How much water should I drink a day?

The famous eight glasses a day has no scientific origin anyone can find. The actual guidance, from the National Academies of Sciences, is about 15.5 cups (3.7 liters) of total daily fluid for men and 11.5 cups (2.7 liters) for women. The key word is total: roughly 20 percent comes from food, and coffee, tea, and milk all count.

For healthy people, thirst is a genuinely reliable signal, which is boring but true. Drink when thirsty, more in heat or during exercise. Pale yellow urine means you are fine; dark amber means drink up.

You can also overdo it. Drinking extreme amounts in a short window dilutes blood sodium, a condition called hyponatremia that occasionally hospitalizes marathoners. Hydration is a habit, not a competition.