Why there is no single number
Needs differ with body size, food, temperature, activity, disease and medication. Excessive water intake can also be dangerous.
A practical approach
Keep water accessible, drink to thirst and add fluids with heat or activity. Notice headache, dry mouth and unusual fatigue, but do not automatically label them dehydration.
When to seek advice
With heart or kidney disease, electrolyte disorders or prescribed fluid restrictions, follow clinical guidance.
Important: Practical suggestions are for education and self-observation and do not replace medical care.