You've heard "drink 8 glasses a day" your entire life. The reality is more nuanced — and more interesting. Here's the actual science behind daily water intake, how to calculate what's right for you, and the simplest way to track it. How Much Water Do You Actually Need? The "8 glasses a day" rule has no scientific basis. It originated from a 1945 US Food and Nutrition Board recommendation that was widely misunderstood. The science-based recommendations: The National Academies of Sciences recommends: Men: 3.7 litres (125 oz) of total water per day Women: 2.7 litres (91 oz) of total water per day Crucially: This includes water from all sources — drinks and food. About 20% of water intake comes from food (fruits, vegetables, soups, and even solid foods contain significant water). Net drinking targets: Men: ~3.0 litres (12–13 cups) of beverages Women: ~2.2 litres (9–10 cups) of beverages These are averages. Your personal requirement depends on several factors.…