Most 6-month-old boys weigh around 17.5 pounds (7.9 kg), and most 6-month-old girls weigh around 16 pounds (7.3 kg). These are the 50th percentile values on the WHO growth charts used by pediatricians in the United States, meaning half of healthy babies weigh more and half weigh less. A wide range is perfectly normal: boys between about 14.5 and 21 pounds and girls between about 13.5 and 20 pounds all fall within typical growth curves.
Average Weight by Sex
The WHO growth standards, recommended by the CDC for children from birth to age 2, are based on data from six countries and reflect growth patterns among healthy, well-nourished infants. Here’s how 6-month-old weights break down by percentile:
- Boys, 5th percentile: about 14.5 lbs (6.6 kg)
- Boys, 50th percentile: about 17.5 lbs (7.9 kg)
- Boys, 95th percentile: about 21 lbs (9.5 kg)
- Girls, 5th percentile: about 13.5 lbs (6.1 kg)
- Girls, 50th percentile: about 16 lbs (7.3 kg)
- Girls, 95th percentile: about 19.8 lbs (9 kg)
A baby at the 20th percentile is just as healthy as one at the 80th. What matters more than where your baby falls on the chart is whether they’re following a consistent curve over time.
How Fast Babies Gain Weight at This Age
Weight gain slows significantly by 6 months. In the early weeks, babies typically gain about 1 ounce (28 grams) per day. By 4 months, that drops to about 20 grams a day, and by 6 months many babies are gaining 10 grams or less per day. That’s roughly a third of a pound per week in the early months, tapering to less than a quarter pound per week by the half-year mark.
A common benchmark pediatricians use: most babies double their birth weight by around 4 to 5 months. So a baby born at 7.5 pounds would typically be around 15 pounds or more by their 6-month visit. If your baby was smaller or larger at birth, their 6-month weight will reflect that starting point.
Breastfed vs. Formula-Fed Babies
Healthy breastfed infants typically put on weight more slowly than formula-fed infants during the first year. The difference becomes more noticeable after about 3 months, when formula-fed babies tend to gain weight faster. This pattern continues even after solid foods are introduced around 6 months. Length growth, however, is similar between the two groups.
This means a breastfed baby who looks lighter than a formula-fed peer of the same age may be growing exactly as expected. The WHO growth charts were designed with breastfed infants as the standard, so they’re a better reference for breastfed babies than older charts that were based mostly on formula-fed populations.
Premature Babies and Adjusted Age
If your baby was born early, pediatricians use “corrected age” rather than calendar age to track growth. A baby born two months premature, for example, would be compared to the 4-month benchmarks at a calendar age of 6 months. This adjustment continues until age 2.
Extremely low-birthweight babies often track near or below the 5th percentile, and that can be completely healthy as long as their growth runs parallel to the standard curve. The goal for many premature infants isn’t necessarily reaching the 50th percentile. It’s following their own consistent trajectory. A growth curve that plateaus, drops off, or shows weight loss is what warrants further evaluation.
When the Number on the Scale Matters Less
Growth charts are one tool, not a diagnosis. The CDC specifically notes that they aren’t meant to be used as a sole measure of a child’s health. A single weight measurement tells you very little on its own. What pediatricians look for is a pattern across multiple visits, ideally at least a month apart.
Other signs your baby is growing well at 6 months include gaining about half an inch to 1 inch in length each month, head circumference increasing by about half an inch per month, producing plenty of wet diapers throughout the day, and hitting developmental milestones like sitting with support and reaching for objects.
What Counts as a Growth Concern
Pediatricians watch for a pattern sometimes called failure to thrive, where a baby’s weight gain slows enough that they steadily fall away from their established growth curve. This isn’t defined by a single low weight reading. It requires tracking valid weight measurements over time and seeing a downward trend that crosses percentile lines.
A baby who has always been at the 15th percentile and stays there is growing normally. A baby who was at the 60th percentile and drops to the 15th over two or three visits is a different story. That shift in trajectory, not the number itself, is what prompts a closer look at feeding, calorie intake, and overall health. If your baby’s weight has been stable along any percentile curve, even a low one, that’s generally reassuring.