A 5-month-old baby typically weighs between 12 and 18 pounds, depending on sex, birth weight, and feeding patterns. The average (50th percentile) is about 15.2 pounds for boys and 14.1 pounds for girls. But “average” covers a wide range of perfectly healthy babies, so understanding where your baby fits matters more than hitting one specific number.
Average Weight by Sex
The World Health Organization growth standards, which the CDC recommends using for all children under 2, place the 50th percentile weight for 5-month-old girls at 6.9 kg, or roughly 15.2 pounds. The range between the 5th and 95th percentiles for girls runs from about 12.3 pounds (5.6 kg) to 18.5 pounds (8.4 kg). Boys tend to run slightly heavier at every percentile, with the 50th percentile landing closer to 16.5 pounds.
Any weight within that 5th-to-95th range is considered normal. A baby tracking along the 10th percentile is just as healthy as one at the 80th, as long as they’re following a consistent curve over time. Pediatricians pay far more attention to the pattern of growth than to any single weigh-in.
The Birth Weight Doubling Milestone
You may have heard that babies should double their birth weight by around 5 to 6 months. Research confirms that the average infant actually hits this milestone a bit earlier, around 3.8 months (119 days). So if your baby was born at 7.5 pounds, you’d expect them to be somewhere near 15 pounds by their 5-month checkup, and possibly already past that mark. Premature babies or those with a lower birth weight may reach this milestone on a slightly different timeline, which is normal.
How Fast 5-Month-Olds Gain Weight
Between 4 and 6 months, most babies gain about 1 to 1.25 pounds per month. That works out to roughly 4 to 5 ounces per week. This is noticeably slower than the rapid gains of the first three months, when many babies put on closer to 1.5 to 2 pounds monthly. The slowdown is completely expected and reflects a shift toward more proportional growth as babies get longer and more active.
At 5 months, average length is about 26 inches for boys and 25.2 inches for girls. Weight and length together give a fuller picture of growth than weight alone, which is why your pediatrician plots both on the growth chart.
Breastfed vs. Formula-Fed Growth Patterns
Breastfed and formula-fed babies grow differently, and the gap becomes noticeable right around this age. Formula-fed infants typically gain weight more quickly after about 3 months, so by 5 months they often appear heavier than their breastfed peers. Breastfed babies tend to put on weight more slowly throughout the first year, and these differences persist even after solid foods enter the picture.
This doesn’t mean one group is healthier than the other. The WHO growth charts were built primarily from data on breastfed infants, so a breastfed baby tracking along a lower percentile is following the expected pattern. If your breastfed baby looks smaller than a formula-fed cousin of the same age, that’s a normal variation, not a red flag.
When Growth Is a Concern
The number on the scale matters less than the trajectory. A baby who has always tracked along the 15th percentile is growing normally. A baby who drops from the 60th to the 15th over a couple of months deserves a closer look. Pediatricians call this “falling off the growth curve,” and it’s the single most important pattern they watch for.
Some babies are naturally smaller and gain weight slowly but steadily. These babies typically wake on their own, seem alert and engaged, breastfeed 8 to 12 times in 24 hours, and produce the same number of wet and dirty diapers as faster-growing babies. That pattern of healthy behavior alongside slow-but-steady gain is reassuring.
Signs that weight gain may actually be insufficient include gaining less than 1 pound per month during the first four months, a dramatic drop from a baby’s established growth curve in weight or length, or a noticeable decrease in wet diapers. Babies who seem unusually sleepy and difficult to wake for feedings, or who aren’t interested in eating, also warrant attention. These patterns are different from simply being on the smaller side, and your pediatrician can distinguish between the two with a few data points over time.
What Percentiles Actually Mean
A common source of anxiety is misunderstanding what percentiles represent. If your baby is at the 25th percentile, that means 25% of healthy babies the same age weigh less and 75% weigh more. It does not mean your baby is getting a “low grade.” Percentiles describe where a baby falls in a population of healthy, well-nourished children. The 5th percentile and the 95th percentile are both within the normal range.
What matters is consistency. A baby born at the 30th percentile who stays near the 30th percentile at every visit is growing exactly as expected. Genetics play a large role here: smaller parents tend to have smaller babies, and those babies often track along lower percentiles from birth onward. If your baby is healthy, active, meeting developmental milestones, and following their own curve, the specific number of pounds is far less important than the trend line.