There is no single number that fits everyone, but two common rules of thumb are eight 8-ounce glasses a day, about 2 liters, or half your body weight in pounds expressed as ounces, so a 160 pound person would target around 80 ounces. Needs rise with exercise, heat and body size, and a free water intake calculator personalizes the estimate from your weight and activity, right in your browser.
Free water intake calculator. Estimate how much water to drink per day based on your body weight and exercise, shown in liters, ounces and cups.
Open Water Intake Calculator → Free toolFree water intake calculator in ounces. Estimate daily fluid needs from body weight and activity, shown in ounces, cups, and litres, in your browser.
Open Daily Water Intake in Ounces Calculator →The 8 glasses figure is a rounded population-level baseline, not a personal prescription, and the half-body-weight rule is a heuristic that scales with size. Real needs depend on body weight, activity, sweat rate and climate, which is why two healthy people can have genuinely different targets. Food also contributes fluid, so drinking is not the only source.
Sweating raises fluid needs, so add water around workouts and on hot days rather than forcing a fixed total. A weight and activity based calculator gives you a working daily figure, and thirst plus paying attention to how you feel fills in the rest. For medical conditions that affect fluid balance, follow your doctor guidance over any rule of thumb.
Yes, drinks with caffeine still contribute fluid overall, they are just weaker contributors than plain water.
No. It is a rounded rule of thumb. Smaller people and sedentary days need less, larger bodies, exercise and heat need more.
Yes, extreme intakes in short periods are dangerous. Spread drinking through the day and let thirst guide the upper end.