Key Takeaways

From around 6 months, offer sips of water with meals, in a cup. The AAP suggests 4 to 8 ounces a day, which is 120 to 240 mL ¹².

Water for a baby over 6 months does not need to be boiled, according to the NHS ¹. Formula, at any age, does ³.

The six-month line is not one rule. Fully breastfed babies need no water at all, but formula-fed babies under 6 months can be given small sips of cooled boiled water in hot weather ¹⁴.

Water intoxication is rare and it is not caused by a spoonful. The reported cases are 20 to 30 ounces a day for days, or formula mixed too weak ⁵⁶⁷.

Juice is not a water substitute. The AAP advises no fruit juice under 12 months, and says it should not be used for dehydration or diarrhea ⁸⁹.

The short answer

Six months is the line, and the water is a skill, not a fluid target. A baby that age still gets almost all of their liquid from milk.

Your baby	Water	How much	What to use
0 to 6 months, fully breastfed	Not needed	Not needed	Breast milk only ¹
0 to 6 months, formula fed	Small sips of cooled boiled water in hot weather	A little, alongside normal feeds	Cup or bottle ¹⁴
6 to 12 months	Sips with meals	4 to 8 oz a day, 120 to 240 mL ²	Open or free-flow cup ¹
12 months and over	As wanted	Water becomes the default drink	Open cup; bottles discouraged ¹

Two things in that table surprise parents. The first is that a formula-fed 4-month-old may have water at all. The second is that a breastfed one may not. Both are correct, and the reason is in the next section.

Why the six-month line exists

The rule is usually stated as a prohibition, but the reasons behind it are specific and they do not all apply equally.

Milk is already mostly water. The NHS states that breast milk is the only food or drink babies need for the first 6 months ¹. Formula, mixed to the instructions on the package, is the same idea. There is no fluid deficit to correct in a healthy baby who is feeding well.

A young baby cannot concentrate urine well. The kidneys of a newborn are less able to excrete a water load. Excess free water therefore dilutes the sodium in the blood rather than being cleared. That is the mechanism behind water intoxication, and it is why the risk is highest in the first months and falls as the kidneys mature ⁵⁷.

Water takes up space that milk should occupy. A stomach is small. Fluid that contains no calories displaces fluid that does, and in the first months that trade is not neutral: it is a calorie deficit in a period of rapid growth. The newborn feeding guide covers how much milk that trade is protecting.

Those three reasons are why the advice exists. They are also why it softens at the edges, which is the part most articles leave out.

The exception nobody mentions

NHS guidance for hot weather is more specific than the headline rule. Fully breastfed babies do not need water, and during hot weather they may want to breastfeed more than usual. But if you are bottle feeding, you can give a little cooled boiled water alongside the usual milk feeds ⁴.

So the two halves of the same page say different things, and the difference is deliberate. A breastfed baby regulates intake at the breast: more frequent feeds are the answer, and offering water reduces the feeds that build supply. A formula-fed baby cannot do that, so a small amount of water is a reasonable addition in heat.

The UK health visiting guidance puts it more bluntly. Babies under 6 months should not be offered additional water, and you should speak to a health visitor or GP before doing so ¹⁰. Read together, the position is: not routinely, and not without advice, but not an absolute prohibition for a formula-fed baby in a heatwave.

What no source supports is water for a breastfed newborn, or water instead of a feed at any age.

How much water at six months

Four to eight ounces, which is 120 to 240 mL, or half a cup to a cup ². That is the AAP figure, and it is a daily total, not a per-sitting target.

Put it beside the milk. At 4 to 6 months, most infants take 20 to 40 ounces of formula a day ¹¹, and our formula feeding chart by age shows how that volume changes. Four to eight ounces of water is therefore somewhere between a tenth and a third of the milk volume. It is a taste and a skill, not a source of hydration.

This is why the practical advice is sips with meals rather than a cup on demand. Water at mealtime does three things at once. It washes food down. It gets the baby used to the taste of plain water before sweet things arrive. And it builds the habit of drinking from a cup. The NHS framing is exactly this, sips of water with meals from around 6 months ¹.

Three corollaries that follow from the number:

Do not chase a quota. A baby who drinks 30 mL and tips the rest down their front has done the job. The goal is familiarity.
Do not replace a feed with water. If a baby seems hungry after a meal, the next thing is milk, not more water.
Do not add anything to it. No juice, no sugar, no cereal, no flavoring. The NHS is explicit that nothing but breast milk, formula or water goes in a bottle or trainer cup ¹.

Does it need to be boiled?

This is the question with the most confident wrong answers on the internet, because three different questions get merged into one.

