A 9-month-old should drink 4 to 8 ounces of water per day, which is roughly half a cup to one cup. That’s less than most parents expect. At this age, breast milk or formula still supplies the vast majority of your baby’s hydration, so water is a small supplement rather than a primary drink.
Why the Amount Is So Small
Babies get most of their fluid from breast milk or formula, both of which are mostly water already. A 9-month-old drinking 24 to 32 ounces of milk per day is already well hydrated before water enters the picture. The 4 to 8 ounces of water recommended between 6 and 12 months is really just meant to help your baby get comfortable with the taste of water and practice drinking from a cup while they transition to solid foods.
Solid foods also contribute some water, especially fruits, vegetables, and purees. So even on days when your baby only takes a few sips of plain water, they’re likely getting enough total fluid.
What Happens if a Baby Drinks Too Much Water
Giving a baby too much water can cause a dangerous condition called water intoxication. It happens when excess water dilutes the sodium in a baby’s blood, causing cells to swell. In infants, symptoms include unusual irritability or sleepiness, low body temperature, puffiness, and in severe cases, seizures. These symptoms appear when total body water increases by roughly 7% to 8% or more in a short period.
A baby’s kidneys are still maturing and can’t flush large volumes of water as efficiently as an adult’s. That’s why the guideline is capped at 8 ounces per day. You don’t need to measure every sip precisely, but avoid offering water as a way to fill your baby up between feedings or using it to replace a milk feeding.
How to Offer Water at 9 Months
Nine months is a great time to practice cup skills. Most babies are developmentally ready to start learning to drink from a cup between 6 and 9 months, right around the time they begin eating solid foods. Your baby should be able to sit up without support before using a cup.
An open cup or a small cup with a lid (but no spout) is ideal for building proper drinking skills. Sippy cups with spouts help avoid spills, but they can become a hard habit to break if used too long. If you do use a sippy cup, let your baby drink from it only while sitting. Carrying a sippy cup around while crawling or walking creates a risk of mouth injury during a fall. The goal is to transition away from spouted sippy cups by around 18 months and to a completely lidless cup between ages 2 and 3.
Offer small amounts of water with meals or snacks. A few sips at a time is perfectly fine. There’s no need to push your baby to finish a set amount.
Tap Water, Bottled Water, and Fluoride
Plain tap water is fine for most 9-month-olds, but fluoride is worth thinking about. If your tap water is fluoridated and your baby also drinks powdered or liquid concentrate formula mixed with that same water, the combination can slightly increase the chance of mild dental fluorosis (faint white marks on the teeth that develop later). This is a cosmetic issue, not a health risk, but you can reduce it by occasionally using low-fluoride bottled water to mix formula. Look for bottles labeled purified, distilled, demineralized, or de-ionized, as these contain little to no fluoride unless it’s listed as an added ingredient.
For the small amounts of plain drinking water you’re offering at meals, regular tap water is typically fine. If your local water supply has fluoride levels above 2.0 mg/L (your water utility can tell you), consider using an alternative water source for your baby’s drinking water as well.
Signs Your Baby Needs More Fluid
Even though you’re limiting plain water, you still want to make sure your baby is getting enough total fluid from all sources. Dehydration in babies shows up as:
- Fewer wet diapers than usual
- A sunken soft spot on top of the head
- Sunken eyes
- Few or no tears when crying
- Unusual drowsiness or irritability
If your baby has diarrhea, vomiting, or a fever, fluid losses increase and dehydration can develop faster. In these situations, the right response is usually more breast milk or formula rather than large amounts of plain water. Oral rehydration solutions designed for infants are another option during illness, since they replace both water and electrolytes.
A Typical Day of Fluids at 9 Months
Putting it all together, a 9-month-old’s daily fluid intake looks something like this: several breast milk or formula feedings totaling roughly 24 to 32 ounces, plus a few small sips of water offered in a cup at mealtimes. The water portion stays in that 4 to 8 ounce range. Juice isn’t recommended at this age, and no other beverages are needed.
If your baby seems uninterested in water some days, that’s normal. They’re getting plenty of hydration from milk. The cup practice matters more than the volume at this stage.