The average 5-month-old boy weighs about 16.5 pounds, and the average 5-month-old girl weighs about 15 pounds. But “average” is just the middle of a wide, healthy range. A 5-month-old anywhere between the 5th and 95th percentiles on the WHO growth charts is typically growing normally, which means healthy weights at this age can span from roughly 12 to 20 pounds depending on sex and individual factors.
Average Weight by Sex
The World Health Organization growth standards, which are the charts most pediatricians in the U.S. use for children under 2, place the 50th percentile weight for a 5-month-old boy at about 16.5 pounds (7.5 kg) and for a 5-month-old girl at about 15.2 pounds (6.9 kg). At the lower end of normal, a boy at the 10th percentile weighs around 14 pounds, while a girl at the 10th percentile is closer to 13 pounds. At the higher end, a boy at the 90th percentile comes in around 19 pounds and a girl around 17.5 pounds.
These numbers represent snapshots on a curve, not pass/fail thresholds. Your baby’s individual number matters far less than where it falls relative to their own growth pattern over time.
Why the Range Is So Wide
Several factors explain why two perfectly healthy 5-month-olds can differ by several pounds. Birth weight is one of the biggest influences. A baby born at 9 pounds will almost certainly be heavier at 5 months than one born at 6.5 pounds, even if both are growing at a normal rate. Research shows that higher birth weight also raises the likelihood of being on a higher growth curve throughout infancy and into childhood.
Genetics play a role too. Taller parents tend to have longer, heavier babies. Sex matters as well: boys consistently weigh more than girls at every point in the first year. Gestational age at birth, maternal health during pregnancy (including whether gestational diabetes was present), and feeding method all shape where a baby lands on the growth chart.
The Birth Weight Doubling Milestone
One of the simplest benchmarks parents hear is that babies double their birth weight by about 6 months. At 5 months, most babies are closing in on that milestone. If your baby was born at 7.5 pounds, you’d expect them to be somewhere in the range of 13 to 15 pounds at 5 months, on track to hit roughly 15 pounds by 6 months.
Between 4 and 6 months, babies typically gain 1 to 1.25 pounds per month. That’s a noticeable slowdown from the rapid gains of the first 3 months, and it’s completely normal. Growth naturally decelerates as babies get older.
Breastfed vs. Formula-Fed Growth Patterns
If your baby is breastfed and seems lighter than a formula-fed baby the same age, that’s expected. Healthy 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 after about 3 months, and those differences persist even after solid foods are introduced. Both patterns are normal, which is one reason the WHO charts (based on breastfed infants) are the preferred standard for babies under 2.
Length growth, on the other hand, is similar regardless of feeding method. So a breastfed baby may be leaner but just as long as a formula-fed peer.
What Percentiles Actually Mean
A percentile tells you how your baby compares to other babies the same age and sex. A baby at the 25th percentile weighs more than 25% of babies and less than 75%. That’s not a grade. A baby consistently tracking along the 15th percentile is just as healthy as one tracking along the 80th, as long as they’re following their own curve.
What pediatricians watch for is crossing percentile lines over time. Some shifting is normal, especially in the first few months as babies settle into their genetic growth trajectory. But when weight drops across multiple percentile lines while length stays the same, it often signals that a baby isn’t taking in enough calories. Conversely, when both weight and length fall off at the same rate and the proportions stay normal, the concern shifts toward a possible underlying issue affecting overall growth rather than just nutrition.
Your pediatrician also looks at weight relative to length, not just weight alone. This comparison helps distinguish between a baby who is naturally small all over and one who is thin for their frame. For babies under 6 months, BMI (yes, even infants have one on their charts) can actually be a better indicator of nutritional status than weight-for-length alone.
Signs of Slow Weight Gain
Between 3 and 6 months, gaining less than about two-thirds of an ounce per day (roughly 0.67 ounces) is considered below the expected range. Over a week, that works out to less than about 4.5 ounces. If you’re tracking feedings and your baby seems to be eating well, one slow week isn’t cause for alarm. A sustained pattern over several weeks is worth discussing with your pediatrician.
Beyond the scale, certain behavioral signs can point to inadequate nutrition in a 5-month-old: excessive sleepiness that goes beyond normal napping, a noticeable loss of interest in their surroundings, persistent fussiness that doesn’t resolve with comfort or feeding, and falling behind on physical milestones like rolling over. Wet diaper output staying consistent with your baby’s usual pattern is a reassuring sign that they’re getting enough fluid.
What Matters More Than the Number
The single weight at your baby’s 5-month checkup is less informative than the trend across their 2-month, 4-month, and upcoming 6-month visits. A baby who has tracked steadily along the 30th percentile since birth is thriving. A baby who was at the 70th percentile at 2 months and drops to the 30th by 5 months warrants a closer look, even though both numbers fall squarely in the “normal” range.
If your baby is alert, active, producing a steady number of wet diapers, and hitting developmental milestones in a reasonable timeframe, their weight is very likely fine, wherever it falls on the chart. Growth charts are tools for spotting trends, not scorecards.