What you are doing	Does the water need boiling?
Giving a drink to a baby over 6 months	No, tap water is fine ¹
Making up powdered formula, at any age	Yes, and it should still be at least 70°C when it meets the powder ³
Using bottled water for a baby	Not sterile, so treat it as unsafe for formula unless boiled ³¹⁰
Sterilising equipment	Separate question, and it applies to bottles, not to drinking water

The distinction that matters: boiling for a drink is about killing what is in the water, and UK tap water does not need that for a baby over 6 months. Boiling for formula is about killing what is in the powder. That is why the water has to be hot at the moment of mixing, and not merely boiled at some earlier point ³. The powdered formula guide covers why the powder is the risky part.

Bottled water is its own trap. The NHS advises against it for making up formula, because it is not sterile and may contain too much salt or sulfate ³. If you are traveling and the local water is unsafe, health visiting guidance gives the numbers to check on the label. Look for less than 200 mg a liter of sodium, written as Na, and less than 250 mg a liter of sulfate, written as SO4 ¹⁰. A low-sodium bottled water is not a safer choice for a baby. It is the opposite, and that is the next section.

Water intoxication, and how little it takes

Water intoxication means hyponatremia from drinking more free water than the kidneys can clear. Sodium in the blood falls, cells swell, and in a baby the first visible signs can be lethargy, vomiting, and a seizure.

It is rare, and the case reports are consistent about the size of the overdose. In one, a 2-month-old was fed 20 to 30 ounces of water a day for three days while formula was withheld during vomiting and diarrhea. The baby arrived with seizures, hyponatremia and hypothermia ⁶. That is roughly 600 to 900 mL a day, in a baby whose entire intake should have been milk.

A 1997 paper in Pediatrics is more useful still, because it describes the pattern rather than a single case. Hyponatremic seizures from water intoxication were being reported with increasing frequency, particularly in infants fed fluids other than formula. Most often the fluid was tap water, given as a supplement or as over-diluted formula, over a relatively short period. Less often it was juice, soda, or tea ⁵.

That paper also documented something that should be better known. Two infants developed hyponatremic seizures after being fed the same commercial bottled drinking water marketed for use in babies ⁵. The water was not contaminated. It was simply low in sodium, and it was being used as a feed.

A French case report describes the other common route: a dilute formula plus free water, presenting as status epilepticus, apnea and hypothermia ⁷.

Two practical conclusions follow, and neither is “never give water”.

The risk comes from volume and dilution, not from a sip. A teaspoon with a meal is not the mechanism described in any of these reports.
Never water down formula, and never stretch a feed. If a baby seems hungry and the formula is running low, the answer is a new tub, not a weaker bottle.

The cup matters more than the water

If the water is optional and the volume is tiny, why bother at 6 months? Because the cup is the point.

The NHS recommends introducing a cup from around 6 months, and specifically an open cup or a free-flow cup without a valve. That design makes a baby sip rather than suck, and it is better for their teeth ¹. A valved sippy cup works more like a bottle, and the NHS describes what that means for teeth. Drinks flow slowly through a nipple, so a sweetened drink stays in contact with them for longer ¹. Free flow is the design that avoids the problem.

The rest of the sequence is equally specific:

Open cup first, lidded cup second, and move on as soon as the baby can manage it ¹.
Bottles should be discouraged from 12 months ¹.
Nothing but milk, formula or water in a bottle or trainer cup, and no added sugar, cereal, or chocolate powder ¹.
Expect a mess. The NHS says so directly: it will be messy at first, and the baby will learn ¹.

A straw cup is a reasonable middle step and the AAP lists open, sippy and straw cups as acceptable ². The principle is free flow, not a particular product. The 6-month solids guide has the same cup advice in the mealtime context.

Juice is not a water substitute

The AAP position is unambiguous: fruit juice should not be given to infants under 12 months ⁸⁹. Juice offers no nutritional benefit in that age group, and it raises the risk of tooth decay while building a preference for sweet drinks over plain water ⁹.

Two further points are worth carrying into toddlerhood. From 12 months, 4 ounces of juice a day as part of a meal may be reasonable, and between 1 and 6 years the limit is 4 to 6 ounces ⁹. And juice should not be used to treat dehydration or manage diarrhea, which is the claim that keeps it in the house ⁹.

The practical version: water with meals, milk for the rest, and fruit instead of juice. If a baby refuses water and takes juice happily, the juice is the reason, and the fix is to stop offering it rather than to sweeten the water.

What water does not fix

Water is a drink, not a treatment, and several common problems get worse when it is used as one.

Constipation. Extra water helps at the margins, and only once solids are established. It is not a first-line treatment; the guide to poop after starting solids covers amounts and the fiber side. The short version is that a constipated baby usually needs a dietary review, not a bottle of water.

Hiccups. They stop on their own. The usual advice is to feed, or to wait a few minutes, and there is no evidence that water shortens them.

