The average 6-month-old baby weighs about 16.5 pounds (7.5 kg) for boys and 15 pounds (6.8 kg) for girls, based on World Health Organization growth standards. But there’s a wide range of healthy weights at this age, and your baby’s individual growth pattern matters more than hitting one specific number.
Average Weight at 6 Months
At the 50th percentile on WHO growth charts (the midpoint where half of babies weigh more and half weigh less), a 6-month-old boy weighs roughly 17.4 pounds (7.9 kg) and a girl weighs about 16 pounds (7.3 kg). Babies anywhere between the 5th and 95th percentiles are generally considered within the normal range, which means a healthy 6-month-old could weigh anywhere from about 13 to 21 pounds depending on sex and build.
The AAP and CDC recommend using the WHO growth charts for all infants during the first two years of life. These charts are based on data from breastfed babies across multiple countries and represent how children grow under optimal conditions. Pediatricians look at where your baby falls on these curves and, more importantly, whether they’re following a consistent trajectory over time.
How Fast Babies Gain Weight at This Stage
Between 4 and 6 months, most babies gain about 1 to 1.25 pounds per month. That’s noticeably slower than the first few months of life, when weight gain can be twice as fast. This natural slowdown is completely normal and reflects the shift from rapid early growth to a more gradual pace.
A common rule of thumb is that healthy, full-term babies double their birth weight by about 4 months. So by 6 months, most babies are well past that doubling mark. By their first birthday, most will have tripled their birth weight. If your baby was born at 7 pounds, for example, you’d expect them to be around 14 pounds by 4 months and somewhere near 21 pounds by 12 months, with the 6-month weight falling somewhere in between.
Breastfed vs. Formula-Fed Growth Patterns
How you feed your baby can affect how quickly they put on weight. Breastfed infants typically gain weight more slowly than formula-fed infants during the first year, with the difference becoming more noticeable after about 3 months. Formula-fed babies tend to gain faster from that point on, and the difference in weight patterns continues even after solid foods are introduced around 6 months.
This doesn’t mean breastfed babies are underfed. The WHO growth charts were designed around breastfed infants and reflect a healthy growth pattern. Length growth is similar regardless of feeding method. If your breastfed baby seems to be gaining more slowly than a formula-fed peer, that’s expected and not a reason for concern on its own.
What Matters More Than the Number
Pediatricians care less about one weigh-in and more about the curve. A baby who has been tracking along the 25th percentile since birth is growing perfectly well, even though they weigh less than average. What raises a flag is a sudden change: a baby who was cruising along the 60th percentile and drops to the 15th over a couple of months, for instance.
Some variability is normal. Crossing a single percentile line can happen for all sorts of benign reasons, like a growth spurt that hasn’t arrived yet or a recent illness. The pattern that prompts concern is when weight drops first and then length starts dropping a few months later, which can signal that a baby isn’t getting enough calories. If weight and length both decline at the same time while the ratio between them stays normal, that points toward a different kind of issue, potentially something hormonal affecting overall growth.
The AAP uses the 2nd and 98th percentiles as boundary markers for “potentially suboptimal growth,” meaning babies who fall below or above those lines deserve a closer look. Between those boundaries, the focus is on consistency rather than a specific number.
Premature Babies and Adjusted Age
If your baby was born early, the 6-month weight expectations shift. Pediatricians use “corrected age” for premature infants, meaning they subtract the weeks of prematurity from your baby’s actual age. A baby born two months early would be compared to the growth chart for a 4-month-old when they’re 6 months old chronologically. This adjustment continues until age 2.
Premature babies also have their own specialized growth charts. Catch-up growth, where preemies close the gap with full-term peers, typically happens by 12 to 18 months of corrected age but can continue for several years. Head circumference catches up first, followed by weight and then length. For babies who were very small at birth due to growth restriction in the womb, the goal may not be to reach the same percentiles as full-term babies but simply to follow their own steady curve.
What Can Affect Your Baby’s Weight
Genetics play the biggest role. Tall, larger parents tend to have bigger babies, and smaller parents tend to have smaller ones. Birth weight itself sets a baseline: a baby born at 9 pounds is going to chart differently than one born at 6 pounds, and both can be perfectly healthy.
Around 6 months, most babies are starting solid foods, which can temporarily affect weight gain in either direction. Some babies take to solids enthusiastically and gain a bit faster. Others are slow to warm up and may briefly plateau. Illness, teething, and sleep disruptions can also cause short-term dips in appetite and weight gain that resolve on their own.
The introduction of solids is also the point where the AAP notes that weight-for-length measurement becomes especially useful. For babies under 6 months, BMI may actually be a better indicator of excess relative weight, but after 6 months, weight-for-length plotted on the growth chart gives a clearer picture of whether a baby’s size is proportionate.