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). Most healthy babies double their birth weight by this age, so if your baby was born at 7 pounds, you can expect them to be somewhere around 14 pounds at the 6-month mark.
Average Weight by Sex
Boys and girls follow slightly different growth curves from birth. At 6 months, boys tend to be about a pound to a pound and a half heavier than girls. The World Health Organization growth charts, which the CDC recommends for all children under age 2, place the 50th percentile (the true statistical middle) at roughly 17.5 pounds for boys and 16.1 pounds for girls.
These numbers represent the midpoint, not a target. A baby at the 25th percentile is just as healthy as one at the 75th. What matters more is that your baby’s weight follows a consistent curve over time rather than hitting one specific number at one specific visit.
The Normal Range Is Wide
There’s a big spread between a lighter-but-healthy baby and a heavier-but-healthy baby at 6 months. Boys between roughly 14.5 and 20.5 pounds fall within the 5th to 95th percentile range. For girls, that range runs from about 13.5 to 19.5 pounds. A 6-pound gap between the lightest and heaviest “normal” babies is completely expected.
Pediatricians watch for babies who fall below the 2nd percentile or above the 98th percentile on the WHO charts, as these extremes can signal potential nutritional concerns. But even then, there’s no formal definition of underweight or overweight for children younger than 2. Context matters: a baby who has tracked along the 8th percentile since birth is in a very different situation than one who dropped from the 50th to the 8th over two months.
How Fast Babies Gain Weight
Between 4 and 6 months, most babies gain about 1 to 1.25 pounds per month. That’s roughly 4 to 5 ounces per week. This pace is actually slower than the first few months of life, when weight gain tends to be more rapid. Growth naturally decelerates as babies approach the second half of their first year, so a slight flattening of the curve around this age is normal.
At the same time, babies are getting longer. The average 6-month-old boy is about 26.8 inches (68 cm) long, while the average girl is about 25.5 inches (66 cm). Pediatricians look at weight relative to length, not weight in isolation, to get a fuller picture of whether a baby’s growth is proportional.
Breastfed vs. Formula-Fed Babies
How your baby eats can influence where they land on the growth chart. Breastfed babies typically gain weight more slowly than formula-fed babies, especially after the 3-month mark. This difference persists even after solid foods are introduced around 6 months. It doesn’t mean breastfed babies are underfed. The WHO growth charts were built using data from breastfed infants as the standard, which is one reason the CDC recommends them for children under 2.
If your breastfed baby seems to be “falling” on a chart designed around formula-fed infants (like older growth references), the issue may be the chart rather than the baby. Make sure your pediatrician is using the WHO standards for this age group.
When Babies Were Born Early
If your baby was born premature, the number on the scale at 6 months of calendar age won’t line up with the averages listed above. Pediatricians use “corrected age” for preemies during the first two years. To calculate it, take your baby’s actual age in weeks and subtract the number of weeks they arrived early. A baby born at 34 weeks (6 weeks early) who is now 6 months old would be compared to growth standards for a 4.5-month-old.
This adjustment makes a real difference. A preemie who looks small for 6 months may be right on track for their corrected age. Growth milestones, developmental milestones, and feeding expectations should all be measured against corrected age until at least age 2.
What Affects Your Baby’s Weight
Genetics plays the biggest role. Larger parents tend to have larger babies, and smaller parents tend to have smaller ones. Birth weight itself sets the trajectory: a baby born at 6 pounds will likely weigh less at 6 months than one born at 9 pounds, even if both are perfectly healthy and growing at the same rate.
Other factors that influence weight at this age include how often and how much a baby eats, whether they’ve started solid foods, how active they are (some 6-month-olds are rolling and scooting constantly, burning more calories), and whether they’ve been sick recently. A stomach bug or a stretch of poor feeding can cause a temporary dip that corrects itself within a few weeks.
What Pediatricians Actually Look For
Your baby’s 6-month checkup will include a weight measurement, but your pediatrician isn’t comparing your baby to the national average and calling it a day. They’re plotting your baby’s weight on a growth curve and looking at the trend across multiple visits. A baby who has been at the 30th percentile at every checkup is growing exactly as expected. A baby who jumped from the 30th to the 85th, or dropped from the 50th to the 10th, warrants a closer look regardless of where they fall relative to the average.
Pediatricians also look at weight-for-length, which compares how heavy your baby is relative to how long they are. This measurement is more informative than weight alone because it accounts for the fact that taller babies naturally weigh more. A “heavy” baby who is also very long may have a perfectly proportional build.