When to introduce water — and how — is a question that many parents have, partly because conflicting advice circulates and partly because the reasoning behind the guidance (particularly why water can actually be harmful to young babies) is not always explained. The guidance is clear and evidence-based, and the reasoning is rooted in the physiology of young infants rather than arbitrary rules.
Healthbooq supports parents with evidence-based guidance on infant nutrition and hydration across the first year.
Why Water Is Not Appropriate Under Six Months
Babies under six months have kidneys that are not yet mature enough to handle large quantities of plain water. Breast milk and formula contain exactly the right balance of fluid and electrolytes for the infant's physiological needs. Offering plain water in volumes beyond very small sips can dilute the sodium in the baby's blood, causing hyponatraemia — a dangerous electrolyte imbalance that can, in severe cases, cause seizures. This is known as water intoxication, and while it is uncommon, it is a genuine risk in young babies given too much plain water.
For breastfed babies, breast milk is approximately eighty-seven percent water and is the sole appropriate fluid under six months in any weather conditions — including hot weather. A breastfed baby who needs more fluid in hot weather will signal this by feeding more frequently, and the breastfeeding parent's milk supply adjusts accordingly. Offering water as a supplemental fluid is not necessary and potentially harmful.
Formula-Fed Babies in Hot Weather
Formula-fed babies are fully hydrated by their formula in normal conditions. In very hot weather, some guidance suggests offering small amounts (thirty to sixty millilitres) of cooled boiled water between feeds to provide additional fluid. This is a modest exception to the general under-six-months rule, not a significant additional hydration requirement. Tap water used for this purpose should be boiled and cooled before offering to babies under six months, as tap water is not sterile.
Introducing Water at Six Months
Once solid foods are introduced at around six months, small amounts of water can be offered alongside food in an open cup or free-flow beaker. Water offered at mealtimes helps familiarise the baby with drinking from a cup and supports digestion without displacing milk intake, which remains the primary nutrition source for the first year.
The amount offered is small initially — a few sips — and can be increased gradually as the baby becomes more interested and competent with cup drinking. Tap water is appropriate from six months without boiling. Water should be plain and unadorned; fruit juice, flavoured drinks, or squash are not appropriate for babies under one year and not recommended as a regular drink at any age in early childhood.
What Counts as Adequate Hydration
Through the first year, milk — breast milk or formula — is the primary fluid and the primary measure of hydration. Signs of adequate hydration in a baby include: at least six wet nappies per day after the first week; a moist mouth; urine that is pale yellow to clear; and normal energy and alertness. A baby with fewer wet nappies, dark yellow urine, or signs of lethargy in hot conditions should be seen by a GP or health visitor.
Key Takeaways
Water is not appropriate for babies under six months — breast milk or formula provides complete nutrition and hydration, and offering water can dilute the sodium concentration in the blood, causing hyponatraemia (water intoxication) in very young babies. From six months, small amounts of water can be offered with meals from a cup. Formula-fed babies in hot weather may benefit from small amounts of cooled boiled water between feeds from around six weeks, but breast milk alone is adequate hydration for breastfed babies in any conditions. Tap water is appropriate for babies over six months; before six months, water used in formula preparation should be boiled and cooled.
