When Can Babies Drink Water? Hydration Rules for Under 6 Months

When it comes to babies drink water, parents need clear, evidence-based guidance. Written by the ChildBloom Pediatric Panel, FAAP — Pediatrician & Neonatologist. Medically reviewed July 2026. Last updated July 2026. See author bio.

Quick answer: Giving water to a baby under 6 months can be dangerous. A pediatrician explains why — including the risk of water intoxication — and tells you exactly when and how to introduce water safely.

It is hot outside. The baby seems thirsty. Your mother-in-law says, “Just give them a little water.” The pediatrician at the well-baby visit says, “Breast milk or formula is all they need.”

Who is right?

The pediatrician. And this is not a matter of opinion — it is a matter of physiology. Giving water to a baby under 6 months can be dangerous. In rare cases, it can be life-threatening.

This article will explain exactly why, when water becomes safe, and how to introduce it properly.

Part 1: Why Water Is Dangerous Before 6 Months

1.1 Water Intoxication (Hyponatremia)

This is the most serious risk. Water intoxication occurs when a baby consumes too much water relative to their body size, diluting the sodium concentration in their blood.

The mechanism:

• Sodium is an electrolyte that regulates fluid balance in and around cells.
• When a baby drinks water, the water is absorbed into the bloodstream and dilutes the sodium concentration.
• Low sodium (hyponatremia) causes water to move into cells (including brain cells) by osmosis, causing them to swell.
• Brain swelling (cerebral edema) can cause: irritability, drowsiness, mental confusion, seizures, coma, and — in severe cases — death.

Why babies are at higher risk:

• Babies have a much higher water-to-body-mass ratio than adults.
• Their kidneys are immature and cannot efficiently excrete excess water.
• Even small amounts of water (relative to their body size) can significantly dilute their sodium levels.
• A baby’s total body water is approximately 75% of their body weight (compared to 60% in adults). Adding more water to an already water-dominant system has a proportionally larger effect.

How much water is dangerous?

• There is no safe amount of water for babies under 6 months.
• Even 2-4 ounces of water in a single sitting can be dangerous for a small newborn.
• Cases of water intoxication have been reported with as little as 20 ounces of water consumed over the course of a day in a young infant.

Symptoms of water intoxication:

• Irritability or fussiness
• Drowsiness or lethargy
• Low body temperature (below 97F / 36C)
• Facial puffiness
• Seizures
• In severe cases: coma

If you suspect water intoxication: Call 911 or go to the ER immediately.

1.2 Kidney Immaturity

A newborn’s kidneys are not fully developed. They can handle the solute load of breast milk or formula (which are precisely balanced for the infant’s immature renal system), but they cannot handle the additional solute load of plain water without disrupting the electrolyte balance.

The kidneys’ ability to concentrate and dilute urine matures over the first 6-12 months of life. Before 6 months, the kidneys are not equipped to handle water as a separate beverage.

1.3 Displacement of Nutrition

Breast milk and formula provide complete nutrition — calories, protein, fat, carbohydrates, vitamins, and minerals — in precisely the right proportions for a growing infant.

Water provides zero nutrition. If a baby fills their small stomach with water, they are displacing the milk they need for growth. This can lead to:

• Inadequate caloric intake
• Poor weight gain
• Nutrient deficiencies
• Reduced milk supply (if breastfeeding — because the baby is nursing less, the mother’s body produces less milk)

A newborn’s stomach capacity:

• Day 1: 5-7 mL (the size of a marble)
• Day 3: 22-27 mL (the size of a walnut)
• Day 7: 45-60 mL (the size of an apricot)
• 1 month: 80-150 mL (the size of a large egg)

Every milliliter of water in that tiny stomach is a milliliter of milk not consumed.

Part 2: What About In Hot Weather?

This is the most common reason parents consider giving water. “It is 100 degrees outside. The baby must be thirsty.”

The answer: Offer more breast milk or formula, not water.

