At 5 months old, the average baby boy weighs about 16.5 pounds (7.5 kg) and the average baby girl weighs about 15.2 pounds (6.9 kg). These are 50th percentile values on the WHO growth charts, meaning half of all babies weigh more and half weigh less. A healthy 5-month-old can fall anywhere from roughly 13 to 19 pounds depending on sex, birth weight, and feeding patterns.
Average Weight Ranges by Sex
Boys tend to be slightly heavier than girls at every stage of infancy, and the gap is already noticeable by 5 months. Here’s what the growth chart percentiles look like at this age:
- Boys, 25th to 75th percentile: about 14.8 to 18.1 pounds (6.7 to 8.2 kg)
- Girls, 25th to 75th percentile: about 13.6 to 16.8 pounds (6.2 to 7.6 kg)
Most pediatricians consider any weight within the 5th to 95th percentile range normal, as long as the baby is growing steadily along their own curve. A baby consistently tracking the 15th percentile is just as healthy as one tracking the 80th. What matters more than a single number is whether the trajectory stays consistent over time.
Birth Weight Doubling
You may have heard that babies double their birth weight by 5 or 6 months. In reality, most babies hit that milestone a bit earlier. A study of 357 infants found the average doubling time was 119 days, or about 3.8 months. Boys doubled slightly faster than girls (111 days versus 129 days), and formula-fed babies doubled sooner than breastfed babies (113 days versus 124 days).
So by 5 months, most babies have already passed the doubling mark. If your baby was born at 7 pounds, you’d expect them to be well above 14 pounds by now. A baby born at a lower weight, say 6 pounds, may still be working toward their doubled weight at 5 months, and that’s perfectly normal too.
Breastfed vs. Formula-Fed Differences
During the first 6 to 8 weeks of life, breastfed and formula-fed babies grow at nearly identical rates. After about 2 months, formula-fed infants start gaining weight and length more rapidly. By 5 months, this gap is becoming noticeable on the growth chart, though both groups are still within normal range.
At around this age, there isn’t a consistent difference in body fat between the two groups. That changes later in the first year, when breastfed babies tend to be leaner. The key point: if your breastfed baby seems lighter than a formula-fed baby of the same age, that difference is expected and not a sign of a problem.
Pediatricians now use the WHO growth charts for children under 2, which are based on breastfed infants as the standard. Older CDC charts were built from a mix of feeding types, which sometimes made breastfed babies look like they were falling behind when they were growing exactly as expected.
How Much a 5-Month-Old Eats
At this age, babies are still getting virtually all their nutrition from breast milk or formula. According to Johns Hopkins Medicine, formula-fed babies between 3 and 5 months typically take 6 to 7 ounces per feeding, with five to six feedings per day. That works out to roughly 30 to 42 ounces of formula daily.
Breastfed babies regulate their own intake, so volumes are harder to pin down, but the caloric needs are the same. A 5-month-old needs approximately 82 calories per kilogram of body weight per day. For a 16-pound baby, that translates to roughly 600 calories daily. Both breast milk and formula deliver about 20 calories per ounce, so the math lines up with those 30-plus ounces.
Some parents begin introducing solid foods around this time, but at 5 months, solids are more about practice than nutrition. The calories that drive weight gain are still coming almost entirely from milk.
Weekly Weight Gain at This Age
Weight gain slows down considerably after the first 3 months. During the newborn period, babies often gain 5 to 7 ounces per week. By 5 months, the typical rate drops to about 3 to 5 ounces (roughly 85 to 140 grams) per week. Some weeks your baby might gain more, other weeks less. Short-term fluctuations are normal and can be affected by minor illness, a growth spurt, or even the time of day the baby was weighed.
This slowdown surprises some parents, especially if their baby’s weight curve starts to flatten. A gradual shift from a higher percentile to a slightly lower one over the course of months isn’t automatically concerning. Rapid drops, where a baby crosses two or more major percentile lines on the growth chart, are what prompt closer evaluation.
Signs of Healthy Growth
Weight is one piece of the picture, but it’s not the only one. A 5-month-old who is growing well typically has good muscle tone, is alert and responsive, produces at least 6 wet diapers a day, and is meeting developmental milestones like reaching for objects and rolling over.
Pediatricians also look at length and head circumference alongside weight. A baby who is light but also short is proportionally small, which often reflects genetics rather than a feeding issue. A baby whose weight is dropping while length holds steady is a different clinical picture and more likely to need attention.
When Weight Gain Is Too Slow
The American Academy of Pediatrics now uses the term “faltering weight” instead of the older “failure to thrive.” A baby may be diagnosed with faltering weight if their weight-for-length falls below the 5th percentile, their rate of weight gain drops below the 2.3rd percentile for their age, or their weight drops by one full standard deviation or more on the growth chart.
In practical terms, the red flags parents are most likely to notice include actual weight loss between visits, a baby who seems uninterested in feeding or frequently chokes and gags during feeds, blood or mucus in the stool, or unusually foul-smelling and bulky stools. Persistent lethargy or a baby who doesn’t seem to be getting stronger and more active over time also warrants attention.
When faltering weight is suspected, the initial evaluation focuses on feeding history, developmental assessment, and a thorough physical exam. The AAP recommends against routine lab tests unless there are specific symptoms pointing to an underlying condition. In most cases, the cause turns out to be related to feeding volume or technique rather than a hidden medical issue.
Why Your Baby’s Number Might Differ
Genetics play a bigger role in infant size than most parents realize. If both parents are on the shorter or lighter side, a baby tracking the 20th percentile is doing exactly what their genes dictate. Premature babies also follow adjusted growth charts, meaning their expected weight at 5 calendar months is based on their corrected age, not their birth date.
Babies born large, often those of mothers with gestational diabetes, sometimes drift downward on the growth chart during the first several months as they settle into their genetically programmed size. This kind of percentile crossing is normal and distinct from the rapid drops that signal a problem. Your pediatrician tracks these patterns over multiple visits specifically to tell the difference.