How Much Water Should a Child Drink a Day?

Children do not follow the adult rule. Their water need per kilogram is higher and falls as they grow, so the reference is organized by age. Pick an age band below for an estimate of total water per day — food included.

US reference values, by age

These are the Adequate Intakes published by the Institute of Medicine. The left column is total water from everything; the right column is what actually has to be drunk.

Age	Total water per day	Of that, beverages
0 to 6 months	0.7 L (from milk)	—
7 to 12 months	0.8 L	~0.6 L (about 3 cups)
1 to 3 years	1.3 L	~0.9 L (about 4 cups)
4 to 8 years	1.7 L	~1.2 L (about 5 cups)
Boys 9 to 13	2.4 L	~1.8 L (about 8 cups)
Girls 9 to 13	2.1 L	~1.6 L (about 7 cups)
Boys 14 to 18	3.3 L	~2.6 L (about 11 cups)
Girls 14 to 18	2.3 L	~1.8 L (about 8 cups)

The AAP’s practical version lines up with the beverage column: about 4 cups of fluids a day for ages 1 to 3, around 5 cups for 4 to 8, and 7 to 8 cups for older children, adjusted for activity, heat and humidity. Milk counts toward those cups.

A note on the tool above. The age-band calculator on this page is built on the European (EFSA) reference values, which sit slightly below the US ones — 1.6 L for ages 4 to 8 against the IOM’s 1.7 L, and 1.9/2.1 L for 9 to 13 year olds against the IOM’s 2.1/2.4 L. Where they differ, the table above is the US figure and the one to use.

Under 6 months: milk only

The AAP states it without hedging: infants under 6 months old should be drinking only breast milk or formula, and additional water is not recommended at that age. Milk is overwhelmingly water and covers the entire requirement. Offering water earlier displaces milk the baby needs and, in volume, can disturb electrolyte balance.

Around 6 months, water can be introduced with complementary foods — about 4 to 8 ounces a day until the first birthday, since the rest still arrives as milk or formula. If you are worried an infant is not getting enough to drink, the AAP’s advice is to call the pediatrician immediately rather than to improvise.

Why the adult formula does not transfer

Children carry a higher proportion of body water than adults — StatPearls puts total body water at roughly 65% to 80% in infants and children against 60% to 65% in older children — and they lose proportionally more of it to insensible losses, because their surface area is large relative to their mass. The result is a higher requirement per kilogram that declines through childhood.

Applying an adult per-kilogram rule to a small child underestimates badly. That is the reason every published pediatric reference is banded by age.

Signs of dehydration in a child

Children dehydrate faster than adults and often do not stop playing to ask for a drink, so the watching is on you. The AAP’s lists:

Infants — fewer wet diapers than the typical six to eight; unusually sleepy; sunken soft spot (fontanelle); no tears when crying.

Older children — dry lips or a sticky mouth; less urination or dark-colored urine (it should be very light yellow, almost clear); sleepy and irritable; flushed skin.

Teens — dry lips or mouth, lightheadedness, cramps, thirst, dark or reduced urine, headache, rapid pulse, flushed skin, feeling excessively hot or cold.

StatPearls grades pediatric dehydration as mild at 3% to 5% of body weight, moderate at 6% to 10%, and severe above 10%. Mild cases may show nothing but reduced urine output; dry mucous membranes, poor skin turgor, prolonged capillary refill, fast heart rate and irritability appear at moderate; altered mental status and lethargy at severe. Fever, vomiting, diarrhea or intense heat accelerate all of it, and a child who is getting worse needs medical assessment without delay.

Sports, heat and getting kids to drink

For vigorous activity, the AAP gives concrete numbers: children aged 9 to 12 need about 3 to 8 ounces every 20 minutes, and teens about 34 to 50 ounces an hour. Drink before, during and after. Past an hour of vigorous exercise, or with heavy sweating, electrolyte-containing drinks may be warranted.

Day to day, what works is access and example: a bottle or cup within reach at their height, water offered at intervals rather than on request, water and milk as the default drinks, and adults in the house drinking it too. The AAP is blunt about juice — none at all under a year, no more than 4 oz a day for ages 1 to 3, 4 to 6 oz for ages 4 to 6, and 8 oz for ages 7 to 18, and only when whole fruit is not available.

For adults and older teens, the per-weight approach on the daily water intake calculator applies. At the other end of life, thirst becomes unreliable in a different way — see how much water should seniors drink.

This content is informational. A child’s fluid needs are assessed by a pediatrician.