes — hyponatremia (dangerously low blood sodium from excessive water intake) can occur, mainly in endurance athletes who drink large amounts without sodium. For most people, kidneys can process about 1 liter/hour, making overhydration rare with normal drinking patterns.
Do other drinks count toward hydration?
Yes. Coffee, tea, juice, and milk all contribute to fluid intake. Caffeinated drinks were once thought to cause net fluid loss (diuresis) but research shows moderate caffeine consumption still results in net hydration. Alcohol is genuinely dehydrating and doesn't count.
How does dehydration affect performance?
Even mild dehydration (1-2% body weight) impairs cognitive function, reduces endurance by 10-15%, and increases perceived effort. Severe dehydration (5%+) can cause heat exhaustion. Staying ahead of thirst is particularly important during exercise in heat.