A typical 6-month-old weighs about 16.1 pounds (7.3 kg) if they’re a girl or 17.5 pounds (7.9 kg) if they’re a boy, based on the 50th percentile of WHO growth standards. But healthy babies at this age can range widely, from around 13 pounds on the lower end to over 20 pounds on the higher end, depending on genetics, feeding method, and birth size.
Average Weight by Sex
Boys tend to be slightly heavier than girls at 6 months. At the 50th percentile, which represents the midpoint where half of babies weigh more and half weigh less, a 6-month-old boy weighs roughly 17.5 pounds and a girl weighs about 16.1 pounds. The 50th percentile is not a target. A baby tracking steadily along the 20th percentile is just as healthy as one tracking along the 80th.
Length follows a similar pattern. The average 6-month-old boy measures about 26.6 inches (67.6 cm), while the average girl is about 25.9 inches (65.7 cm). Weight and length together give a fuller picture of growth than weight alone.
The Birth Weight Rule of Thumb
A simple benchmark most pediatricians use: babies typically double their birth weight by 6 months. So if your baby was born at 7.5 pounds, you’d expect them to be around 15 pounds at the half-year mark. This is a rough guide, not a hard rule. A baby born at 5 pounds after a premature birth or one born at 9.5 pounds will naturally hit different numbers at 6 months. For premature babies, doctors adjust for gestational age until age 2, meaning a baby born a month early would be compared against 5-month-old growth standards.
Breastfed vs. Formula-Fed Babies
Feeding method affects weight gain patterns more than many parents realize. Breastfed babies typically put on weight more slowly than formula-fed babies during the first year. The difference becomes noticeable after about 3 months, when formula-fed infants start gaining weight more quickly. This gap continues even after solid foods enter the picture around 6 months.
This doesn’t mean breastfed babies are underweight or that formula-fed babies are overweight. Both patterns are normal. The key distinction is that breastfed and formula-fed babies grow at similar rates in terms of length. It’s specifically fat accumulation that differs. If your breastfed baby looks leaner than a formula-fed baby of the same age, that’s expected.
What Growth Percentiles Actually Mean
Percentiles rank your baby compared to a large reference population. A baby at the 30th percentile weighs more than 30% of babies the same age and sex. Being at a low percentile is not automatically a problem. What matters most is the trajectory over time. A baby who has tracked along the 15th percentile since birth is following their own growth curve, and that’s completely normal.
The red flag is a significant drop, not a low number. Clinically, doctors look for a baby whose weight crosses two major percentile lines on the growth chart after previously growing on track. For example, a baby who was at the 60th percentile at 2 months and falls to the 10th percentile by 6 months would warrant a closer look. A single measurement that seems low is rarely enough to diagnose a problem, because normal variation between babies is so wide. Weight is the most sensitive indicator of nutritional status, though, so your pediatrician tracks it at every visit for exactly this reason.
Growth Spurts Around 6 Months
Six months is one of the common ages for a growth spurt, which can temporarily change your baby’s eating and sleeping patterns. During a spurt, you may notice increased hunger, more fussiness than usual, and disrupted sleep. In babies under a year, these spurts typically last up to three days. Your baby might suddenly want to nurse or take a bottle more frequently, then settle back to their normal routine.
A growth spurt can also mean a noticeable jump in weight between two pediatric visits. If your baby seemed to gain more than expected in a short window, this is a likely explanation. Other common spurt windows during the first year include around 3 weeks, 6 weeks, 3 months, and 9 months.
When Weight Gain Might Be Too Slow
Slow weight gain isn’t always a concern. Some babies are simply smaller than average, and their growth curve reflects that consistently. The pattern that raises questions is when a baby who was gaining well suddenly slows down or stalls. Possible reasons range from simple ones, like not getting enough milk during a feeding transition, to medical causes that a pediatrician can evaluate.
If your baby’s weight drops sharply on the growth chart, your doctor will likely want to see you more frequently for weight checks before drawing conclusions. Starting solid foods around 6 months can also temporarily affect weight patterns as babies adjust to new calorie sources. The introduction of complementary foods is a normal transition, not a sign of trouble, but it’s worth mentioning to your pediatrician if you notice a stall in weight gain that coincides with the shift.