Most 6-month-old boys weigh between 14 and 21 pounds (6.4 to 9.7 kg), while most girls weigh between 13 and 20 pounds (5.8 to 9.0 kg). These ranges cover the 5th to 95th percentiles on the WHO growth charts, which are the standard used in the United States for children under 2. A baby right in the middle of the chart, at the 50th percentile, weighs roughly 17.5 pounds for boys and 16 pounds for girls.
What the Average Really Means
Those numbers are useful as a ballpark, but your baby’s “right” weight depends heavily on what they weighed at birth. Healthy, full-term newborns typically double their birth weight by around 4 months and triple it by their first birthday. So a baby born at 6 pounds might weigh around 12 pounds at 4 months and be approaching 15 or 16 pounds by 6 months, while a baby born at 9 pounds could be well over 20 pounds at the same age. Both are perfectly normal.
What matters more than hitting a specific number is staying on a consistent growth curve. A baby tracking along the 25th percentile from birth is growing exactly as expected, even though they weigh less than most babies their age. Pediatricians look at the pattern over multiple visits, not a single weigh-in.
How Fast 6-Month-Olds Gain Weight
Weight gain slows significantly in the second half of infancy. In the first few months of life, babies gain about 1 ounce (28 grams) per day. By 4 months that drops to around 20 grams a day. By 6 months, many babies are gaining 10 grams or less per day, which works out to roughly 1 to 1.5 pounds per month. This slowdown is normal and coincides with babies becoming more active, rolling, sitting up, and burning more energy.
Parents sometimes worry when the numbers on the scale seem to plateau compared to those dramatic early months. But the shift from rapid to steady gain is a predictable part of development, not a sign that something is wrong.
Breastfed vs. Formula-Fed Babies
Feeding method affects weight gain patterns, especially after 3 months. Breastfed infants typically put on weight more slowly than formula-fed infants during the first year. Formula-fed babies tend to gain weight more quickly starting around 3 to 4 months, and this difference continues even after solid foods are introduced around 6 months.
This doesn’t mean breastfed babies are underfed. The WHO growth charts were developed using primarily breastfed infants as the reference population, so breastfed growth is considered the biological norm. If your pediatrician uses older CDC charts (designed for a mixed-feeding population), a healthy breastfed baby might appear to “drop off” their curve when they’re actually growing exactly as expected. It’s worth confirming which chart your provider uses.
Premature Babies and Adjusted Age
If your baby was born early, their weight at 6 calendar months won’t match the ranges above, and it isn’t supposed to. Doctors use “adjusted age” (also called corrected age) to account for prematurity. You calculate it by subtracting the number of weeks your baby was born early from their actual age. A baby born at 32 weeks, which is 8 weeks early, has an adjusted age of about 4 months when the calendar says 6 months. Their weight is then compared to growth charts for a 4-month-old, not a 6-month-old.
To illustrate: a girl born at 32 weeks weighing 1,420 grams (about 3 pounds) might weigh around 5.5 kg (12 pounds) at 7.5 calendar months, which corresponds to a corrected age of 5.5 months. Plotted against the WHO chart for her corrected age, she can be tracking normally even though she looks small compared to full-term peers. Most pediatricians continue using adjusted age for growth monitoring until a child is about 2 years old, when the gap typically closes.
When Weight Gain Is a Concern
Doctors watch for a pattern called failure to thrive, which means a baby isn’t gaining enough usable nutrition over time. The key indicator isn’t a single low number on the scale. It’s a downward trend, where a baby’s weight steadily falls away from the curve they’ve been following. A baby who has always been at the 15th percentile is different from a baby who was at the 50th percentile at 2 months and dropped to the 15th by 6 months.
This is why consistent well-baby checkups matter. Diagnosing a growth problem requires valid weight measurements taken over several visits, not a one-time comparison. Signs that typically prompt further evaluation include crossing two or more major percentile lines downward, losing weight between visits, or not regaining birth weight by 2 weeks of age.
Practical Tips for Tracking Weight
Home scales aren’t reliable enough for tracking infant growth in small increments. The measurements at your pediatrician’s office, taken on a calibrated infant scale with the baby undressed, are the ones to trust. If you’re curious between appointments, that’s fine, but avoid comparing a clothed, post-meal home weigh-in to a stripped-down office measurement.
Keep in mind that day-to-day weight can fluctuate based on when your baby last ate, had a diaper change, or even how hydrated they are. A single weigh-in that seems high or low is rarely meaningful. The trend line across weeks and months tells the real story. If your baby is feeding well, meeting developmental milestones like sitting with support and reaching for objects, producing plenty of wet diapers, and seems alert and content, their weight is very likely right where it should be for them.