Water Intake Calculator — Daily Hydration Needs
This free water intake calculator estimates how much water you should drink each day based on your body weight and activity level. It uses the common guideline of roughly 30–35 millilitres of water per kilogram of body weight, then adjusts for exercise, climate, and other factors. Remember that food supplies 20–30% of your daily fluid, so you rarely need to drink the full figure, and urine colour is the most reliable everyday gauge. All calculations run locally in your browser for complete privacy.
How is daily water intake calculated?
This calculator uses the body-weight method: Water (ml) = Weight (kg) × 35 ml. An 80 kg person needs approximately 2,800 ml (2.8 litres) per day under normal conditions. For imperial: multiply weight in lbs by 0.5 to get ounces, or weight in lbs × 67 ml.
The 35 ml/kg baseline applies to a sedentary adult in a temperate climate. Exercise increases needs significantly: add approximately 500–750 ml for every hour of moderate exercise, more in hot weather or at high altitude. Total water intake includes all beverages — tea, coffee, milk, and juice count — plus water in solid food (typically 20% of total intake). Plain water remains the best source: it has no calories, no caffeine, and is immediately available. Thirst is generally a reliable indicator of hydration in healthy adults, but the elderly often have a blunted thirst response and should monitor intake more deliberately.