The popular "8 glasses a day" rule is a simplification that ignores body size, activity, and climate. Official guidelines from the European Food Safety Authority (EFSA) and the US National Academies set adequate intakes by age and sex. A common rule of thumb scales intake with body size: 30 to 35 ml per kilogram of body weight, adjusted for activity and temperature.
Our calculator multiplies your weight in kilograms by 0.033 L, then by an activity multiplier and a climate multiplier. A 70 kg sedentary adult in a temperate climate gets about 2.3 L a day. The same person running daily in summer heat may need 3 L or more. The activity and climate multipliers are practical adjustments, not official values.
EFSA's adequate intakes (2.0 L a day for women and 2.5 L for men) cover total water, and about 20% of it usually comes from food, so part of your target is met by what you eat (EFSA, 2010, Scientific Opinion on Dietary Reference Values for water).
Thirst is a late indicator of dehydration: by the time you feel thirsty, your body is already running low on water. Earlier signals include: urine darker than pale yellow, headaches (one of the most common early signs), difficulty concentrating, mild fatigue, and dry mouth. A review by Masento et al. (2014, British Journal of Nutrition) links mild dehydration to weaker attention and memory.
Front-load your hydration: drink a large glass upon waking (you lose water 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: seeing your progress through the day makes it easier to keep going.