Water Intake Calculator

The calculator starts from a common rule of about 35 ml of water per kilogram of body weight per day, which matches the reference amounts for adults of average size. It adds about half a litre for every hour of exercise to replace sweat, and half a litre for hot or humid weather. About a fifth of the total typically comes from food — fruit, vegetables, soups and dairy are mostly water — so the amount to drink is shown separately.

The European Food Safety Authority sets adequate total water intakes from drinks and food of 2.0 litres a day for adult women and 2.5 litres for adult men, in moderate climates and with moderate activity. The US National Academies give higher figures — about 2.7 litres for women and 3.7 litres for men — because they describe what people typically consume rather than a minimum. Pregnant and breastfeeding women need more. All of these include water from food, and all drinks count, including tea, coffee and milk; caffeine in moderate amounts does not cause dehydration.

For healthy people, thirst is a reliable guide most of the time. Pale yellow urine is a good sign; dark yellow urine, headaches, tiredness and dizziness can be signs of dehydration. Older adults feel thirst less strongly and may need to drink by habit. Children, athletes training for long periods, and people working outdoors in the heat need to pay particular attention, and long endurance events may require drinks with salt as well as water. Drinking far more than you need is rarely a problem, but drinking very large amounts in a short time can dilute the salt in your blood, which is dangerous.

People with heart failure, kidney disease or some liver conditions may be told to limit fluids, and some medicines change how the body handles water. If that applies to you, follow your doctor's advice rather than a general estimate. The same applies if you are always thirsty or pass a lot of urine, which can be a sign of diabetes.