Where the numbers come from
The European Food Safety Authority sets an adequate intake of water from everything — drinks and food — at 2.0 litres a day for women and 2.5 litres for men, plus 0.3 litres in pregnancy and 0.7 litres while breastfeeding. About a fifth of it usually comes with food, so drinks make up the rest.
Exercise and heat add to it: sweat losses are often 0.5–1 litre an hour of exercise, more in the heat. This calculator adds about 0.5 litres for each hour of exercise and 0.5 litres on a hot day.
What counts
Every drink counts — water, tea, coffee, milk, juice. Coffee and tea do not dry you out at normal amounts. Soups, fruit and vegetables bring a lot of water too: a bowl of soup or a big salad can be a glass's worth.
Sugary drinks and alcohol are poor ways to get water: they bring calories, and alcohol makes you lose fluid.
Too little, too much
Mild dehydration shows as thirst, dark urine, a headache or tiredness. Drinking far more than you need is rare but possible — mostly during very long endurance events. For healthy people, drinking to thirst and a little more with exercise is enough. People with heart or kidney conditions should follow their doctor's limits.
Questions
- Do I really need eight glasses of water a day?
- Not as a rule. The "8×8" figure has no clear scientific source. Women need about 1.6 litres from drinks and men about 2 litres on an ordinary day, and all drinks count.
- Does coffee count towards my water?
- Yes. At normal amounts coffee and tea hydrate almost as well as water.
- How much water should I drink when I exercise?
- Roughly 0.4–0.8 litres an hour, more in the heat or if you sweat heavily. Weighing yourself before and after a long session shows your own sweat loss.
- Should I drink more water to lose weight?
- A glass of water before a meal may help some people eat a little less, and water instead of sugary drinks saves calories. Water itself does not burn fat.