Night waking. A baby who wakes at night is usually hungry, thirsty for milk, or in a sleep pattern that water will not change. The NHS advice for a bottle-fed baby who wakes is to offer the usual milk, and cooled boiled water only if the milk feeds have already been had ⁴.

A hot baby. In hot weather the answer for a breastfed baby is more breastfeeds, not water ¹⁴. For a formula-fed baby over 6 months, ordinary water is fine.

A “thirsty” baby who is not drinking. If a baby is refusing fluids, the question is why, and the answer is a clinical assessment rather than a different cup.

When water is the wrong fluid

Dehydration is the one situation where the instinct to give water is understandable and wrong.

The NHS signs in a baby are a sunken soft spot on the head, sunken eyes, few or no tears when crying, fewer wet diapers than usual, and drowsiness or irritability ¹². Those are signs to get advice the same day, not signs to push water.

For a baby losing fluid through vomiting or diarrhea, the replacement is an oral rehydration solution, which contains the sugar and salts that water does not ¹². Plain water alone does not correct that loss and in a young baby can worsen the sodium picture.

What you see	What to give
Sunken soft spot, sunken eyes, no tears, fewer wet diapers	Same-day advice; oral rehydration solution if recommended ¹²
Vomiting or diarrhea with ongoing losses	Oral rehydration solution, not water or juice ¹²
Fever with poor drinking	Milk first; fluids and advice if intake is falling
Hot weather, feeding well, wet diapers	Normal feeds, plus water with meals over 6 months ¹⁴

How to tell it is enough

You do not need to count milliliters. Three observations cover it.

Wet diapers. A baby who is having regular wet diapers and pale urine is hydrated. Fewer wet diapers than usual is the signal, and the NHS lists it as a dehydration sign ¹².
The soft spot. A sunken fontanelle is a late sign, and a normal one is reassuring rather than diagnostic.
Behaviour at the feed. A baby who feeds with interest and settles afterwards is getting what they need. A baby who pushes the breast or bottle away repeatedly is a different problem, and worth raising.

The one number worth keeping in mind is the upper bound, not the lower one. At 240 mL a day, the top of the AAP range for this age, water is still a small fraction of what a baby drinks ².

What is worth recording

Almost nothing, which is the point. Water at this age is a habit rather than a quantity, so the only record that earns its place is the one you would need at an appointment.

That means two columns: what went in, and what came out. A week of wet diapers beside a week of feeds answers the hydration question faster than any count of milliliters, and it is the pattern a clinician asks for. Our water tracker logs a drink in one tap and sits beside feeding and diapers, so the two sides of the ledger are in the same place. If logging is starting to feel like a job, tracking without data anxiety is the counterweight.

The other thing worth noting is a reaction: a new cup, a first taste of water, a bout of gagging or coughing. Those are the details that get forgotten by the next appointment.

Frequently asked questions

Can a 6 month old drink tap water?

Yes, in the UK. The NHS says drinking water for babies over 6 months does not need to be boiled first ¹. If you are traveling somewhere the tap water is not safe, use bottled water that is low in sodium and sulfate and treat it as not sterile ¹⁰.

How much water should a 6 month old drink?

Four to eight ounces a day, which is 120 to 240 mL, offered in sips with meals ². Most of it will end up on the bib at first, and that is expected.

What happens if you give a 4 month old water?

A small taste is unlikely to cause harm, but it is not recommended, and the risk rises with volume. The reported cases of water intoxication involved 20 to 30 ounces a day, or formula mixed too weak ⁵⁶⁷. If a formula-fed baby is hot under 6 months, NHS guidance allows small sips of cooled boiled water alongside normal feeds ⁴.

Do I need to boil water for a 6 month old?

No, not for drinking ¹. Yes, for making up powdered formula, at any age, and the water should still be at least 70°C when it is mixed with the powder ³. Bottled water is not sterile and needs boiling if it is used for formula ³.

Can I give my baby water at night instead of milk?

Only if the baby has already had their usual milk feeds. The NHS advice for a bottle-fed baby who wakes is to offer milk first, and cooled boiled water as well only after the normal feeds ⁴. Replacing night feeds with water reduces calorie intake in a baby who may still need it.

Is water or juice better for constipation?

Neither is a treatment. Water helps a little once solids are established, and juice is not recommended under 12 months at all ²⁹. The poop guide covers what actually changes stool, and persistent constipation needs a clinician rather than a drink.

Disclaimer: This article is for informational purposes only and does not constitute medical advice. How much water a baby needs depends on their age, feeding, health, and the weather. Follow the guidance from your own health visitor or pediatrician, and seek same-day advice for a baby with signs of dehydration, a fever under 3 months, or a baby who is difficult to rouse.

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