"Drink eight glasses a day" is the advice everyone has heard, but it isn't based on much. A 50 kg person and a 100 kg marathon runner clearly don't need the same amount. A better estimate scales with your body weight and how much you sweat. Here's a practical way to find your number.
A weight-based starting point
A widely used rule of thumb ties intake to body weight:
Daily water (ml) ≈ 30–35 ml × body weight (kg)
So for a 70 kg adult:
- 70 × 33 ≈ 2,310 ml, or about 2.3 liters per day
In imperial terms, a common version is about half an ounce to one ounce of water per pound of body weight. Either way, you land in a similar ballpark for most people.
Add for activity and heat
Exercise and hot weather increase fluid loss through sweat, so add to your baseline:
- Light exercise: +about 350 ml
- Moderate exercise (30–60 min): +about 500–700 ml
- Intense or long sessions: +700 ml or more, replacing what you sweat out
- Hot or humid climates: add extra throughout the day
A 70 kg person doing a moderate workout might aim for roughly 2,300 + 600 ≈ 2,900 ml that day.
Remember food and other drinks count
You don't have to drink every milliliter as plain water. Roughly 20% of daily fluid comes from food — fruits, vegetables, soups — and other beverages like milk, tea, and coffee contribute too. The goal is total fluid balance, not a quota of water bottles.
Signs you're well hydrated:
- Pale, straw-colored urine (dark yellow suggests you need more)
- Infrequent thirst
- Steady energy rather than afternoon headaches
How to use it
Get a personalized estimate with the Water Intake Calculator. Enter your weight and activity level, and it returns a daily target in liters or ounces.
Hydration ties into your wider routine. The Calorie (TDEE) Calculator uses the same activity level you'd pick here, so the two work well side by side for planning active days. And if you run, the Running Pace Calculator helps you gauge session length — longer, faster runs mean more sweat and a higher fluid need.
Don't overdo it
More isn't always better. Drinking very large volumes in a short time can dilute blood sodium (a rare condition called hyponatremia), so spread intake across the day rather than gulping liters at once. Most healthy adults can simply drink to thirst and check their urine color.
These are general estimates, not medical advice. People with kidney, heart, or other conditions may have different needs and should follow their doctor's guidance.