Most six-month-old boys weigh around 17.5 pounds (7.9 kg), and most girls weigh around 16.1 pounds (7.3 kg). These are the 50th percentile values on the WHO growth charts, meaning half of all healthy babies weigh more and half weigh less. A wide range is perfectly normal: boys between roughly 14.5 and 21 pounds and girls between roughly 13.5 and 19.5 pounds all fall within typical growth curves.
What the Averages Actually Mean
Pediatricians don’t focus on a single “ideal” number. They use growth chart percentiles, which show how your baby’s weight compares to thousands of other babies the same age and sex. A baby at the 15th percentile isn’t unhealthy, and neither is a baby at the 90th. What matters most is that your baby follows a consistent curve over time. A baby who has been tracking along the 25th percentile since birth and continues to do so at six months is growing exactly as expected.
A common rule of thumb: most healthy, full-term babies double their birth weight by about four months and triple it by around one year. So if your baby was born at 7.5 pounds, you’d expect somewhere around 15 pounds by four months and roughly 22.5 pounds by their first birthday. At six months, weight typically falls somewhere between those two milestones.
How Fast Weight Gain Slows Down
Babies don’t gain weight at the same pace throughout their first year. In the first few months, they pack on about one ounce (28 grams) per day. By four months, that slows to roughly 20 grams a day. And by six months, many babies are gaining 10 grams or less per day. This slowdown is completely normal and reflects a shift in how babies use energy: they’re becoming more active, rolling, sitting, and burning more calories through movement.
Parents sometimes worry when the scale seems to barely move between the four-month and six-month checkups. If your baby is alert, meeting developmental milestones, producing plenty of wet diapers, and staying on their growth curve, a slower rate of gain at this age is expected, not a red flag.
Boys vs. Girls
Boys tend to be slightly heavier and longer than girls at six months. The average boy measures about 26.6 inches (67.6 cm) in length, while the average girl is about 25.9 inches (65.7 cm). Weight differences are modest, typically a pound or so at the 50th percentile. Your pediatrician uses sex-specific growth charts to account for these differences, so your baby is always being compared to the right reference group.
Breastfed vs. Formula-Fed Babies
Growth patterns differ depending on how your baby is fed, and this is one of the most common sources of unnecessary worry. Healthy breastfed infants typically gain weight more slowly than formula-fed infants during the first year. Formula-fed babies tend to pick up weight more quickly after about three months of age, and these differences persist even after solid foods are introduced around the six-month mark.
Importantly, length (linear growth) is similar regardless of feeding method. The difference is in weight gain, not overall development. The WHO growth charts, which most pediatricians now use, were designed based on breastfed infants and are the recommended standard. If your doctor is using older CDC charts, a breastfed baby may look like they’re “falling behind” when they’re actually growing normally. It’s worth asking which chart your pediatrician uses.
When Percentile Drops Matter
No single weigh-in tells the full story. Pediatricians look for patterns across multiple visits. The concern isn’t where your baby sits on the chart but whether they’re crossing downward across percentile lines over time. A baby who drops from the 75th percentile to the 25th percentile over two or three visits, for example, warrants closer evaluation, even though the 25th percentile is technically “normal.”
This kind of sustained downward shift can signal feeding difficulties, food sensitivities, or other issues that are usually very treatable when caught early. On the flip side, a single visit where your baby’s weight dips slightly can reflect something as simple as the timing of their last feeding or a recent illness. Context matters more than any one number.
Premature Babies Need Adjusted Age
If your baby was born early, their weight at six calendar months won’t match the charts for a full-term six-month-old, and it shouldn’t. Pediatricians use “adjusted age” (sometimes called “corrected age”) to account for prematurity. You calculate it by subtracting the number of weeks your baby was born early from their actual age. A baby born eight weeks premature who is now six months old would be assessed as a four-month-old for growth purposes.
Most doctors continue using adjusted age for growth monitoring until around two years old. If your preemie’s weight seems low compared to other six-month-olds, make sure the comparison is being made using corrected age. The NICHD Neonatal Research Network offers an online calculator where you enter your baby’s birth date, visit date, and gestational age at birth to get the adjusted age automatically.
What Your Baby’s Weight Tells You (and Doesn’t)
Weight is one piece of a much bigger picture. At the six-month checkup, your pediatrician is also looking at length, head circumference, developmental milestones, feeding patterns, and overall alertness. A chubby baby isn’t necessarily healthier than a lean one. Genetics play a significant role: tall, slim parents tend to have babies who track along lower weight percentiles, and that’s exactly where those babies belong.
The six-month visit also typically coincides with the introduction of solid foods, which can temporarily affect weight gain in either direction. Some babies take to solids quickly and gain a bit faster. Others are slow to warm up and may eat less breast milk or formula during the transition. Both patterns usually even out within a few weeks.