At 6 months old, the average baby weighs about 16 to 17 pounds and measures around 26 inches long. Boys tend to be slightly heavier and longer than girls at this age, but there’s a wide range of normal. Here’s what to expect and how to tell if your baby’s growth is on track.
Average Weight and Length at 6 Months
Most 6-month-old boys weigh between 14 and 21 pounds, with the average falling around 17.5 pounds. Girls typically weigh between 13 and 19 pounds, averaging closer to 16 pounds. For length, boys average about 26.5 inches while girls average about 25.8 inches.
These numbers come from the WHO growth charts, which pediatricians use to track infant development. Your baby doesn’t need to hit the exact middle of the range to be healthy. A baby consistently tracking along the 20th percentile is growing just as normally as one tracking along the 80th. What matters is that your baby follows a steady curve over time rather than suddenly jumping up or dropping down.
How Fast Babies Grow at This Age
From birth through 6 months, babies grow roughly 1 inch per month in length. By the time they reach 6 months, that pace starts to slow. Between 4 and 6 months, average length gain drops to about half an inch to 1 inch per month.
Weight gain follows a similar pattern. Babies gain weight fastest in their first few months of life, then gradually taper off. A healthy full-term baby typically doubles their birth weight by 4 months. So if your baby weighed 7.5 pounds at birth, you’d expect them to be around 15 pounds by 4 months and continuing to climb from there. By their first birthday, most babies have tripled their birth weight.
Breastfed vs. Formula-Fed Differences
Breastfed and formula-fed babies don’t grow at the same rate, and that’s normal. After the first 3 months, formula-fed babies typically gain weight more quickly than breastfed babies. This difference continues even after solid foods are introduced around 6 months. The CDC notes that breastfed infants generally put on weight more slowly throughout the first year, which is why pediatricians use the WHO growth charts (based on breastfed infants) rather than older charts that were based primarily on formula-fed babies.
If your breastfed baby seems smaller than a formula-fed baby the same age, that alone isn’t a concern. The two feeding methods simply produce different growth patterns.
What This Means for Clothing Sizes
Baby clothing sizes are based on weight and length, but they vary between brands and rarely match a baby’s actual age. A 6-month-old who weighs 14 pounds might still fit comfortably in clothes labeled “6 months” (designed for 12 to 16 pounds), while a bigger baby the same age could already need “9 months” sizes (designed for 16 to 20 pounds).
The best approach is to check the weight range printed on the tag rather than going by the age label. Some brands run small and others run large, so a “6 months” onesie from one company may fit very differently than the same label from another.
Diaper Sizing at 6 Months
Most 6-month-olds wear a Size 2 or Size 3 diaper. Size 2 fits babies between 12 and 18 pounds, while Size 3 covers 16 to 28 pounds. A baby right around 16 to 17 pounds could go either way, so the better guide is fit rather than the number on the box. If you’re seeing frequent blowouts or red marks around the legs and waist, it’s time to size up regardless of what the package says your baby “should” be wearing.
Growth Expectations for Premature Babies
If your baby was born early, their size at 6 months of calendar age will likely be smaller than the averages listed above. Pediatricians use “adjusted age” (also called corrected age) to evaluate a preemie’s growth. You calculate it by subtracting the number of weeks your baby was born early from their actual age in weeks.
For example, a baby born 5 weeks early who is now 6 months old (24 weeks) has an adjusted age of 19 weeks, or about 4 months and 3 weeks. Their doctor would compare their size to the growth charts for a 4- to 5-month-old, not a 6-month-old. This adjustment is used until age 2, by which point most preemies have caught up to the typical range for their age.
Signs Your Baby’s Growth Is on Track
Size alone doesn’t tell the full story. A baby who is small but consistently small, following their own curve on the growth chart, is typically growing well. Pediatricians look for red flags like a baby crossing two or more percentile lines downward, not gaining weight for several weeks, or losing weight. At 6-month checkups, your baby’s doctor will plot weight, length, and head circumference to see how all three measurements relate to each other and to previous visits.
Genetics plays a significant role. Tall parents tend to have longer babies, and smaller parents tend to have smaller babies. Your baby’s birth weight, gestational age, and feeding method all influence where they land on the chart. Two perfectly healthy 6-month-olds can differ by several pounds and still both be growing exactly as they should.