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The "drink 8 glasses a day" advice doesn't fit everyone — water needs depend on your weight and how much you move. A 50 kg person and a 90 kg person can't need the same amount, and someone who exercises loses more through sweat. Instead of a blanket rule, a target based on your own body helps you drink enough without overdoing it.
Enter your weight and activity level, and your daily water need is calculated in litres and glasses.
How is it calculated?
Calculation
Daily water need is based on body weight: roughly 33 ml per kilogram as a baseline, rising with activity.
| Activity level | ml/kg |
|---|---|
| Lightly active | 33 ml |
| Moderate | 38 ml |
| Intense exercise | 43 ml |
For a 70 kg lightly active person: 70 × 33 = 2,310 ml ≈ 2.3 litres (about 12 glasses).
Not just water
Part of your fluid comes from food and other drinks (soup, fruit, milk). But sugary drinks and excess caffeine don't substitute for water; coffee and tea have a mild diuretic effect. It's healthiest to meet most of your need from plain water.
What increases your needs
- Hot/humid weather and sweating
- Fever, diarrhoea, vomiting (fluid loss)
- Pregnancy and breastfeeding
- A high-protein diet
- High altitude
Too much is also harmful
Drinking excessive water in a short time (litres per hour) can dangerously lower blood sodium (hyponatremia). Spread your intake through the day. Kidney or heart conditions may require fluid restriction; if you have a chronic illness, consult your doctor.
For your calorie and macro needs, use a calorie calculator and a protein calculator.
Worked example
A person weighing 70 kilograms who is lightly active: 70 × 33 = 2,310 ml, about 2.3 litres (≈12 glasses) a day. If the same person does regular moderate exercise the need rises to 70 × 38 = 2,660 ml (≈13 glasses), and on intense training days to 70 × 43 = 3,010 ml (≈15 glasses). As you can see, activity alone can add about 0.7 litres to the daily need — which is why a blanket "8 glasses for everyone" falls short.
FAQ
How much water should I drink a day?+
It depends on your weight and activity: about 33 ml per kilogram if lightly active. A 70 kg person should drink about 2.3 litres (≈12 glasses); exercise increases this.
How is daily water intake calculated?+
Your weight is multiplied by a factor for your activity level (33–43 ml/kg). The result is in millilitres, converted to litres and glasses. Just enter weight and activity.
Is the 8-glasses rule correct?+
It's a rough generalisation that doesn't fit everyone. A 90 kg person who exercises needs far more than a 50 kg sedentary person. A personalised figure is more accurate.
Do tea, coffee and other drinks count as water?+
Partly — some fluid comes from food and drinks. But sugary drinks and excess caffeine aren't healthy choices; it's best to meet most of your need from plain water.
When do I need more water?+
In hot weather, while sweating, during fluid loss (fever, diarrhoea, vomiting), in pregnancy and breastfeeding, on a high-protein diet, and at high altitude.
Is drinking too much water harmful?+
Excess water in a short time (litres per hour) can dangerously lower blood sodium. Spread intake through the day. Those with kidney or heart conditions should consult a doctor about fluid limits.