The average 6-month-old boy weighs about 17.5 pounds (7.9 kg), and the average 6-month-old girl weighs about 16.1 pounds (7.3 kg). These numbers come from the WHO growth charts, which pediatricians in the United States use to track infant growth through the first two years. But “average” is just the midpoint on a wide spectrum of healthy weights, and where your baby falls on that spectrum matters less than how consistently they’re growing over time.
Typical Weight Range at 6 Months
A healthy 6-month-old can weigh anywhere from about 13 to 21 pounds and still be perfectly on track. The WHO growth charts show weight as a curve of percentiles, not a single target number. A baby at the 15th percentile and a baby at the 85th percentile are both growing normally, as long as each one is following their own curve without dramatic drops or spikes.
A useful rule of thumb: most babies double their birth weight by 6 months. So if your baby was born at 7.5 pounds, you’d generally expect them to be somewhere around 15 pounds at the half-year mark. Babies who were born smaller or larger will land in different spots on the scale, and that’s expected.
How Fast Babies Gain Weight
Weight gain isn’t steady throughout infancy. It follows a clear pattern of slowing down. In the first few months, babies gain roughly 1 ounce (28 grams) per day. By around 4 months, that pace drops to about 20 grams a day. By the time they reach 6 months, many babies are gaining 10 grams or less per day.
This slowdown is completely normal and catches some parents off guard. If your baby seemed to pack on weight quickly in the early months and then leveled off, that’s the typical trajectory. Babies who are starting to roll, sit, and move more are burning more energy, which naturally slows their rate of gain.
Boys vs. Girls
Boys tend to weigh slightly more than girls at every age during infancy. At 6 months, the gap is roughly 1 to 1.5 pounds on average. This difference is reflected in the WHO growth charts, which have separate curves for boys and girls. Your pediatrician will plot your baby’s weight on the chart that matches their sex.
Breastfed vs. Formula-Fed Babies
How your baby is fed can influence what the scale shows at 6 months. Healthy breastfed infants typically put on weight more slowly than formula-fed infants, and this difference becomes noticeable after about 3 months of age. It persists even after babies start solid foods.
This is one reason the AAP and CDC recommend using the WHO growth charts for all infants under 2. The WHO charts were built from data on breastfed babies and reflect a growth pattern that pediatricians consider the biological norm. A breastfed baby who appears to “drop” on an older growth chart may actually be growing exactly as expected on the WHO curves.
What Affects Your Baby’s Weight
Several factors shape where a 6-month-old lands on the growth chart, and most of them were set before your baby was even born.
- Genetics: Tall, large-framed parents tend to have bigger babies. Smaller parents tend to have smaller babies. By 6 months, genetic influence on size is increasingly visible.
- Birth weight: Babies who were born heavier generally remain on a higher percentile curve. Research also suggests that a higher birth weight raises the risk of childhood obesity, though being a big baby doesn’t guarantee weight problems later.
- Gestational age: Babies born prematurely often weigh less at 6 months of actual age. Pediatricians typically use a “corrected age” for preemies, adjusting expectations based on their original due date rather than their birth date.
- Gestational diabetes: Babies born to mothers who had gestational diabetes may have had a higher birth weight, which can shift their starting point on the growth curve.
- Feeding method: As noted above, breastfed and formula-fed babies follow slightly different weight trajectories in the first year.
How Pediatricians Evaluate Growth
At the 6-month wellness visit, your pediatrician will weigh your baby, measure their length, and check their head circumference. All three measurements get plotted on the WHO growth chart. The goal isn’t to hit a specific number. It’s to see a consistent pattern over multiple visits.
Doctors look for two main red flags. The first is a weight (or weight-for-length) that falls below the 5th percentile. The second, and often more concerning, is a drop across two or more percentile lines over time. For example, a baby who was at the 50th percentile at 2 months and falls to the 10th percentile by 6 months would warrant a closer look, even though the 10th percentile is technically within the normal range. A single measurement tells you very little. The trend across visits is what matters.
There’s one important exception: babies who were born large for gestational age often drift down on the growth chart during their first several months as they settle into their genetically programmed size. This kind of downward shift is compensatory and not a sign of a feeding problem.
Signs of Healthy Growth Beyond the Scale
Weight is just one piece of the picture. At 6 months, a baby who is growing well is also hitting developmental milestones. They’re likely reaching for objects, rolling in at least one direction, sitting with some support, babbling, and showing interest in the world around them. They’re producing plenty of wet diapers (six or more per day is a common benchmark) and seem satisfied after feedings.
If your baby is on the lighter or heavier end of the range but is active, alert, meeting milestones, and following a consistent growth curve, their weight is almost certainly fine. The number on the scale matters far less than the overall pattern of thriving.