A typical 4-month-old weighs around 14 to 15 pounds, though healthy babies at this age can range from about 12 to 18 pounds depending on sex, birth weight, and how they’re fed. At 4 months, most babies are approaching double their birth weight, which is one of the earliest and most reliable signs that growth is on track.
Average Weight at 4 Months
The World Health Organization growth standards, which the CDC recommends for all children under 2, place the 50th percentile (the statistical middle) for a 4-month-old boy at roughly 15.4 pounds (7.0 kg) and for a 4-month-old girl at roughly 14.1 pounds (6.4 kg). But “average” is just the midpoint of a wide bell curve. A baby sitting at the 15th percentile is just as healthy as one at the 85th, as long as they’re growing consistently along their own curve.
Here’s how the range breaks down by percentile for 4-month-olds:
- 10th percentile: about 12.6 lbs (boys) / 11.5 lbs (girls)
- 25th percentile: about 13.9 lbs (boys) / 12.8 lbs (girls)
- 50th percentile: about 15.4 lbs (boys) / 14.1 lbs (girls)
- 75th percentile: about 16.8 lbs (boys) / 15.5 lbs (girls)
- 90th percentile: about 18.1 lbs (boys) / 16.8 lbs (girls)
Boys tend to be slightly heavier than girls at every percentile, and that gap widens over the first year.
The Birth Weight Doubling Milestone
One of the most commonly cited benchmarks is that babies double their birth weight between 4 and 6 months. Research looking at large groups of infants found the average doubling happens at about 119 days, or just under 4 months. Boys tend to reach it a bit sooner (around 111 days) than girls (around 129 days), and formula-fed babies typically double earlier than breastfed babies: 113 days versus 124 days.
So if your baby was born at 7 pounds, you’d expect them to be somewhere around 14 pounds by 4 months. If they were a smaller newborn at 6 pounds, a 4-month weight of 12 pounds is perfectly proportional. Birth size matters a lot when interpreting a single weight measurement, which is why pediatricians track the trend over time rather than focusing on one number.
How Fast 4-Month-Olds Gain Weight
Weight gain actually slows around this age. In the first three months, babies typically put on about 1 ounce (28 grams) per day. By 4 months, that rate drops to roughly 20 grams per day, and by 6 months many babies are gaining 10 grams or less daily. This slowdown is completely normal and catches some parents off guard, especially if they’ve been watching rapid early gains.
In practical terms, a 4-month-old gaining around 4 to 5 ounces per week is right on track. If your baby’s weight gain seems to have stalled compared to the first couple of months, that shift alone isn’t a concern.
Breastfed vs. Formula-Fed Growth
If your 4-month-old is breastfed and seems lighter than a formula-fed baby of the same age, that’s expected. Breastfed infants typically put on weight more slowly than formula-fed infants during the first year, and the difference becomes most noticeable after about 3 months. Formula-fed babies tend to gain weight faster from this point forward, and the gap persists even after solid foods are introduced later.
This is why the CDC recommends using the WHO growth charts for all babies under 2. The WHO charts were built from data on predominantly breastfed infants and reflect a more natural growth pattern. Older charts based mostly on formula-fed babies can make a normally growing breastfed infant look like they’re falling behind when they’re not.
What Matters More Than the Number
A single weight at 4 months tells you much less than the pattern of weights over time. Growth charts are percentile curves, and the key question is whether your baby is following their own curve consistently. A baby who has tracked along the 20th percentile since birth is growing well. A baby who was at the 60th percentile at 2 months and drops to the 15th by 4 months warrants a closer look, even though the 15th percentile is technically “normal.”
Pediatricians flag potential concerns when a baby’s weight falls below the 5th percentile for age, or when weight crosses downward across two or more major percentile lines on the growth chart. The American Academy of Pediatrics published updated guidelines in 2026 specifically addressing “faltering weight” in young children, with criteria focused on weight trends rather than isolated measurements. A single weigh-in that looks low isn’t diagnostic of anything on its own.
Signs That Growth Is on Track
Beyond the scale, several everyday signs tell you your 4-month-old is getting enough nutrition. Steady diaper output is one of the simplest: six or more wet diapers a day and regular bowel movements suggest adequate intake. Your baby should seem alert and active during wakeful periods, with good muscle tone and energy for the developmental work they’re doing at this age (reaching, grasping, rolling, babbling).
Outgrowing clothes is another informal but useful signal. A baby who needed 0-3 month sizes at 2 months and is filling out 3-6 month outfits by 4 months is almost certainly growing well, even if their percentile isn’t in the 50th or higher range. Length growth, head circumference, and developmental milestones all factor into the full picture alongside weight.
When Weight Gain May Be Too Slow
Truly inadequate weight gain at 4 months usually comes with other visible signs. The baby may appear thin, with loose skin around the thighs and buttocks. They may seem unusually fussy or lethargic, or have difficulty with feeding, whether that’s trouble latching, frequent spitting up, or refusing to eat. These patterns together paint a different picture than a healthy baby who simply sits at a lower percentile.
Common reasons for slow weight gain at this age include insufficient milk supply, difficulty with latch or bottle feeding, frequent illness, or, less commonly, an underlying medical condition affecting nutrient absorption. Most of the time, the issue is straightforward and correctable with feeding adjustments. If your baby has dropped significantly on their growth curve and you’re noticing any of the signs above, that combination is worth raising at your next pediatric visit or sooner.