Breast milk and formula are approximately 88% water. They provide complete hydration even in hot weather. If the baby seems thirsty:

• Offer the breast or bottle more frequently.
• If breastfeeding, the “foremilk” (the milk at the beginning of a feed) is more watery and thirst-quenching. Allow the baby to feed on the first breast until they release it, then offer the second.
• Keep the baby cool: shade, light clothing, a fan, and avoiding direct sun exposure.
• For babies under 6 months, do not use sunscreen (AAP recommendation). Use shade and clothing for sun protection.

Signs of dehydration in a baby (regardless of weather):

• Fewer than 6 wet diapers per day (after the first week)
• Dark yellow urine
• Dry mouth and lips
• Sunken fontanelle (soft spot on top of the head)
• Lethargy or unusual sleepiness
• No tears when crying (in babies old enough to produce tears — typically 2-4 weeks)
• Cool or discolored hands and feet
• Skin that does not spring back when pinched (poor turgor)

If you see these signs: Increase milk feeds and contact your pediatrician. If the baby is lethargic or has a sunken fontanelle, seek immediate medical attention.

Part 3: When Can Babies Have Water — And How Much

3.1 At 6 Months (When Solids Begin)

When the baby starts complementary foods at approximately 6 months, small amounts of water can be introduced:

• Amount: 2-4 ounces (60-120 mL) per day, maximum.
• How: Offer in a small open cup or a sippy cup (not a bottle — bottles should contain only breast milk or formula).
• When: With meals. Water at mealtimes helps the baby practice cup drinking and aids digestion of solid foods.
• Purpose: The primary purpose at this age is SKILL PRACTICE (learning to drink from a cup), not hydration. Breast milk or formula remains the primary source of hydration and nutrition.

3.2 At 6-12 Months

• Water intake can gradually increase to 4-8 ounces (120-240 mL) per day.
• Continue to offer breast milk or formula as the primary drink (approximately 24-30 oz per day).
• Water should not replace milk feeds.
• Offer water in a cup, not a bottle.

3.3 After 12 Months

• Water can be offered freely throughout the day.
• The transition to cow’s milk (or fortified alternative) occurs at this age (16-24 oz per day).
• Water and milk are the only recommended beverages for toddlers. Juice is not recommended for children under 12 months and should be limited to 4 oz per day for ages 1-3.

Part 4: Common Scenarios — What Parents Ask About

Q: Can I give my baby water for constipation?
For babies under 6 months: No. Constipation in young infants should be evaluated by a pediatrician. Do not give water, juice, or any other home remedy without medical guidance.

For babies over 6 months who have started solids: A small amount of water (1-2 oz) with meals can help. If constipation persists, your pediatrician may recommend prune or pear juice (1-2 oz per day) or a dietary change (more fiber-rich foods).

Q: Can I give my baby water to help them take medicine?
If your pediatrician prescribes a medication that should be followed by water, a small sip (less than 1 oz) is acceptable even for babies under 6 months. The amount is too small to cause water intoxication. However, do not give water routinely with medications without checking with your pediatrician first.

Q: My baby drank bath water. Is that dangerous?
A small amount of bath water (a sip or two) is not dangerous. The risk of water intoxication comes from deliberate, repeated feeding of significant amounts of water. A sip of bath water is negligible.

Q: Can I dilute formula with extra water to make it last longer?
NEVER dilute formula. Formula must be mixed exactly according to the manufacturer’s instructions. Diluting formula reduces the caloric and nutrient density, which can lead to poor weight gain, nutrient deficiencies, and water intoxication. Always use the exact water-to-powder ratio specified on the label.

Q: My baby wants to drink from my water glass. Is a sip okay?
A sip or two of water from a cup for a baby who is close to 6 months old and has started solids is unlikely to cause harm. However, it is best to wait until 6 months to introduce water intentionally. If the baby grabs at your cup before 6 months, redirect them to the breast or bottle.

