General estimate
Water Intake Calculator
Offers a starting fluid estimate adjusted for activity.
How this estimate is calculated
A starting band of 30–35 ml per kilogram of body weight, plus roughly 12 ml for each minute of sweating activity.
How to use the result
Treat it as a starting point to compare against how you actually feel and what you actually eat. Estimates are most useful for spotting a large gap, not for chasing a precise number.
What this cannot tell you
Food and other drinks contribute fluid, and heat, altitude and illness raise losses. Some heart, kidney and endocrine conditions come with a prescribed fluid limit that overrides any estimate.
Estimates, not measurements: this tool runs a population equation, so it describes a typical person with your inputs rather than measuring you. Where a number drives a decision about a diagnosed condition or medication, have a dietitian or clinician sanity-check it against your history.