At six months old, most babies weigh between 14 and 18 pounds and measure about 25 to 27 inches long. Boys tend to run slightly larger than girls, but there’s a wide range of normal at this age. Here’s what to expect in terms of size, growth patterns, and the practical details parents actually need.
Average Weight and Length at Six Months
The World Health Organization growth standards, which the CDC recommends for all children under two, put the median weight for a six-month-old boy at roughly 17.5 pounds (7.9 kg) and for a girl at about 16 pounds (7.3 kg). For length, boys average around 26.5 inches (67.6 cm) and girls about 25.8 inches (65.7 cm).
These are 50th percentile numbers, meaning half of healthy babies are bigger and half are smaller. A baby in the 25th percentile for weight is just as healthy as one in the 75th. What matters most is that your baby follows a consistent curve over time rather than hitting one specific number. A baby who has tracked along the 20th percentile since birth is growing perfectly normally.
How Fast They’re Growing
Growth slows noticeably around the six-month mark. From birth to six months, babies typically grow about 1 inch (2.5 cm) per month in length. That pace drops to roughly half an inch per month during the second half of the first year. Weight gain also tapers: by six months, many babies are gaining about 10 grams or less per day, compared to the rapid gains of the first few months.
Most babies have doubled their birth weight by around four to five months and will triple it by their first birthday. So if your baby was born at 7.5 pounds, expect them to be somewhere in the 15-pound range by six months, on their way to about 22 pounds by age one. Premature babies often follow a slightly different timeline, and pediatricians track their growth using an adjusted age that accounts for how early they arrived.
Breastfed vs. Formula-Fed Babies
If your breastfed baby seems leaner than a formula-fed baby of the same age, that’s expected. Healthy breastfed infants typically put on weight more slowly than formula-fed infants during the first year, with the difference becoming more noticeable after about three months. Formula-fed babies tend to gain weight more quickly from that point onward, and the gap persists even after both groups start solid foods.
Length is a different story. Linear growth patterns are similar regardless of how a baby is fed. The WHO growth charts were developed using data from breastfed infants, which is one reason the CDC recommends them for children under two. If your pediatrician uses these charts, a breastfed baby’s growth will be plotted against an appropriate reference population.
What Clothing Size to Buy
Baby clothing sizes are notoriously inconsistent across brands, but as a general guide, most six-month-olds fit into either “6 month” or “6-9 month” labeled clothing. A typical size chart breaks it down like this:
- 3-6 months (labeled 6M): fits babies 12.5 to 17 pounds, 24 to 26.5 inches long
- 6-9 months (labeled 9M): fits babies 17 to 21 pounds, 26.5 to 28.5 inches long
Many parents find their six-month-old is already wearing 9-month clothing, especially if the baby is long-torsoed or on the higher end of the weight range. If you’re buying ahead or receiving gifts, sizing up is almost always the safer bet. Babies grow out of clothes fast and can wear slightly roomy outfits, but they can’t squeeze into something too small.
Diaper Sizing at Six Months
Diaper sizes are based on weight rather than age. Most six-month-olds fall into size 3 diapers, which typically cover the 16 to 28 pound range depending on the brand. Some smaller six-month-olds may still be in size 2 (roughly 12 to 18 pounds). The clearest sign you need to size up isn’t your baby’s age. It’s frequent leaks, red marks on the thighs or waist, or difficulty fastening the tabs comfortably.
When Size Varies From the Average
Genetics plays the biggest role in where your baby lands on the growth chart. Tall parents tend to have longer babies. Parents with smaller frames often have babies that track lower percentiles. Ethnicity, birth weight, and gestational age all influence size as well.
Pediatricians pay less attention to a single measurement and more attention to the pattern over time. A baby who drops from the 60th percentile to the 15th over two visits is more concerning than a baby who has consistently been in the 10th percentile. Similarly, a sudden jump upward can sometimes signal overfeeding, though some babies naturally shift percentiles as they settle into their genetic growth trajectory during the first year.
At the six-month well visit, your pediatrician will plot weight, length, and head circumference on a growth chart. Head circumference at this age is typically around 16.5 to 17.5 inches (42 to 44.5 cm), and it’s tracked because steady head growth reflects healthy brain development. All three measurements together give a much fuller picture than any single number.