How much water should I drink a day?

The two figures are not comparable quantities, which is the whole of the confusion. The IOM's number is total water intake from all beverages and food. Eight 8-ounce glasses is 1.9 litres of drinking water alone. Comparing them as though they answered the same question makes the guideline look like a stretch when it is actually higher than it appears.

Valtin traced the eight-glasses rule in 2002 and found no scientific basis for it. Its likeliest origin is a 1989 recommendation of 1 mL of water per kcal of energy expenditure - whose very next sentence noted that most of that water is already contained in food. That sentence got dropped in transmission, and the remainder became a rule nobody had written.

Caffeinated drinks count. The IOM figure is total water from all beverages, and the report treats caffeinated drinks as contributing to it. So does the water in food, which is why the total-water figure is the one this site leads with rather than a plain-water target.

Two other conventions exist and are shown as comparison lines rather than answers: 1 mL per kcal, and 30 mL per kg of body weight, which is used mostly in clinical settings for older adults and which the review documenting it notes works badly for very thin or very heavy people. The IOM also sets total water at 3.0 litres a day in pregnancy and 3.8 during lactation. All of these are Adequate Intakes for a population, not prescriptions for a person - thirst and clinical advice both outrank a table.