Babies under 6 months old should not drink water at all. Their hydration comes entirely from breast milk or formula. Starting at 6 months, you can introduce 4 to 8 ounces of water per day, gradually increasing as your child grows into toddlerhood. Getting these amounts right matters more than you might expect, because even small quantities of plain water can cause problems for a very young infant.
Why Babies Under 6 Months Don’t Need Water
Breast milk and formula are roughly 80% to 90% water, so a young baby who is feeding well is already fully hydrated. Their stomachs are tiny. At one month old, a baby’s stomach is about the size of an egg. Filling any of that limited space with plain water means less room for milk, which contains the calories, vitamins, minerals, and proteins a baby needs to grow.
This isn’t just a nutrition concern. Proper weight gain is one of the most important markers of infant health in the early months. Replacing even a small portion of milk feeds with water dilutes a baby’s calorie intake and can slow growth. For the same reason, you should never add extra water to powdered formula beyond what the package instructions call for, even if you’re trying to stretch your supply.
Ages 6 to 12 Months: Starting Small
Once your baby begins eating solid foods, typically around 6 months, you can offer 4 to 8 ounces of water per day. That’s roughly half a cup to one cup. The goal isn’t to replace any milk feeds. It’s to help your baby get comfortable with water alongside their new diet of purees or soft foods, and to support digestion as solids are introduced.
This is also a good time to start cup training. Babies can begin learning to drink from an open cup, sippy cup, or straw cup around 6 months. Offering water in a cup rather than a bottle builds oral motor skills and makes the eventual transition away from bottles easier. Don’t worry if your baby only takes a few sips at first. The amount matters less than the habit at this stage.
After Age 1: Increasing Water Intake
As your child moves past their first birthday and breast milk or formula becomes a smaller part of their diet, water and milk become their primary drinks. Toddlers between 1 and 3 years old generally need about 1 to 4 cups of water per day, depending on their size, activity level, and how much fluid they get from milk and food. Whole milk typically fills the rest of their hydration needs during this period.
Adjusting for Hot Weather
Heat changes the equation slightly. For babies under 6 months in hot weather, breastfed babies may simply want to nurse more often. If you’re bottle feeding, you can offer a little cooled boiled water between regular feeds during genuinely hot days. From 6 months onward, offer additional water outside of mealtimes when it’s warm. After age 1, frozen lollies made from plain water or very diluted fruit juice can help keep toddlers hydrated when they’re reluctant to drink.
The Risk of Too Much Water
Water intoxication is rare, but infants are uniquely vulnerable to it. When a baby takes in too much plain water, their blood sodium levels drop rapidly. This happens because their kidneys are still immature and can’t flush excess water the way an older child’s or adult’s kidneys can. If total body water increases by roughly 7% to 8% or more, cells in the brain begin to swell.
The signs include unusual irritability or sleepiness, low body temperature, facial or body puffiness, and in severe cases, seizures. Babies under 6 months are at the highest risk because of their small size, immature kidneys, and strong hunger drive, which can lead them to swallow large amounts of any liquid offered to them. Sticking to the recommended limits (zero water before 6 months, 4 to 8 ounces from 6 to 12 months) keeps this risk essentially off the table.
Water Safety for Formula Preparation
If you use powdered formula, tap water is safe for mixing in most cases. You can use it filtered or unfiltered, following the manufacturer’s instructions. However, extra precautions apply for babies who are younger than 2 months, were born premature (before 37 weeks), or have a weakened immune system. For these infants, boil the water first and let it cool for about five minutes before mixing it with the powder. This kills bacteria like Cronobacter that can contaminate powdered formula.
During emergencies like floods, hurricanes, or water main breaks, switch to bottled water until local authorities confirm the tap supply is safe. If the water is contaminated with chemicals rather than bacteria, boiling won’t help. Use bottled water or ready-to-feed formula instead.
Fluoride and Formula
Most municipal water contains fluoride, which is beneficial for dental health at the right levels. The optimal range in drinking water is 0.7 to 1.2 parts per million. But because formula-fed babies consume so much more water relative to their body weight than adults do, they can get excess fluoride exposure. Over time, this can cause dental fluorosis, a condition that disrupts enamel formation and creates white spots or streaks on the teeth. If formula is your baby’s main nutrition source, you can alternate between fluoridated tap water and low-fluoride or fluoride-free bottled water to reduce this risk.
How to Tell if Your Baby Is Well Hydrated
The simplest check is diaper output. A well-hydrated infant produces at least six wet diapers in a 24-hour period. Fewer than six is a sign of possible dehydration, especially if combined with a dry mouth, fewer tears when crying, or unusual fussiness and sleepiness. The urine itself should be pale and relatively odorless. Dark or strong-smelling urine in a baby is worth paying attention to, particularly during illness or hot weather when fluid losses increase.