'Eight glasses a day' is a slogan, not science — your actual water needs depend mostly on body weight and how much you sweat. This calculator gives a personalized daily target from your weight and exercise minutes.
How it works
It starts from a widely used baseline of about 35 ml per kilogram of body weight, then adds roughly 350 ml for every 30 minutes of exercise to replace sweat losses.
The formula
Weight in kg, exercise in minutes per day. Hot climates, altitude, pregnancy, and illness raise needs further; food (fruits, soups) typically covers ~20% of intake on top of drinks.
Worked example
At 70 kg with 30 minutes of daily exercise: 70 × 0.035 = 2.45 L baseline, plus 0.35 L for the workout ≈ 2.8 liters per day.
Frequently asked questions
Do coffee and tea count toward the total?
Mostly yes — their diuretic effect is mild at normal consumption, and their water content largely counts. Sugary drinks hydrate too but bring calories along.
What's the simplest sign I'm drinking enough?
Urine color: pale straw yellow suggests good hydration; dark yellow suggests you're behind. Thirst works too, though it lags in older adults and during intense exercise.
Can I drink too much water?
Yes — drinking many liters in a short period can dilute blood sodium (hyponatremia), which is dangerous. Spread intake through the day and, during long endurance events, include electrolytes.
General guidance for healthy adults. Kidney, heart, and certain other conditions change fluid needs — follow your clinician's advice.