Why children's targets are age-based
Adult calculators use body weight, but for children the reference values from bodies like the European Food Safety Authority are set by age band, and from age nine they split by sex. That's because children's water turnover relative to size is higher than adults', and it shifts as they grow. Our article on how hydration needs change with age explains the full picture from infancy to old age.
Drinks versus total water
Official values are for total water, including what's in food. Kids who eat plenty of fruit, vegetables, soup and yogurt get a meaningful share that way, so we assume about 80% of the total needs to come from drinks. Milk counts. Juice counts too, but watch the sugar; water should be the default between meals.
Hot days and sports days
Children overheat more easily than adults and often forget to drink while playing. The calculator adds fluid for minutes of sweaty activity and for hot weather, but the practical fix is simpler than any number: a bottle they like, a drink before they go out, and a refill at every natural break. Our guide to hydration in hot weather has more.
Watch the signs, not just the number
Because thirst is unreliable in busy kids, learn the early signs of dehydration: dark urine, dry lips, grumpiness, a headache after school. If your child is unwell, vomiting or has diarrhea, fluid needs rise sharply — follow your pediatrician's advice rather than this calculator.
