How much water should you drink a day?

The US National Academies of Sciences set adequate total water intakes at about 3.7 litres a day for men and 2.7 litres for women. Crucially, this counts water from all sources, including tea, coffee, milk and food, which typically supplies about 20 percent of the total. The popular rule of eight glasses a day has no firm scientific basis. Needs rise with heat, exercise, illness, pregnancy and breastfeeding. For most healthy people, drinking when thirsty and aiming for pale yellow urine is a reliable guide, while drinking extreme amounts in a short time can dangerously dilute sodium in the blood.