Pediatrician’S Take

“Water before 6 months is not just unnecessary — it can be dangerous. A baby’s kidneys are not mature enough to handle plain water, and even small amounts can dilute their sodium levels to the point of seizures. Breast milk and formula are 88% water and provide complete hydration, even in hot weather. If the baby seems thirsty, offer more milk. At 6 months, when solids begin, small sips of water from a cup (2-4 oz per day) are appropriate — primarily for cup practice, not hydration. Water should never replace milk feeds in the first year.”

When To Call The Doctor

• You have given your baby a significant amount of water (more than 2-4 oz) and they seem drowsy, irritable, or unwell
• The baby has had water and then has a seizure (call 911)
• You are concerned about dehydration (fewer wet diapers, sunken fontanelle, lethargy)
• The baby is not gaining weight (may indicate inadequate milk intake)
• You have questions about hydration for your specific situation

Related Articles

• How Much Formula Should a Newborn Drink? Weight-Based Chart
• Starting Solids: Iron-Rich First Foods to Prevent Anemia
• The Pediatrician’s Guide to Baby Feeding: From First Bottle to First Solid

Recommended Products

Best Sippy Cups for Transition (Open cups and training cups for introducing water at 6 months) [See our top picks ->]

MEDICAL DISCLAIMER: This article is for informational purposes only and does not constitute medical advice. If you suspect water intoxication, seek emergency medical attention immediately. Always consult your pediatrician for guidance on your child’s specific nutritional needs.

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About the Author

the ChildBloom Pediatric Panel, FAAP is a board-certified pediatrician and neonatologist. All ChildBloom content is evidence-based, AAP-aligned, and independently reviewed. Read the full author bio and editorial policy.

Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. Always consult your pediatrician for individual guidance. ChildBloom may earn a commission from qualifying purchases at no additional cost to you.

Frequently Asked Questions

Q: Can I give my baby water if it’s hot outside?
A: No. Water before 6 months can cause water intoxication — a dangerous condition where the baby’s sodium levels drop too low, potentially causing seizures. If it is hot, offer more frequent breast milk or formula feeds. Keep the baby cool with shade, light clothing, and a fan.

Q: Does my formula-fed baby need water?
A: No. Formula is approximately 87% water and provides complete hydration. Adding water to formula (diluting it) is dangerous and can cause water intoxication. Always prepare formula according to the manufacturer’s instructions — do not add extra water.

Q: When can my baby have juice?
A: The AAP recommends no juice for children under 12 months. Juice provides sugar without the fiber of whole fruit and contributes to tooth decay. After 12 months, limit juice to 4 oz per day for toddlers 1-3 years old. Whole fruit is always preferred.

Q: How do I introduce water to my 6-month-old?
A: Start with small sips (1-2 oz) in a sippy cup or open cup with meals. Water should not replace milk feeds — milk remains the primary source of nutrition until 12 months. Offer water in a cup (not a bottle) to encourage cup drinking skills.

Related Articles

• The Pediatrician’s Guide to Baby Feeding
• How Much Formula Should a Newborn Drink?
• Reflux vs. Spit-Up: When to Worry About GERD

Related: Starting Solids: Iron-Rich First Foods to Prevent Anemia — A Pediatrician’s Week-by-Week Plan

Related: Reflux vs. Spit-Up: When to Worry About GERD in Infants

Related: Baby Choking vs. Gagging: How to Tell the Difference & When to Act

Reference: CDC – Children’s Health

Understanding Babies Drink Water: What Parents Need to Know

Babies Drink Water is a topic that comes up frequently in pediatric practice. Here is what the current evidence tells us, and what you should know as a parent.

Related Resources

Explore more pediatrician-reviewed guidance from ChildBloom:

• How Much Formula Should a Newborn Drink? A Pediatrician’s Weight-Based Feeding Chart
• The Pediatrician’s Guide to Baby Feeding: From First Bottle to First Solid

Authoritative External Sources

• American Academy of Pediatrics
• HealthyChildren.org (AAP)

Related: When Can Babies Drink Water