Water is the body's operating fluid: it makes up roughly 50 to 65 percent of adult body weight and is involved in temperature regulation, digestion, nutrient transport, joint lubrication, and waste removal through urine. Even mild dehydration — a loss of just 1 to 2 percent of body weight in fluid — can bring on headaches, fatigue, and measurably worse concentration and physical performance. The U.S. National Academies' widely cited guidance sets adequate total water intake at about 3.7 liters per day for men and 2.7 liters per day for women, but that figure includes all sources: drinking water, other beverages, and food, which supplies roughly 20 percent of the total. The old 'eight glasses a day' rule (about 1.9 liters) is a reasonable minimum for many people, not a personalized prescription.
A practical personalized rule of thumb is about 35 milliliters of water per kilogram of body weight per day from fluids. That scales the target to body size: a 60 kg person lands near 2.1 liters, an 80 kg person near 2.8 liters. Then adjust for reality: add roughly 500 to 1,000 ml per hour of sweaty exercise, more in hot or humid weather or at altitude, and more during fever, vomiting, or diarrhea. Pregnant and breastfeeding women need extra fluids as well. Your simplest daily gauge needs no calculator at all — pale yellow urine and rarely feeling thirsty generally mean you are drinking enough, while dark urine and frequent thirst mean you are behind.
Overhydration is rare in everyday life but real: drinking extreme amounts of water in a short time can dilute blood sodium (hyponatremia), a particular risk for endurance athletes who overdrink during long events. People with heart failure, kidney disease, or other conditions affecting fluid balance should follow their doctor's limits rather than any general rule. This calculator gives an informational estimate from your weight and activity — useful for building a drinking habit, not a medical directive. When in doubt, especially with a medical condition, ask your healthcare provider what target fits you.