“Drink eight glasses of water a day” is one of the stickiest pieces of health advice ever, repeated so often it feels like settled science. In reality there is no strong evidence behind the exact figure — but the underlying message, that hydration matters, is sound. The honest answer to “how much should I drink?” is: it depends, and your body is better at telling you than any fixed rule.
What actually determines your needs
- Body size: a larger body simply holds and uses more water. A common rough estimate is about 30–35 ml per kilogram of body weight per day, from all sources.
- Activity: sweat loss during exercise can add anywhere from a few hundred milliliters to more than a liter per hour.
- Climate: heat and humidity raise sweat loss; dry air and high altitude increase it in less obvious ways.
- Diet: a lot of your water comes from food — fruits, vegetables, and soups can supply a meaningful share.
Food and other drinks count
A persistent myth is that only plain water “counts.” In fact roughly 20% of most people’s water intake comes from food, and beverages like tea, coffee, and milk all contribute despite mild diuretic effects. The total fluid balance is what matters, not the source.
The simplest gauge: your urine
You do not need to track milliliters. Pale, straw-colored urine generally signals good hydration; dark yellow suggests you should drink more. Thirst is also a reliable cue for most healthy adults — the idea that you are “already dehydrated by the time you feel thirsty” is overstated for everyday life.
When to pay closer attention
Hydration needs rise with fever, vomiting, diarrhea, pregnancy and breastfeeding, and intense or prolonged exercise. On the other end, drinking far too much water in a short time can dilute blood sodium dangerously — a rare condition called hyponatremia that mostly affects endurance athletes. Balance, not maximization, is the goal.
Frequently asked questions
Does coffee dehydrate me?
Only mildly. The fluid in a cup of coffee more than offsets its diuretic effect, so it still contributes to your daily total.
Is clear urine the target?
Not necessarily — completely clear can mean you are drinking more than you need. Pale yellow is the sweet spot.
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Results are for general information only and are not professional financial, medical, or legal advice.