A weight-based fluid estimate
The displayed estimate multiplies body weight by an entered millilitres-per-kilogram assumption. At 75 kg and 35 mL/kg, 75 × 35 is 2,625 mL, or 2.63 litres per day. It is a simple starting figure, not a measure of hydration status.
Conditions that change needs
Heat, humidity, altitude, exercise duration, sweat rate, fever, vomiting, diarrhoea, pregnancy, breastfeeding, and food water content can all change fluid needs. A desk-day baseline cannot price in an endurance event or an acute illness.
Hydration is not water alone
Beverages and moisture in food contribute to total fluid intake, while substantial sweating can also involve electrolyte replacement decisions. The calculator does not measure sodium balance, urine concentration, kidney function, or losses from a particular activity.
Why excess can be harmful
Forcing large volumes quickly can be unsafe, particularly in endurance settings or for people with conditions affecting fluid balance. Follow medical guidance for heart, kidney, liver, endocrine, or medication-related restrictions, and seek care for signs of illness or dehydration.
Plan from the day in front of you
For routine planning, record conditions such as activity, weather, and access to drinks beside the baseline. Adjusting an estimate from those facts is more defensible than treating one litres-per-day number as a rule for every season and body.
Check the arithmetic in context
The 2.63-litre display is 2,625 mL before rounding. It is not an instruction to consume that amount at once or a target that must be reached from plain water alone. For a long hot activity, duration, sweat loss, access to fluids, and any medical advice are more relevant inputs than body mass by itself. For an ordinary day, thirst, meals, and routine beverage habits provide context the equation cannot collect.
Some people have clinician-directed fluid limits or replacement plans. In those cases, a generic mL-per-kilogram result should be set aside in favour of that instruction. The calculation also does not account for caffeine, alcohol, or medication effects; it only multiplies two user-supplied values to make the baseline visible. A bottle size is not a medical unit: if using a bottle to plan, write its actual millilitres on the record and avoid assuming that every container holds the same amount.
It is informational only and is not medical or professional advice.