A 4-month-old baby typically weighs around 14 to 15 pounds, though healthy weights range from about 12 to 17 pounds depending on sex, birth weight, and feeding method. Boys tend to be slightly heavier than girls at this age, averaging closer to 15 pounds while girls average closer to 14.
Average Weight by Sex
The WHO growth charts, which pediatricians use for children under 2, place the 50th percentile for a 4-month-old boy at roughly 15.2 pounds (6.9 kg) and for a girl at about 13.9 pounds (6.3 kg). But “average” is just the midpoint of a wide healthy range. A baby at the 25th percentile is just as healthy as one at the 75th. What matters more than any single number is whether your baby is following a consistent curve over time.
The normal range spans several pounds in either direction. A 4-month-old boy between the 5th and 95th percentiles could weigh anywhere from about 12.5 to 18.5 pounds. For girls, that range is roughly 11.5 to 17.5 pounds. If your baby falls anywhere in that window and is tracking steadily along their own curve, their weight is considered normal.
The Birth Weight Doubling Rule
One of the simplest benchmarks: most babies double their birth weight around 4 months. Research puts the average doubling time at about 119 days, or just under 4 months. Boys tend to hit this milestone a bit earlier (around 111 days) while girls take slightly longer (around 129 days). Formula-fed babies also reach it sooner than breastfed babies, at roughly 113 days versus 124 days.
So if your baby was born at 7 pounds, you’d expect them to be in the neighborhood of 14 pounds by their 4-month checkup. This isn’t a hard rule, but it’s a useful ballpark that your pediatrician will likely reference.
How Growth Rate Slows at This Age
Babies gain weight fastest in the first three months, then the pace starts to taper. By around 4 months, the typical weight gain slows to about 20 grams per day, which works out to roughly 4.5 to 5 ounces per week. That’s noticeably less than the rapid gains of the newborn period. Parents sometimes worry when weight gain seems to decelerate, but this slowdown is completely expected.
Breastfed vs. Formula-Fed Babies
Breastfed and formula-fed babies follow different growth curves, and the gap becomes more visible right around 3 to 4 months. Formula-fed infants typically gain weight more quickly after about 3 months of age, so they often look heavier on the growth chart at the 4-month mark. Breastfed babies put on weight more slowly through the first year, and this pattern continues even after solid foods are introduced.
This doesn’t mean breastfed babies are underweight. The WHO growth charts were specifically developed using breastfed infants as the standard, which is why pediatricians use them for children under 2. If your breastfed baby’s weight looks lower compared to a friend’s formula-fed baby of the same age, that difference is normal and expected.
What Premature Babies Should Weigh
If your baby was born early, the number on the scale at 4 calendar months won’t match the standard charts, and it shouldn’t. Pediatricians use “corrected age” for premature babies, subtracting the weeks of prematurity from the baby’s actual age. A baby born 6 weeks early who is now 4 months old would be evaluated as a 2.5-month-old on the growth chart. This adjusted-age approach is used until age 2.
Preterm infants at a corrected age of 4 months typically gain between 15 and 25 grams per day. Healthy preemies tend to catch up in head size first, then weight and length. Extremely low-birthweight babies often track near or below the 5th percentile, but as long as their growth runs parallel to the normal curve, this pattern is usually healthy. Babies born small for their gestational age may show less catch-up growth overall, and their pediatrician may set individualized goals rather than targeting a specific percentile.
When a Weight Percentile Drop Matters
It’s common for babies to shift between percentile lines during the first two to three years. A baby who started at the 60th percentile and drifts to the 40th is probably just settling into their genetic blueprint. Not every child is meant to be on the 50th percentile, and a child’s growth is strongly influenced by the size of their parents.
The signal that something needs attention is crossing two major percentile lines on the WHO growth chart. Dropping from the 75th percentile to the 25th, for example, warrants a closer look. A pediatrician would typically check for underlying causes and assess whether the baby is getting enough calories. In the absence of any medical issue, the reassurance is straightforward: optimize feeding, monitor the curve, and trust that the baby is growing within their genetic potential.
How to Weigh Your Baby at Home
Between pediatrician visits, you can track your baby’s weight at home with a few simple approaches. A digital baby scale on a hard, flat surface (not carpet) gives the most accurate reading. Make sure your baby’s legs don’t hang off the end of the scale, which will throw off the number. Weigh them naked, ideally before a feed, and try to measure at roughly the same time of day for consistency.
If you don’t have a baby scale, you can use regular bathroom scales. Weigh yourself first, then weigh yourself holding your naked baby, and subtract. Another option is placing a baby bath on the scale, zeroing it out, and then setting the baby inside, though this only works if the bath fits entirely on the scale without touching the floor. To check your scale’s accuracy, weigh something with a known weight, like a 1 kg bag of sugar, and confirm the reading.
Keep in mind that home scales aren’t as precise as what your pediatrician uses. They’re useful for spotting general trends, but small day-to-day fluctuations are normal and don’t mean much. The weights recorded at your baby’s well-child visits remain the most reliable data points for tracking growth.