A six-month-old can safely drink 4 to 8 ounces of water per day, according to the CDC. That’s roughly half a cup to one cup total across the entire day, not per feeding. This small amount is enough to help your baby get used to water while keeping breast milk or formula as their primary source of nutrition and hydration.
Why the Limit Is So Low
At six months, your baby’s kidneys are still maturing. The channels that move water across cell membranes don’t reach adult capacity until around 15 months of age, and the kidneys’ overall ability to concentrate urine keeps developing through the first two years of life. This means a young baby’s body is slower to process and excrete excess water than an older child’s or adult’s body.
Breast milk and formula already contain all the water a baby needs. Plain water is supplemental at this age. Its purpose is to familiarize your baby with the taste and the experience of drinking from a cup, not to meaningfully hydrate them.
The Risk of Too Much Water
Giving a baby significantly more water than recommended can cause a condition called water intoxication. When too much plain water enters a baby’s system, it dilutes the sodium in their blood. If sodium drops low enough, cells in the brain begin to swell. Symptoms include unusual irritability or sleepiness, low body temperature, puffiness, and in serious cases, seizures. These symptoms tend to appear after a rapid sodium drop and an increase in total body water of about 7% to 8% or more.
This doesn’t mean a few extra sips will cause harm. Water intoxication in infants typically involves much larger volumes, such as when water is used to stretch formula or replace feedings. But the margin of safety is smaller in babies than in adults, which is why sticking to the 4 to 8 ounce range matters.
Breast Milk and Formula Come First
One practical reason to keep water intake low is that babies have tiny stomachs. Water fills space that would otherwise go to breast milk or formula, which provide calories, fat, protein, and vitamins your baby needs to grow. If a baby drinks too much water before or during a feeding, they may take in less milk, and over time that can slow weight gain.
Think of water as a companion to solid foods at this stage, not a replacement for any part of a milk feeding. A few sips with a meal is the right scale.
Hot Weather and Illness
Parents often wonder whether babies need extra water in the heat. For babies under six months who are exclusively breastfed, research consistently shows that breast milk alone maintains normal hydration even in hot conditions, with no supplementary water needed. The WHO and UNICEF both maintain this recommendation regardless of climate.
Once your baby hits six months and starts solids, the same 4 to 8 ounce daily guideline applies in warm weather. If you’re concerned about dehydration during a fever or illness, offering small amounts of water is fine within that range, but breast milk or formula remains the better hydration source because it replaces electrolytes along with fluid.
How to Introduce Water Practically
Six months is a great time to start practicing with a cup. Open cups are worth trying early, even though they’re messier. A small, two-handled training cup works well because your baby can grip it independently. Start with just a tiny amount of water in the cup so spills stay manageable and your baby can focus on the skill of sipping rather than coping with a flood of liquid.
Sippy cups, straw cups, and closed feeding cups are useful for reducing mess, especially on the go. But practicing with an open cup builds oral motor skills that sippy cups don’t require, so mixing both types into your routine gives your baby the most practice. Offer a few sips of water at mealtimes when your baby is eating solids. There’s no need to offer water between meals or carry a water bottle for your baby throughout the day at this age.
What Changes After Six Months
The 4 to 8 ounce daily range covers the entire period from 6 to 12 months. As your baby eats more solid food and their kidneys continue to mature, water gradually becomes a bigger part of their diet. After the first birthday, when whole milk or continued breastfeeding replaces formula, water intake naturally increases and the strict upper limits loosen. Until then, small sips with meals are all your six-month-old needs.