How Much Water Should You Drink? Calculate by Weight, Not by “8 Glasses”
“Eight glasses a day” is catchy, but a 50 kg person and a 90 kg person clearly don't need the same. Weight and activity are better guides.
“Drink eight glasses (about 2 liters) a day” is everywhere, but it's a rounded, rough guideline that ignores the variables that matter most: body weight, climate, and activity.
Calculate from body weight
A common estimate: daily water in ml = body weight in kg × 30 to 35. A 60 kg adult needs roughly 1,800–2,100 ml per day. The Water Intake Calculator gives you a personalized number.
When you need more
- Exercise and sweat: add roughly 500–700 ml per hour of activity.
- Heat or humidity: sweat losses rise sharply, so intake should follow.
- High altitude or dry aircraft cabins: water is lost faster through breathing and skin.
- Fever, diarrhea, or vomiting: losses accelerate, so replace fluids and electrolytes promptly.
- Pregnancy and breastfeeding: needs rise clearly — confirm the amount with your doctor.
Water isn't only plain water
Many assume only plain water counts. In reality food counts too — fruits and vegetables are often over 90% water — as do soup, milk, tea, and coffee. Coffee's mild diuretic effect offsets part of its fluid, but it doesn't cause a net loss.
More isn't always better
Water is not the more the better. Drinking very large amounts quickly can cause hyponatremia, which is rare but can be life-threatening. People with kidney failure or heart failure need doctor-set fluid limits and shouldn't apply a generic formula.