A friendly older relative will, at some point in your baby's first six months, offer to "give the baby a little drink of water" on a hot day. The instinct is kind. The advice is wrong, and the reason matters: a young baby's kidneys cannot handle a meaningful volume of plain water without diluting their blood sodium, and the consequences in the most serious cases can include seizures.
Once that line is understood, the rest of the guidance is simple.
Healthbooq covers infant nutrition and hydration across the first year.
Why Water Is Not Right Before Six Months
A newborn's kidneys are still maturing. They cannot efficiently excrete a sudden load of water without flushing out sodium with it. If a baby under six months is given a few ounces of plain water — particularly across a few feeds — the sodium concentration in their blood can drop into a dangerous range. The clinical name is hyponatraemia. The lay name is water intoxication. In severe cases it presents with irritability, sleepiness, vomiting, and seizures.
It is uncommon, but it is real, and it is the reason every paediatric body in the UK and US is unambiguous on this point: under six months, no plain water in any meaningful volume.
The other half of the answer is that babies under six months do not need water. Breast milk is around 87% water, and formula prepared correctly is similar. Whatever the weather, both are complete hydration. A baby in hot weather will simply feed more often — that is how the system works, and the breastfeeding parent's supply adjusts within a day or two to match.
The One Exception: Formula-Fed Babies in a Heatwave
If you are formula feeding and the weather is genuinely hot — a sustained heatwave, not just a warm afternoon — and the baby seems extra unsettled, NHS guidance does allow a small amount (around 30 to 60 ml) of cooled, boiled water between feeds. The water has to be properly prepared: tap water is not sterile for very young babies, so it should be boiled and then cooled to room temperature.
This is a small adjustment for a specific situation, not a hidden requirement that parents are forgetting. Most formula-fed babies, in most British summers, get all the water they need from their formula.
From Six Months: A Cup of Water at Meals
Once solids start, water enters the picture as a sensible accompaniment to meals. From six months:
- Tap water is fine, no boiling needed.
- Offer it in an open cup or a free-flow beaker, not a bottle.
- Small amount — a few sips. Most of it ends up on the bib for the first weeks.
- Offer it at mealtimes rather than as something to graze on between meals.
Water at meals helps your baby get used to the cup, helps with swallowing solids, and keeps the meal as a defined event with a beginning and an end.
What does not belong in the cup before age one (and is best kept rare even after that):
- Fruit juice — sugar load, acid load, no nutritional benefit a baby cannot get from whole fruit.
- Squash, cordial, fizzy drinks, flavoured "kids' drinks" — same problem, often worse.
- Tea, coffee, anything caffeinated.
- Cow's milk as a main drink before twelve months. (In cooking and food, fine from six months. As the main drink, twelve months and over.)
If your toddler does drink juice occasionally, keep it diluted (one part juice to ten parts water), keep it to mealtimes, and use a cup rather than a bottle. The single thing that wrecks toddler teeth fastest is sweet drinks sipped slowly across the day.
Knowing When a Baby Is Hydrated
Through the whole first year, milk is the measure of hydration, not water. The everyday signs that a baby is well hydrated:
- Six or more wet nappies a day after the first week
- Pale yellow or near-clear urine
- A moist mouth and tongue
- Normal energy, normal feeding, normal alertness
Signs that suggest a baby is dehydrated and should be seen the same day:
- Fewer than six wet nappies in 24 hours, or noticeably drier nappies
- Dark yellow, strong-smelling urine
- A sunken fontanelle (the soft spot on top of the head)
- Sunken eyes, dry mouth
- Drowsiness or floppiness, particularly in hot weather or with a tummy bug
A baby with these signs needs a GP or 111 the same day, sooner if they are also vomiting and unable to keep feeds down.
Key Takeaways
Under six months, water is not just unnecessary — it can be harmful. Breast milk and formula are designed to hydrate. Plain water in any volume can dilute the baby's blood sodium and, in severe cases, cause seizures. From six months, small sips of tap water with meals are fine and useful for cup practice. Formula-fed babies in genuinely hot weather are the only exception under six months: a small amount (30–60 ml) of cooled boiled water between feeds is reasonable.
