A 5-month-old boy weighs about 16.5 pounds (7.5 kg) on average, while a 5-month-old girl weighs about 15.2 pounds (6.9 kg). But “average” is just the 50th percentile on a growth chart, and healthy babies come in a wide range. A boy anywhere from roughly 13.5 to 19.5 pounds or a girl from 12.5 to 18 pounds can be perfectly on track. What matters more than any single number is whether your baby is following a consistent growth curve over time.
Average Weight by Sex
The World Health Organization growth standards, which the CDC recommends for all children under 2, break down expected weights by sex. At 5 months, here’s what the key percentiles look like:
- Boys: 10th percentile ~14.2 lb, 25th ~15.2 lb, 50th ~16.5 lb, 75th ~17.9 lb, 90th ~19.2 lb
- Girls: 10th percentile ~13.0 lb, 25th ~14.0 lb, 50th ~15.2 lb, 75th ~16.5 lb, 90th ~17.7 lb
A baby at the 15th percentile is not “too small,” and a baby at the 85th percentile is not “too big.” Both are well within the normal range. The CDC defines truly concerning low weight as below the 2nd percentile for age and sex, and unusually high weight as above the 98th percentile. Everything between those markers is considered normal variation.
The Birth Weight Doubling Milestone
One of the easiest benchmarks to check at home: most babies double their birth weight somewhere between 4 and 6 months. A baby born at 7.5 pounds should weigh roughly 15 pounds by this age. If your baby was premature or had a low birth weight, your pediatrician may use an adjusted age for this milestone, so the timeline can shift.
Between 4 and 6 months, babies typically gain about 1 to 1.25 pounds per month. That’s noticeably slower than the rapid gains of the first three months, when weekly weight jumps were more dramatic. This gradual slowdown is completely normal and expected.
Breastfed vs. Formula-Fed Babies
Breastfed and formula-fed babies often weigh differently at 5 months, even if they started out the same size. Formula-fed infants typically gain weight more quickly after about 3 months of age, while breastfed babies put on weight more slowly throughout the first year. This difference persists even after solid foods are introduced.
The key point: a breastfed baby tracking along the 25th percentile is not behind a formula-fed baby at the 50th. They’re on different but equally healthy trajectories. Length growth, interestingly, stays similar regardless of feeding type. If your pediatrician is using the WHO growth charts (as recommended for children under 2), those charts are based on breastfed infants as the standard, so your breastfed baby’s curve should look right on track.
Why the Trend Matters More Than the Number
Pediatricians care far less about where your baby falls on the chart at any single visit and far more about the pattern over multiple visits. A baby consistently tracking along the 20th percentile is growing exactly as expected. A baby who drops from the 60th percentile to the 15th over two or three visits is the one who needs closer evaluation, even though the 15th percentile is technically “normal.”
This is why growth monitoring relies on a series of accurate measurements over time. One weigh-in can be thrown off by a recent feeding, a wet diaper, or even different scales at different offices. The CDC specifically notes that interpreting growth requires context from multiple data points, not a snapshot. If you’re concerned about a single reading, the most useful thing you can do is come back for a follow-up weight check in two to four weeks to see the direction of the curve.
When poor growth is real and sustained, weight is always affected first. Length slows down later, and head circumference is the last to be impacted. So a baby whose weight is dipping but whose length and head are growing normally may just need a feeding adjustment rather than an extensive workup.
How Much a 5-Month-Old Needs to Eat
At 5 months, babies are still getting all or nearly all of their nutrition from breast milk or formula. Most need about 5 to 7 ounces per feeding, roughly six times a day. That works out to somewhere around 30 to 42 ounces total. Breastfed babies won’t measure their intake in ounces, of course, but frequent nursing sessions (typically every 3 to 4 hours, sometimes more) accomplish the same goal.
Some parents begin introducing solid foods around this age, but at 5 months, solids are more about exposure and practice than calories. Breast milk or formula remains the primary fuel for growth. If your baby seems hungry after finishing a bottle or nurses more frequently for several days in a row, that’s often a growth spurt rather than a sign that milk alone isn’t enough.
Signs That Growth May Be Off Track
A baby who is gaining weight too slowly won’t always look visibly thin. The clearest warning signs come from the growth chart: a steady drop across percentile lines over two or more visits, or weight that has plateaued for several weeks. Other practical signals include fewer than four wet diapers a day, persistent fussiness or lethargy after feedings, and a baby who seems uninterested in eating.
On the other end, rapid weight gain alone at 5 months is rarely a concern. Babies this age can’t overeat in the way older children can, and their activity levels will naturally shift their growth curve once they start crawling and walking. Unless your baby’s weight-for-length is above the 98th percentile, there’s generally no reason to restrict feeding.