A healthy adult needs about 2.7 litres of total fluid a day if female and 3.7 litres if male, counting all drinks and food. Your personal target is roughly 35 millilitres per kilogram of body weight per day, which lets you calculate your own number instead of borrowing an average.

The top-ranking guides, including Mayo Clinic and Healthline, give the same population averages, but they leave three gaps. First, they rarely give a personal calculation from body weight. Second, they seldom split the total into drinks and food. Third, they rarely turn the total into a daily schedule. This guide closes all three.

The population figures come from the U.S. National Academies of Sciences, Engineering, and Medicine: about 15.5 cups, or 3.7 litres, a day for men, and about 11.5 cups, or 2.7 litres, a day for women. Those totals cover all fluids, and about 20% of daily fluid usually comes from food, with the rest from drinks. The familiar 8 glasses a day is a rough shorthand, not a researched rule, and it sits below the male average and near the female average once food is counted.

Worked example — a 70 kg adult. At 35 millilitres per kilogram, the total is 70 x 35 = 2,450 millilitres, about 2.5 litres. Food supplies about 20%, which is 2,450 x 0.20 = 490 millilitres. Drinks must supply the rest, 2,450 - 490 = 1,960 millilitres. With a 250-millilitre glass, that is 1,960 / 250 = 7.84 glasses, so about 8 glasses of drinks a day, plus food.

Worked example — an 85 kg adult. The total is 85 x 35 = 2,975 millilitres, about 3.0 litres. Food supplies about 2,975 x 0.20 = 595 millilitres. Drinks must supply 2,975 - 595 = 2,380 millilitres, which is 2,380 / 250 = 9.52 glasses, so about 9 to 10 glasses of drinks a day. The heavier adult needs roughly 2 more glasses than the 70 kg adult, which is exactly why a single fixed number fits nobody.

A simple daily schedule for the 70 kg example. Drink 2 glasses on waking and with breakfast, 2 glasses before lunch, 2 glasses in the afternoon, and 2 glasses with and after dinner. That spreads 8 glasses across the day instead of leaving them to the evening, when a large late intake mostly disturbs sleep. Keep a bottle visible on your desk, because most shortfalls come from forgetting, not from deciding to drink less.

When to drink more. Heat, humidity, exercise, and sweating all raise fluid needs, so add glasses on hot days and around workouts rather than sticking rigidly to the base number. Illness with fever, vomiting, or diarrhoea also raises needs. Thirst and pale yellow urine are practical checks that the total is about right, and anyone with heart, kidney, or liver disease should follow their clinician's fluid guidance, which can differ from these general figures.

Bottom line: multiply your weight in kilograms by 35 millilitres for your total, take off about 20% for food, and divide the rest by 250 millilitres for your glasses. A 70 kg adult lands near 8 glasses of drinks a day, and an 85 kg adult near 10.

Q: Is 8 glasses of water a day a scientific rule?

A: No. It is a handy shorthand. The researched totals are about 2.7 litres for women and 3.7 litres for men, including food, and your own figure depends on body weight.

Q: Do tea and coffee count toward daily fluid?

A: Yes. The totals cover all fluids from drinks and food, not plain water alone, though plain water remains the simplest way to meet most of the target.

Q: How much of my fluid comes from food?

A: About 20% of daily fluid usually comes from food, especially fruits, vegetables, soups, and other water-rich foods, leaving about 80% to come from drinks.

Q: Should I drink more when exercising or in hot weather?

A: Yes. Heat and sweating raise fluid losses, so add glasses on hot days and around workouts instead of holding rigidly to your base number.