At 4 months old, most babies weigh between 12 and 16 pounds and measure about 24 to 26 inches long. Boys tend to run slightly heavier and longer than girls at this age, but there’s a wide range of normal. By this point, many babies have doubled their birth weight, which is one of the most dramatic growth spurts of their entire lives.
Average Weight and Length at 4 Months
The WHO growth charts, which the CDC recommends for all U.S. children from birth to age 2, place the 50th percentile for 4-month-old boys at roughly 15 pounds and 25 inches. Girls at the 50th percentile are closer to 13.5 to 14 pounds and about 24.5 inches. These are midpoint numbers. A baby at the 25th percentile is smaller and a baby at the 75th is larger, and both are perfectly healthy as long as they’re growing consistently along their own curve.
What matters more than hitting a specific number is the pattern over time. A baby who was born in the 30th percentile and stays near the 30th percentile is growing exactly as expected. Pediatricians pay attention when a baby’s growth suddenly jumps or drops across multiple percentile lines, because that shift can signal a feeding issue or other concern worth investigating.
How Fast They’re Growing
During the fourth month, babies typically gain 1 to 1.25 pounds per month and grow about half an inch to a full inch in length each month. That pace is noticeably slower than the first two months of life, when weight gain can be closer to 2 pounds per month, but it’s still rapid compared to any other stage of childhood. Head circumference also continues to increase, roughly half an inch per month, as the brain grows quickly during this period.
Breastfed and formula-fed babies sometimes follow slightly different growth trajectories. Breastfed babies often gain weight faster in the first three months and then slow down compared to formula-fed babies, who tend to gain more steadily through the second half of the first year. Neither pattern is better. The WHO charts were designed around breastfed infants as the standard, which is one reason the CDC recommends them for children under 2.
How Much They’re Eating
To fuel all that growth, a 4-month-old typically takes in 24 to 30 ounces of breast milk or formula per day, spread across multiple feedings. Individual feedings usually run about 3 to 4 ounces at a time for formula-fed babies. Breastfed babies regulate their own intake at the breast, so exact ounce counts are harder to pin down, but total daily volume tends to fall in the same range.
Most 4-month-olds are not yet eating solid foods. Current guidelines recommend waiting until around 6 months, though some pediatricians give the green light closer to 4 months if a baby shows signs of readiness, like good head control and interest in food. At this stage, milk is still providing all the calories and nutrients your baby needs to grow.
What Their Size Means for Diapers and Clothes
A 4-month-old in the average weight range is typically wearing size 2 diapers, which fit babies between about 12 and 18 pounds. Smaller babies may still be in size 1 (8 to 14 pounds), while bigger babies could be moving into size 3. There’s overlap between sizes, so the best fit depends on whether the diaper leaves red marks on your baby’s thighs or leaks at the waist.
For clothing, most 4-month-olds fit into 3 to 6 month sizes, though longer babies sometimes need to size up to 6 to 9 months for sleepers and onesies just to accommodate their length. Baby clothing sizes are notoriously inconsistent across brands, so going by your baby’s actual weight and length rather than the age label on the tag saves a lot of frustration.
Physical Milestones That Reflect Their Growth
All those extra pounds and inches come with real changes in strength and coordination. By 4 months, most babies can hold their head up steadily, and many are starting to roll over, often from tummy to back first. During tummy time, you’ll likely see your baby pushing up on their arms and lifting their chest off the floor. Some babies at this age even try to bear weight on their legs when held in a standing position.
Arms and legs move with more purpose now. The random flailing of the newborn phase gives way to deliberate kicking and reaching. Babies this age are working on bringing their hands together at midline and may start batting at toys or grabbing objects within reach. These milestones are directly tied to the muscle mass and neurological development that come with healthy weight gain. Encouraging tummy time, even just a few supervised minutes at a stretch, helps your baby build the core and arm strength that rolling, sitting, and eventually crawling all depend on.