The '8 glasses' rule is a rough start. Your real needs scale with your weight, sweat, and climate — here's how to find your number.
A practical, evidence-based starting point is about half an ounce of water per pound of body weight (roughly 30–35 ml per kg). That covers a typical day before you account for exercise or heat. Get your personalized number with the water intake calculator.
A 160 lb person starts at about 160 × 0.5 = 80 ounces a day — roughly 2.4 liters, or ten 8-ounce cups. A heavier or very active person needs more; a smaller, sedentary person needs less.
Hard exercise can cost 16–32 ounces of fluid per hour through sweat, so add roughly 12 ounces for every 30 minutes of activity. Hot, humid, or high-altitude conditions raise needs further. Drink before, during, and after workouts rather than trying to catch up afterward.
The easiest gauge is urine color: pale straw means you're well hydrated, while dark yellow signals you need more. Thirst, headaches, and fatigue are later signs of dehydration. You rarely need to force water far beyond thirst unless you're sweating heavily.
Yes. Despite a mild diuretic effect, coffee and tea still add net fluid, and water-rich foods like fruit and vegetables contribute too. Plain water is best for the bulk of your intake, but you don't have to hit your entire target from a glass.