The popular "8 glasses a day" rule is a simplification that ignores body size, activity, and climate. Evidence-based hydration guidelines from the European Food Safety Authority and the US National Academies of Medicine calculate needs based on body weight and sweat rate. The general formula: 30–35ml per kilogram of body weight, adjusted for activity and temperature.
A 70kg sedentary adult in a temperate climate needs approximately 2.1L. The same person running daily in summer heat may need 3L or more. Our calculator uses this formula with activity and climate multipliers validated by sports medicine research.
Thirst is a late indicator of dehydration — by the time you feel thirsty, you are typically already 1–2% dehydrated, enough to impair cognitive performance. Earlier signals include: urine darker than pale yellow, headaches (one of the most common early signs), difficulty concentrating, mild fatigue, and dry mouth. Cheuvront & Kenefick (2014, Nutrition Reviews) documented that even mild dehydration reduces attention, memory, and psychomotor speed.
Front-load your hydration: drink a large glass upon waking (you lose approximately 500ml overnight through breathing and perspiration), and drink before meals rather than during. Keep a visible water container at your desk. Use this calculator's glass tracker to build the habit — visual progress cues are one of the most effective behaviour-change tools in habit research (Clear, 2018, Atomic Habits).