Babies can start drinking small amounts of water at 6 months old, when they begin eating solid foods. Before 6 months, breast milk or formula provides all the hydration a baby needs, and giving water earlier can be genuinely dangerous. The American Academy of Pediatrics recommends breast milk or formula as the sole source of nutrition for the first six months of life.
Why Babies Under 6 Months Should Not Drink Water
A newborn’s kidneys are not mature enough to process plain water safely. At birth, a baby’s kidneys can only concentrate urine to about 800 mOsm/L, compared to 1,200 mOsm/L by around 8 weeks. Their filtration rate is a fraction of what it will be later, and their ability to handle sodium is still developing. These immature kidneys mean that even a seemingly small amount of water can throw off the delicate balance of electrolytes in a baby’s blood.
The specific risk is called water intoxication, a condition where too much plain water dilutes sodium levels in the blood. When sodium drops rapidly below 125 mEq/L, it causes cells in the brain to swell. Symptoms include unusual irritability or excessive sleepiness, low body temperature, swelling, and in serious cases, seizures. A total body water increase of just 7 to 8 percent can trigger this cascade, which in a small infant doesn’t take much liquid at all.
This is also why you should never water down formula. Over-diluting formula has the same effect: too much water relative to nutrients and electrolytes. It also means the baby gets fewer calories and nutrients per feeding, which can lead to poor growth over time.
What About Hot Weather or Illness?
Parents often worry about dehydration during heat waves or when a baby has a fever, and it’s tempting to offer a little water. But research consistently shows this isn’t necessary before 6 months. A systematic review of studies on infant hydration in hot climates found that exclusively breastfed babies maintained normal hydration levels without supplementary water, even in high temperatures. They didn’t concentrate their urine to maximum levels, meaning they had hydration to spare. Both the WHO and UNICEF recommend exclusive breastfeeding for the first six months regardless of weather conditions.
If your baby under 6 months shows signs of dehydration, such as fewer wet diapers, a dry mouth, or unusual sleepiness, the answer is more frequent breast milk or formula feedings, not water. Dehydration in very young infants can escalate quickly and warrants a visit to a doctor.
How Much Water at 6 to 12 Months
Once your baby hits 6 months and starts solids, you can introduce water in small amounts. The AAP recommends 4 to 8 ounces per day (roughly half a cup to one cup) between 6 and 12 months. This isn’t meant to replace breast milk or formula. It’s a supplement that helps with digestion of solid foods and gets your baby used to drinking water.
Think of it as practice rather than a primary fluid source. A few sips with meals is a good starting point. Breast milk or formula remains the main source of hydration and nutrition throughout the first year.
After the first birthday, water intake increases significantly. Between 12 and 24 months, the recommended range is 8 to 32 ounces per day (1 to 4 cups). By ages 2 to 5, children can drink 8 to 40 ounces daily.
Choosing the Right Cup
The 6-month mark is also a good time to introduce a cup, which doubles as practice for oral motor development. An open cup with no lid is actually the best option, even at this young age. Drinking from an open cup teaches a baby to form their lips around the rim and sip rather than suck, which supports tongue movement and eventually speech development.
If an open cup feels too messy, a straw cup without a spill-proof valve is the next best choice. It allows relatively natural tongue movement compared to hard-spouted sippy cups. Traditional sippy cups with hard spouts can restrict tongue movement when used frequently throughout the day, potentially interfering with oral motor development.
Signs your baby is ready for a cup: they can sit independently, hold objects with both hands, and bring things to their mouth. You can help by lifting the cup to their lips for small sips until they get the hang of it.
Recognizing Dehydration in Older Babies
Once your baby is 6 months and older, it helps to know the signs of dehydration so you can respond appropriately. Mild dehydration shows up as fewer wet diapers than usual. Moderate dehydration brings additional signs: a dry mouth, lips, and tongue, cool hands and feet, darker urine, and a baby who seems quieter and less active than normal.
Severe dehydration is more alarming. Cold, mottled (patchy, reddish-blue) skin, sunken eyes, extreme sleepiness, and very little or no urine output all signal an urgent problem. For babies over 6 months with mild to moderate dehydration, continuing breast milk or formula while offering small amounts of water or diluted apple juice is appropriate. For severe symptoms at any age, seek medical attention promptly.