WATER BY WEIGHT
CALCULATOR
Estimate a daily water intake target based on your body weight, using the 30–35ml-per-kg guideline used by dietitians as a starting point for hydration planning.
Weight-Based Water Formula
A commonly used starting point for hydration planning is 30–35ml of water per kilogram of body weight per day. Under this guideline, a 70kg person would aim for roughly 2.1–2.45L daily at baseline, before accounting for exercise, climate, or individual health conditions. Because the formula scales with body size, it tends to give a more personalized estimate than a single fixed daily target (such as "drink 8 glasses a day") that ignores how much a person weighs.
This calculator applies that formula to the weight you enter and also shows an "active" figure (40ml/kg) for days with exercise or heat exposure, along with an optional lean-mass-based estimate if you know your body fat percentage. Total fluid needs vary from person to person based on activity level, climate, sweat rate, diet, and underlying health conditions, so treat the number below as a reasonable starting estimate rather than a fixed prescription.
How the Numbers Are Calculated
The standard estimate multiplies your body weight in kilograms by 35ml. The active estimate uses the same weight multiplied by 40ml/kg to account for higher fluid turnover during exercise or hot weather. The lean-mass estimate, shown when you enter a body fat percentage, first removes estimated fat mass from your total weight and applies the 40ml/kg figure to the remaining lean mass, since metabolically active tissue accounts for most day-to-day water turnover. None of these figures include water obtained from food, which typically supplies a meaningful share of total daily fluid intake.