Estimate how much water to drink per day from your weight, exercise and heat — calculated in your browser.
The reference used here is 35 ml of water per kilogram of body weight, a clinical rule of thumb for healthy adults that lands close to the adequate intakes published by the European Food Safety Authority (2.5 litres a day for men and 2.0 for women, counting all fluids and food) and by the United States Institute of Medicine. On top of that base the tool adds 500 ml for every 30 minutes of exercise, to replace what you lose through sweat, and 10% more when you tick the hot-weather box. A 70 kg adult with no exercise ends up at about 2.45 litres, or roughly ten glasses of 250 ml.
The arithmetic is deliberately simple and completely visible: weight × 35 ml, plus the proportional exercise allowance, plus the climate adjustment, then divided by 250 to convert into glasses. There is no hidden coefficient and no data collection — the whole calculation runs in JavaScript on your own device. Use it as a target to aim at during the day, particularly if you tend to forget to drink, and adjust it as your routine changes.
Be sceptical of any single number, including this one. The famous advice to drink eight glasses a day has no strong scientific basis — the physiologist Heinz Valtin traced it back to a 1945 recommendation whose next sentence, usually ignored, noted that most of that water already comes from food. Real needs vary a lot with diet, air temperature, humidity, altitude, fever, pregnancy, breastfeeding and kidney function. Every drink counts, coffee and tea included, and food supplies roughly a fifth of your daily water. Thirst and pale-straw urine remain the most practical day-to-day signals for a healthy person; drinking far beyond thirst is not harmless either, since extreme volumes can dilute blood sodium. And if you have kidney or heart disease, or take medication that affects fluid balance, your limit has to be set by your doctor rather than by a formula.
Water (ml) = weight (kg) × 35, plus 500 ml for every 30 minutes of exercise, plus 10% in hot weather. Glasses = total ml ÷ 250 ml. Example: 70 kg × 35 = 2,450 ml, that is 2.45 litres or about 10 glasses.
For a healthy adult, roughly 35 ml per kilogram of body weight — about 2.45 litres for 70 kg — plus around 500 ml for every 30 minutes of exercise and about 10% more on hot days. It is an estimate, not a prescription.
It has no solid scientific basis. It is usually traced to a 1945 recommendation that also said most of that water comes from food, a detail dropped in later retellings. Needs vary far too much between people for one fixed number to fit everyone.
Yes. Every drink contributes to your hydration, and the mild diuretic effect of caffeine does not cancel out the fluid you drank with it. Food adds roughly another 20% of your daily water.
Pale-straw urine and the absence of persistent thirst are the simplest signals. Dark urine, dry mouth, headache and tiredness suggest you should drink more — and if the signals persist, that is a question for a doctor, not a calculator.
Yes, in extreme cases. Drinking litres far beyond thirst in a short window can dilute blood sodium, a condition called hyponatremia that has been documented in endurance events. People with kidney or heart conditions need a fluid limit defined by their doctor.
No. Weight, exercise minutes and results are processed on your device only. Nothing is uploaded, logged or stored.
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