At five 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), based on the World Health Organization growth standards. But “average” is just the midpoint of a wide, healthy range. A five-month-old can weigh anywhere from around 12 to 20 pounds and be perfectly on track, depending on their sex, birth weight, genetics, and how they’re being fed.
Typical Weight Ranges at Five Months
The WHO growth charts, which pediatricians use to track infant development, show the full spread of healthy weights at each age. At five months, the numbers break down like this:
- Boys: The middle 80% of healthy five-month-old boys weigh between roughly 14 and 19 pounds. The 50th percentile (the true average) sits around 16.5 pounds.
- Girls: The middle 80% of healthy five-month-old girls weigh between roughly 12.8 and 17.5 pounds, with the 50th percentile around 15.2 pounds.
A baby in the 15th percentile is just as healthy as one in the 85th percentile. What matters more than any single number is the pattern over time. Your pediatrician is tracking whether your baby follows a consistent curve, not whether they hit one specific weight.
How Fast Babies Gain Weight at This Age
Weight gain slows noticeably around the fourth and fifth months. In the first few months of life, babies typically gain about an ounce a day. By four months, that drops to roughly 20 grams (about two-thirds of an ounce) per day. By six months, many babies gain 10 grams or less per day. So at five months, your baby is right in the middle of this slowdown, and that’s completely normal.
A useful benchmark: most babies double their birth weight by six months. If your baby was born at 7.5 pounds, you’d expect them to be somewhere in the neighborhood of 15 pounds around that half-year mark. At five months, they’re closing in on that milestone but may not have reached it yet.
Breastfed vs. Formula-Fed Babies
How your baby is fed affects their growth pattern, and this is one of the most common reasons parents worry unnecessarily. Breastfed infants typically put on weight more slowly than formula-fed infants during the first year. Formula-fed babies tend to gain weight faster starting around three months of age, and that difference persists even after solid foods are introduced. Both patterns are healthy.
The CDC notes that length (height) growth is similar between breastfed and formula-fed babies. The difference is primarily in how quickly they fill out. If your breastfed five-month-old sits lower on the weight chart than a formula-fed baby of the same age, that’s expected, not a sign of a problem.
Why Percentile Trends Matter More Than Numbers
A single weight measurement tells you very little on its own. What pediatricians look for is your baby’s growth curve: how their weight tracks from visit to visit. A baby who has been in the 25th percentile since birth and stays there is growing beautifully. A baby who drops from the 60th percentile to the 15th percentile over two or three visits is the one who needs a closer look.
Small jumps or dips between visits are normal. Babies don’t grow in perfectly smooth lines. An illness, a growth spurt, or even a feeding transition can shift things temporarily. It’s a sustained change in trajectory, crossing two or more major percentile lines downward, that signals a potential concern.
Signs Your Baby Is Growing Well
The scale is only one piece of the picture. At five months, you can look for several reassuring signs that your baby is getting enough nutrition and developing on track. A well-fed baby produces at least five to six wet diapers a day. They’re alert and engaged during wake periods, making eye contact, responding to your facial expressions, and starting to reach for things. They have good muscle tone and are likely rolling over or working on it.
On the flip side, signs that growth may be faltering include a baby who cries more than expected, sleeps excessively, falls asleep during feedings regularly, or doesn’t interact with people the way you’d expect for their age. These behavioral cues often show up before a concerning drop on the growth chart, so they’re worth paying attention to.
Babies Born Early Need Adjusted Calculations
If your baby was born premature, the five-month weight ranges above don’t apply directly. Preterm babies are measured using their corrected age, not their actual birth date. You calculate corrected age by subtracting the number of weeks your baby arrived early. A baby born eight weeks premature who is now five months old would be evaluated against the growth standards for a three-month-old.
This adjusted age calculation is used until age two. Premature infants also have specialized growth charts designed for their early months. As they approach their original due date and beyond, they transition to the same standard charts used for full-term babies. If your baby was premature, their pediatrician is already factoring this in at every checkup.
What Influences Your Baby’s Weight
Genetics plays a bigger role than most parents realize. Tall, lean parents tend to have babies who are lighter for their age. Shorter, stockier parents often have babies who weigh more. Birth weight itself sets the starting point. A baby born at 6 pounds will almost certainly weigh less at five months than a baby born at 9 pounds, even if both are growing at the same rate.
Sex accounts for roughly a one-pound difference at this age, with boys trending heavier. Feeding frequency, whether your baby has been sick recently, and individual metabolism all contribute. There is no single “right” weight for a five-month-old. The right weight for your baby is the one that fits their personal growth trajectory and overall health.