How much water should I drink a day?
Are the 8 glasses or 2 litres true? What the evidence says about how much water you need, what changes it and how to tell if you're low.
“Drink eight glasses of water a day.” It is one of the most repeated health tips in the world… and also one of the most misunderstood. The amount of water your body needs is not a fixed number, but something that depends on you and on your day. Let’s see what the evidence actually says.
Where the “2 litres” and “8 glasses” come from
The general recommendation from scientific bodies places an adult’s intake at around 2 litres a day, equivalent to about 8 glasses. A slightly more precise rule, mentioned by BRITA, estimates about 35 ml of water per kilogram of body weight. With that formula:
- A person of 50 kg would need ~1.7 litres.
- One of 70 kg, ~2.4 litres.
- One of 80 kg, ~2.8 litres.
These are good starting references, but it is best to take them as guidance, not an exact obligation.
Not all water comes from the glass
An important nuance stressed by Mayo Clinic: not all of your hydration comes from drinking plain water. A considerable part arrives through food (fruit, vegetables, soups) and other drinks. So when people say “you need 2 litres,” you are already covering much of it by eating without realizing.
The factors that change your needs
Your fluid requirement is not the same every day. It rises when:
- You exercise or sweat a lot: you have to replace what you lose before, during and after.
- It is hot or humid: warm weather increases losses.
- You are ill (fever, vomiting, diarrhea) or pregnant or breastfeeding.
In those cases you will need to drink more; on a calm, cool day, perhaps less.
Not too little… and not too much
Falling short on water causes tiredness, headaches and poor concentration. But drinking in excess is not “healthier” either: forcing extra litres does not eliminate more toxins or help you lose weight, and in extreme cases it can dangerously dilute the sodium in your blood (hyponatremia), as explainers like Directo al Paladar warn. Water is essential, but it is not magic.
The best practical guide? Your own body: thirst works well in healthy people, and the color of your urine is a good indicator. Pale yellow usually means good hydration; dark yellow, that you should drink more.
The honest answer
There is no universal number. The 2 litres and 8 glasses are a useful reference, not a law: your age, weight, climate and activity matter more than any round figure. Drink regularly throughout the day, be guided by thirst and the color of your urine, and adjust upward when you sweat or it is hot. And for specific health concerns, always consult a professional.