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 are 50th percentile values from the WHO growth standards, meaning half of all healthy infants weigh more and half weigh less at this age. A wide range is perfectly normal: boys between roughly 14.5 and 20.5 pounds and girls between 13.5 and 19 pounds all fall within typical growth curves.
How 6-Month Weight Relates to Birth Weight
A common rule of thumb is that babies double their birth weight by 5 to 6 months, but the actual average is closer to 4 months. A study of infant growth found the mean age of birth weight doubling was 119 days, or about 3.8 months. Boys tend to hit this milestone a bit earlier (around 111 days) than girls (around 129 days), and formula-fed infants get there slightly sooner than breastfed infants.
So if your baby was born at 7.5 pounds and now weighs 15 to 16 pounds at 6 months, that’s right on track. If they doubled their weight closer to 3 months and have since slowed down, that’s also normal. Weight gain naturally decelerates as babies get older. By the time they reach 6 months, many infants are gaining about 10 grams or less per day, compared to the roughly 20 to 30 grams per day typical of the first few months.
Breastfed vs. Formula-Fed Babies
Breastfed and formula-fed infants follow noticeably different growth patterns, and this matters when you’re comparing your baby’s weight to averages. Healthy breastfed infants typically put on weight more slowly than formula-fed infants during the first year, with the difference becoming more apparent after about 3 months of age. Formula-fed babies tend to gain weight more quickly in this window, and the gap persists even after solid foods are introduced.
This doesn’t mean a lighter breastfed baby is falling behind. The WHO growth charts, which most pediatricians now use for children under 2, were built from data on breastfed infants and reflect their growth pattern as the biological norm. If your pediatrician is using older CDC charts (more common in some practices), a breastfed baby might look like they’re dropping percentiles when they’re actually growing exactly as expected.
What Growth Percentiles Actually Mean
A baby in the 20th percentile isn’t underweight, and a baby in the 85th percentile isn’t overweight. Percentiles describe where your baby falls compared to other babies the same age and sex. A baby who has consistently tracked along the 15th percentile since birth is following their own curve, and that’s healthy. What matters more than the number itself is the trajectory.
Pediatricians pay close attention when a baby’s weight crosses two or more major percentile lines on the growth chart, either up or down. A baby who was in the 60th percentile at 2 months and drops to the 10th percentile by 6 months is a different situation from a baby who has always been near the 10th percentile. The drop signals a change that may need evaluation, while consistently tracking low is often just that child’s normal pattern.
Weight and Length Together
Weight alone doesn’t tell the full story. At 6 months, the average boy is about 26.5 inches (67.6 cm) long, and the average girl is about 25.75 inches (65.7 cm). A baby who is long and lean will weigh less than a shorter, stockier baby, and both can be perfectly healthy. Pediatricians look at weight-for-length as a more meaningful measure than weight alone, because it accounts for body proportions.
Head circumference also gets tracked at this age. All three measurements together, plotted over time, give a much clearer picture of growth than any single number at a single visit.
Signs That Weight May Be a Concern
A weight below the 5th percentile for age, or a weight-for-length ratio significantly below the mean, can indicate a growth issue that warrants investigation. But context matters enormously. A baby born to smaller parents who has tracked along the 4th percentile since birth is in a very different situation from a baby who was at the 50th percentile and dropped to the 4th over two or three months.
More practical signs to watch for include fewer than 4 to 6 wet diapers per day, persistent fussiness or lethargy during feeds, and a baby who seems unsatisfied after nursing or bottle-feeding. These day-to-day observations often catch issues before the next weigh-in does. If your baby is meeting developmental milestones, feeding well, producing plenty of wet diapers, and seems alert and content, their weight is very likely fine, wherever it falls on the chart.