Most 6-month-old babies weigh between 14 and 18 pounds and measure around 25 to 27 inches long, though there’s a wide range of normal. By this age, babies have typically doubled their birth weight, which is one of the clearest signs that growth is on track.
Average Weight at 6 Months
A 6-month-old boy at the 50th percentile weighs about 17.5 pounds (7.9 kg), while a girl at the 50th percentile weighs about 16.1 pounds (7.3 kg). The 50th percentile means half of babies weigh more and half weigh less, so it’s simply the midpoint, not a target. A baby at the 20th or 80th percentile is just as healthy as one right in the middle, as long as they’re following a consistent curve over time.
The general rule of thumb is that babies double their birth weight somewhere between four and six months. If your baby was born at 7 pounds, you’d expect them to be around 14 pounds by now. Babies who were larger or smaller at birth will have different numbers, but the doubling pattern holds as a useful benchmark.
Average Length and Head Size
At 6 months, most babies are between 25 and 28 inches long (64 to 70 cm). Boys tend to be slightly longer than girls, but the overlap is significant. Unlike weight, length is harder to measure accurately in a squirming baby, so a single reading that seems off isn’t necessarily meaningful.
Head circumference matters because it reflects brain growth. For 6-month-old boys, the typical range runs from about 40.8 cm (3rd percentile) to 45.2 cm (97th percentile), with the midpoint at 43 cm. Girls tend to measure about half a centimeter smaller at each percentile. Your baby’s pediatrician tracks head circumference at every well visit for this reason.
Why the Growth Curve Matters More Than a Single Number
A single measurement tells you very little on its own. What matters is the pattern. A baby who has been tracking along the 15th percentile since birth is growing perfectly well. A baby who was at the 70th percentile and has dropped to the 15th over two or three visits is a different story, because that kind of shift can signal a feeding problem or an underlying health issue.
Pediatricians plot your baby’s weight, length, and head circumference on standardized growth charts at each visit. These charts show how your baby compares to thousands of other babies the same age and sex. The goal isn’t to hit a particular percentile. It’s to follow a roughly consistent curve. Babies do cross percentile lines sometimes, especially in the first year, and small shifts are normal. A sustained drop across two or more major percentile lines is what raises concern.
Breastfed vs. Formula-Fed Growth Patterns
If your baby is breastfed and seems lighter than formula-fed babies you know, that’s expected. Breastfed infants typically gain weight more slowly than formula-fed infants, especially after the first three months. Formula-fed babies tend to put on weight faster in the second half of the first year, and this difference persists even after babies start eating solid foods.
Length growth, on the other hand, is similar regardless of feeding method. The weight difference doesn’t mean one approach is better or worse. It’s simply a different pattern. The CDC’s WHO-based growth charts, which are the standard used for children under 2, were designed around the growth of predominantly breastfed infants, so they account for this difference.
Premature Babies Need Adjusted Expectations
If your baby was born early, their growth should be measured against their “adjusted age” rather than their calendar age. Adjusted age is calculated by subtracting the number of weeks of prematurity from their actual age. A baby born two months early who is now 6 months old would be compared to growth standards for a 4-month-old. This adjustment is typically used until age 2 for weight and length, and sometimes until age 3 for head circumference.
Pediatricians use specialized growth charts for preterm infants, particularly during the early months. These charts reflect the reality that premature babies often follow a catch-up growth pattern that looks different from full-term babies. Most preemies do catch up eventually, but the timeline varies.
Signs Growth May Be Falling Behind
Growth faltering (previously called “failure to thrive”) is identified when a baby’s weight or length consistently falls below expected ranges or drops significantly from their established curve. Some signs you might notice at home include a baby who cries more than expected, sleeps excessively, falls asleep during feedings, or doesn’t interact with people the way other babies their age do, like mimicking facial expressions or responding to voices.
Physical signs that a pediatrician checks for include poor muscle mass, changes in fat distribution, and the condition of a baby’s skin and hair. These indicators, combined with growth chart trends, help distinguish a naturally small baby from one who isn’t getting adequate nutrition or who has a medical condition affecting growth. Most cases of growth faltering are related to feeding difficulties or calorie intake and can be addressed once identified.
What Healthy Growth Looks Like Beyond the Scale
Numbers on a growth chart are important, but they’re not the whole picture. A 6-month-old who is growing well will also be hitting developmental milestones: rolling over, reaching for objects, babbling, recognizing familiar faces, and showing curiosity about their surroundings. They’ll have good skin color, alert eyes, and enough energy to play and interact during wakeful periods.
If your baby is eating well, producing plenty of wet diapers (at least six a day), seems content between feedings, and is meeting developmental milestones, their size is very likely right where it should be, whether that’s the 10th percentile or the 90th.