“Everyone needs eight glasses of water a day.”
Strong confidence because authoritative water-intake reference values cover total water from food and all beverages, vary by age and sex, and are population Adequate Intakes rather than a universal eight-glass prescription. Needs also change with climate, activity, illness and pregnancy.
Eight glasses can be a reasonable personal target, but it is not a requirement for everyone. Water from food and other beverages counts, and individual needs vary widely.
The National Academies set Adequate Intakes for total water, not eight glasses of plain water. For young adults, the reference amounts are about 3.7 L/day for men and 2.7 L/day for women from all foods and beverages, based mainly on observed intakes rather than a minimum proven requirement.
Safety note: Too little fluid can cause dehydration, while drinking far beyond need can cause life-threatening hyponatremia. Thirst may be less reliable in older adults, young children, illness, high heat and prolonged exercise, and some medical conditions require fluid limits.
Talk to a healthcare professional if: Ask for a specific plan if you have heart, kidney or liver disease, are pregnant or breastfeeding, take diuretics, have recurrent dehydration or low sodium, or exercise or work for long periods in heat.
Use thirst, urine pattern, activity, climate, diet and medical needs together. Eight glasses is a simple option, not a universal pass-fail rule.
Drink regularly with meals and when thirsty, and increase fluids for heat or activity. Follow a clinician’s plan if your health changes normal fluid needs.