The average weight for a 6-month-old is about 16.5 pounds (7.5 kg) for boys and 15.5 pounds (7 kg) for girls. These figures come from the WHO growth standards, which the CDC recommends using for all children in the U.S. from birth to age 2. But averages only tell part of the story. What matters more is how your baby’s weight tracks over time on their own growth curve.
Average Weight by Sex
Boys and girls follow slightly different growth patterns from birth. At 6 months, the 50th percentile (the statistical middle) for boys sits a full pound above girls. Here’s how the range looks across common percentiles:
- Boys at 6 months: 25th percentile is roughly 15.4 lbs, 50th percentile is about 16.5 lbs, and 75th percentile is around 17.7 lbs.
- Girls at 6 months: 25th percentile is roughly 14.4 lbs, 50th percentile is about 15.5 lbs, and 75th percentile is around 16.7 lbs.
A baby at the 25th percentile is not “too small,” and a baby at the 75th percentile is not “too big.” Percentiles describe where your baby falls relative to other babies the same age and sex. A healthy baby can sit comfortably at the 10th or the 90th percentile, as long as they’re growing consistently along that curve.
How Fast Babies Gain Weight at This Age
Between 3 and 6 months, infants typically gain about 1 ounce every other day, or roughly half a pound per week. That pace is noticeably slower than the first three months of life, when weight gain is more rapid. By around 6 months, most babies have at least doubled their birth weight. Research shows the average infant actually doubles birth weight closer to 4 months (around 119 days), so if your baby has doubled by 6 months, they’re well within normal range.
Boys tend to hit that doubling milestone a bit earlier than girls (around 111 days versus 129 days), and formula-fed babies reach it slightly sooner than breastfed babies (113 days versus 124 days). These are averages, not deadlines. A baby who doubles birth weight at 5 months is just as healthy as one who gets there at 3.5 months.
Breastfed vs. Formula-Fed Weight Differences
Breastfed babies typically put on weight more slowly than formula-fed babies during the first year, and this difference becomes more noticeable after about 3 months of age. It persists even after solid foods are introduced around 6 months. This slower gain is normal and expected. It does not mean a breastfed baby is underfed.
The WHO growth standards were built primarily from data on breastfed infants, which is one reason the CDC recommends them for children under 2. Older CDC growth charts were based heavily on formula-fed babies, which could make a perfectly healthy breastfed infant look like they were falling behind. If your pediatrician is using the WHO charts, they’re comparing your breastfed baby against the right benchmark. Length (height) growth tends to be similar regardless of feeding method.
What Percentile Changes Actually Mean
It’s normal for babies to shift percentiles during the first two to three years. A baby born at the 60th percentile might drift down to the 40th by 6 months as they settle into their own genetic growth pattern. Small shifts like this are common and rarely signal a problem.
What pediatricians watch for is a drop that crosses two or more major percentile lines on the WHO chart (for example, falling from the 75th percentile down past the 25th). That kind of change can indicate an underlying issue with nutrition, absorption, or health and usually prompts further evaluation. A single measurement that looks low is less meaningful than the overall trend across several checkups, which is why your baby’s growth curve over time carries more diagnostic weight than any single number.
How Premature Birth Affects the Numbers
If your baby was born early, the standard weight ranges for 6 months won’t apply to their actual birth date. Premature infants are tracked using “corrected age,” which adjusts for how early they arrived. The calculation is straightforward: take your baby’s actual age in weeks and subtract the number of weeks they were born before 40 weeks.
So a baby born at 34 weeks (6 weeks early) who is now 6 months old would have a corrected age of about 4.5 months. Their weight should be compared against the 4.5-month mark on the growth chart, not the 6-month mark. Pediatricians typically use corrected age for growth tracking until a preemie is about 2 years old, by which point most catch up to their full-term peers.
What to Focus On at the 6-Month Checkup
The 6-month well visit is one of the key growth check-ins during the first year. Your pediatrician will plot weight, length, and head circumference on the WHO growth chart. The most useful thing you can bring to that appointment is context: whether your baby seems satisfied after feedings, is producing enough wet diapers (typically six or more per day), and is meeting developmental milestones like sitting with support and reaching for objects.
A single weight measurement is a snapshot. The pattern across your baby’s 2-week, 2-month, 4-month, and 6-month visits is what reveals whether growth is on track. If your baby has been consistently following the 30th percentile, that’s their normal. Trying to push them toward the 50th would be unnecessary and counterproductive. Healthy babies come in a wide range of sizes, and the “average” is simply the midpoint of that range.