Water Intake Calculator

The common advice of 8 glasses (2 liters) per day is a rough approximation that ignores body size, activity level, and climate. A more evidence-based approach is 30-35ml per kilogram of body weight per day for sedentary adults. A 70kg person needs approximately 2.1-2.45 liters; an 90kg person needs 2.7-3.15 liters. Physical activity adds requirements: moderate exercise (30-60 minutes) adds 500-700ml, and intense exercise or hot-weather workouts can require an additional 1-1.5 liters per hour. The Institute of Medicine recommends 3.7 liters total daily water intake for men and 2.7 liters for women — but this includes water from all sources, including food (which typically provides 20-30% of daily water intake). Fruits and vegetables like watermelon, cucumber, and oranges are 85-95% water. Coffee and tea count toward hydration despite caffeine's mild diuretic effect — research shows the fluid intake more than compensates. This calculator personalizes your target based on your weight, activity level, and climate, processing everything in your browser.

Mild dehydration (1-2% body water loss) causes thirst, darker urine, and reduced concentration — studies show a 1.5% water deficit can impair mood and cognitive performance. Moderate dehydration (3-5%) brings headaches, fatigue, and reduced physical performance by up to 25%. The simplest hydration test is urine color: pale straw indicates good hydration, dark yellow suggests dehydration, and consistently clear urine might indicate overhydration. Hyponatremia (water intoxication) is rare but dangerous — it occurs when excessive water intake dilutes blood sodium below 135 mmol/L. Marathon runners and endurance athletes are most at risk, typically from drinking more than 1.5 liters per hour for extended periods. For most people, spreading intake throughout the day is optimal: drink a glass upon waking (you lose 200-300ml of water during sleep through breathing), before and after meals, and steadily during exercise. Setting hourly reminders can help build the habit. Track your intake pattern to find what schedule works for your routine.