The most widely used clinical rule of thumb is roughly 30 to 40 ml of total water per kilogram of body weight per day for a healthy adult in a temperate climate. Taking the middle of that range, a 70 kg adult lands on 70 × 35 = 2,450 ml, or about 2.5 litres of total water.
The word total is doing a lot of work there. Total water includes everything: plain water, tea, coffee, milk, juice, soup, and the water content of solid food. Roughly 20 to 30% of daily water intake comes from food — fruit and vegetables are 80 to 95% water, and even bread is around 35%. So a 2,450 ml total target means drinking closer to 1,800 to 2,000 ml. Calculators that hand you a total and imply you must drink all of it are overshooting by about a fifth.
For comparison, the US Institute of Medicine set adequate total water intake at about 3.7 litres a day for men and 2.7 for women, again including food. Those are population averages for planning, not individual prescriptions.
Eight eight-ounce glasses is 1.9 litres, and the figure is repeated everywhere without a source. The likely origin is a 1945 US National Research Council recommendation of about 1 ml of water per calorie consumed — which for a 2,000 calorie diet gives 2 litres. But that document explicitly added that most of this quantity is contained in prepared foods. The caveat was dropped somewhere in the retelling and the number became a drinking target. There is no controlled trial establishing eight glasses as a threshold for health.
| Factor | Rough adjustment | Why |
|---|---|---|
| Moderate exercise | +400 to 800 ml per hour | Sweat losses of 0.5 to 2 L per hour depending on intensity |
| Hot or humid climate | +500 to 1,000 ml | Higher baseline sweat rate even at rest |
| High altitude above 2,500 m | +300 to 500 ml | Increased respiratory water loss and diuresis |
| Fever, vomiting, diarrhoea | Substantially more | Losses can outpace thirst; oral rehydration salts may be needed |
| Pregnancy | +300 ml | Increased plasma volume |
| Breastfeeding | +700 to 1,000 ml | Milk is about 87% water |
| High-protein or high-fibre diet | Modestly more | Greater renal solute load; fibre binds water in the gut |
Your kidneys regulate hydration far more precisely than a weight-based estimate can. Two signals are worth more than the number this tool gives you:
This deserves stating plainly because most hydration content omits it. Drinking far beyond your needs, especially quickly, can dilute blood sodium and cause hyponatraemia, which ranges from headache and nausea to seizures and is occasionally fatal. Documented cases cluster in marathon runners who drank plain water heavily during long events, in some water-drinking challenges, and in people with kidney, heart or liver conditions or on certain medications. Healthy kidneys can excrete roughly 0.8 to 1 litre per hour, so sustained intake above that is the danger zone. If you are exercising for more than about 90 minutes, replacing electrolytes as well as water matters more than volume alone.
This is a general-population starting point built on body weight and a few adjustments. It cannot account for your kidney function, heart or liver condition, medications such as diuretics or lithium, sweat sodium concentration, or an actual measured sweat rate. If you have a diagnosed condition affecting fluid balance, or you have been given a fluid restriction, follow your clinician rather than a web calculator. Nothing here is medical advice.
Not at habitual doses. Caffeine is a mild diuretic, but the water in the drink more than compensates, and regular consumers develop tolerance to the diuretic effect. Coffee and tea count toward your intake.
Modestly, and indirectly. Water has no calories, so replacing sweetened drinks with it cuts intake, and a glass before a meal can slightly reduce how much people eat. Water itself does not burn fat.
For exercise, yes — it is the most practical method available. Weigh yourself before and after a session; each kilogram lost is roughly one litre of sweat. Replacing about 125 to 150% of that over the following hours restores balance.
No. Your weight and activity level are used only in your browser to compute the estimate, and nothing is transmitted or saved.
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