For decades, the idea that people should drink 1.5 liters of water per day has been widely accepted, appearing on water bottles, in health magazines, and even in medical advice. However, nephrologists—doctors who specialize in kidney health—are now challenging this common belief, pointing out that there is no solid scientific research to support this exact number. The recommendation actually comes from a misinterpretation of a 1945 U.S. government document, which suggested that adults should consume about 2.5 liters of water per day from all sources, including food and drinks. However, this number has been incorrectly simplified and popularized as the need to drink 1.5 liters of water alone. The reasoning behind the 1.5-liter recommendation is based on the idea that the body loses about two liters of water daily through urine, sweat, and breathing, and that food provides about 0.5 liters of water. This would mean that an additional 1.5 liters from drinks would be needed to maintain balance. However, experts argue that this calculation is not grounded in universal biology and has never been validated by serious clinical studies. Recent research involving thousands of people across four continents has shown that the “eight glasses of water a day” rule has not been supported by any rigorous scientific evidence. A major flaw in the 1.5-liter rule is that it ignores the significant amount of water people consume through food. Fruits, vegetables, soups, and even yogurt contain water, contributing up to about 0.5 liters daily. An older American recommendation even suggested that each calorie consumed corresponds to one milliliter of water, which for a typical 1,900-calorie diet would mean about 1,900 milliliters of water from food and drinks. However, the public often misunderstood this, thinking that an additional two liters of water were necessary, leading to the popular 1.5-liter guideline. Despite an article nearly 20 years ago that debunked this myth, the habit of drinking a fixed amount of water has remained largely unchanged. Experts now stress that individual water needs vary depending on factors such as age, physical activity, temperature, and the water content of one's diet. The 1.5-liter rule applies only to a theoretical average person under average conditions, which is not relevant to any specific individual. Instead, specialists recommend paying attention to the body’s signals—such as the color of urine (which should be pale or clear) and the feeling of thirst—as more reliable indicators of hydration needs. Adjusting water intake based on personal circumstances, such as hot weather, exercise, or illness, is more effective than following a rigid formula. While the 1.5-liter myth is no longer considered valid, it highlights the importance of listening to one’s body rather than adhering to a one-size-fits-all rule.