How it works
A useful range, not a perfect prescription
Water needs change with body size, sweat, weather, food, health, and the rest of your day. The calculator combines three practical estimates and shows the uncertainty instead of hiding it.
1. Body-size baseline
The starting range is 30-35 milliliters per kilogram of body weight, with 32.5 mL/kg as the center estimate. This is a practical product heuristic, not an official formula for every person.
2. Activity that makes you sweat
Planned sweaty activity adds 0.4-0.8 liters per hour, centered at 0.6 L/hour. Actual sweat loss varies widely, so endurance exercise should use changes in body weight and an individual sweat-rate plan.
3. Today's conditions
Warm outdoor time adds 0.2-0.4 liters. Several hours in hot or humid conditions adds 0.4-0.8 liters. These are broad planning adjustments, not temperature-based medical rules.
What the result includes
The result is a practical target for fluid from drinks. Water can be the main source, but coffee, tea, milk, and other non-alcoholic drinks also contribute. Water in food contributes in addition to the displayed target.
Research context
The U.S. National Academies lists adequate total-water intakes of about 3.7 L/day for men and 2.7 L/day for women under typical conditions. EFSA lists 2.5 L and 2.0 L. Both include water from food and beverages and describe population-level adequate intake, not an exact individual requirement.
These sources inform the calculator's general ranges and principles, but they do not validate or endorse this site's combined formula. The result is a practical planning estimate, not a universal scientific or medical standard for an individual.