The average weight for a 4-month-old boy is about 14.2 pounds (6.4 kg), and for a girl it’s about 12.8 pounds (5.9 kg). These figures represent the 50th percentile on the World Health Organization growth charts, meaning half of all babies at this age weigh more and half weigh less. A healthy 4-month-old can fall anywhere between roughly 11 and 18 pounds depending on sex, genetics, and feeding patterns.
Average Weight by Sex
Boys and girls follow slightly different growth curves from birth, and the gap becomes more noticeable by 4 months. Here’s what the 50th percentile looks like at this age:
- Boys: 14.2 pounds (6.4 kg)
- Girls: 12.8 pounds (5.9 kg)
Most healthy babies fall between the 5th and 95th percentiles on standard growth charts. For 4-month-old boys, that range spans roughly 12 to 17.5 pounds. For girls, it’s about 11 to 16 pounds. A baby at the 15th percentile is not less healthy than one at the 80th percentile. What matters more than any single number is whether your baby is following a consistent curve over time.
How Growth Percentiles Work
A percentile ranks your baby against a reference population of the same age and sex. If your 4-month-old is at the 25th percentile for weight, that means she weighs the same as or more than 25% of babies her age and less than 75% of them. The CDC uses percentiles as the primary tool for tracking growth in U.S. children.
A single percentile reading is less important than the pattern across multiple checkups. A baby who has tracked along the 20th percentile since birth is growing normally. A baby who drops from the 70th percentile to the 20th over a couple of months may need a closer look, even though the 20th percentile itself is perfectly fine. Pediatricians watch for dramatic shifts off a baby’s established curve rather than focusing on a specific number.
How Fast Babies Gain Weight at This Age
Weight gain slows down around the 4-month mark. In the first few months of life, babies typically gain about 1 ounce (28 grams) per day. By 4 months, that pace drops to roughly 20 grams per day, or about 4.5 ounces per week. This slowdown is completely normal and reflects a natural shift as your baby’s metabolism changes and they become more active.
A general benchmark for the first four months: babies should gain at least 1 pound per month, measured from their lowest weight after birth (most newborns lose a few ounces in the first days). If your baby isn’t hitting that threshold, or if their growth curve drops sharply from where it’s been tracking, that’s worth discussing with your pediatrician.
Breastfed vs. Formula-Fed Babies
Breastfed and formula-fed babies often weigh about the same at 4 months, but their growth patterns start to diverge right around this point. Healthy breastfed infants typically put on weight more slowly than formula-fed infants during the first year. Formula-fed babies tend to gain weight more quickly after about 3 months of age.
This difference doesn’t mean one feeding method is better. The WHO growth charts, which are based primarily on breastfed infants, reflect a slightly leaner growth pattern as the standard. If your breastfed baby seems smaller than a formula-fed baby of the same age, that’s expected and not a sign of a problem. The CDC recommends using WHO charts for all children under 2 regardless of how they’re fed.
How Much a 4-Month-Old Typically Eats
At 4 months, formula-fed babies generally take about 6 ounces per feeding, spread across four to six feedings a day. The upper limit is 32 ounces of formula in a 24-hour period. Going over that amount consistently can lead to excess weight gain, spitting up, or digestive discomfort. Breastfed babies regulate their own intake at the breast, so volume is harder to measure, but feeding roughly every 3 to 4 hours is typical at this age.
If your baby seems hungrier than usual for a stretch of days, they may be going through a growth spurt. These are common around 3 to 4 months and can temporarily increase appetite before things settle back to normal.
Signs of Healthy Growth Beyond the Scale
Weight is one piece of the picture, but it’s not the only one. At 4 months, a well-nourished baby typically produces at least six wet diapers a day, is alert and active during wake periods, and is meeting developmental milestones like holding their head steady, bringing hands to their mouth, and tracking objects with their eyes. Steady growth in length and head circumference alongside weight gain gives a much fuller picture than weight alone.
Babies who are content between feedings, sleeping in predictable patterns, and gaining skills on a reasonable timeline are almost always growing well, even if their weight sits at the lower end of the chart.
When a Baby Was Born Early
If your baby was born prematurely, their expected weight at 4 months of calendar age will be different from a full-term baby’s. Pediatricians use “adjusted age” to account for the weeks of prematurity. A baby born 6 weeks early, for example, would be compared against growth standards for a 2.5-month-old rather than a 4-month-old. Specialized growth charts (called Fenton charts) track preterm infants’ weight, length, and head circumference by gestational age and then transition to standard WHO charts once the baby reaches their original due date.
Most premature babies catch up to their full-term peers in weight by age 2, though the timeline varies. Until then, adjusted age gives a much more accurate picture of whether growth is on track.
What Slow Weight Gain Looks Like
Occasional weigh-ins that come in a little low aren’t automatically concerning. But certain patterns signal that a baby may not be getting enough nutrition. According to Children’s Hospital of Philadelphia, flags to watch for include gaining less than 1 pound per month during the first four months, failing to regain birth weight by 2 to 3 weeks of age, or a dramatic drop from the baby’s previously established growth curve.
Slow weight gain can stem from feeding difficulties, milk supply issues, reflux, food sensitivities, or underlying medical conditions. It can also simply reflect a smaller genetic build. The distinction usually becomes clear by tracking the pattern over several checkups rather than reacting to one data point.