The average 4-month-old weighs about 14 to 15 pounds, though healthy weights at this age range from roughly 12 to 18 pounds depending on sex, birth weight, and feeding method. At this age, most babies are close to doubling their birth weight, and pediatricians are looking at growth trends rather than any single number on the scale.
Average Weight by Sex
Boys and girls follow slightly different growth curves. At 4 months, the 50th percentile (the statistical middle) for boys is about 15.4 pounds, while for girls it’s around 14.1 pounds. But a baby at the 25th or 75th percentile is just as healthy as one at the 50th. What matters is that your baby is following a consistent curve over time, not where that curve sits on the chart.
The normal range is wide. A 4-month-old girl at the 10th percentile might weigh around 11.5 pounds, while a boy at the 90th percentile could be over 18 pounds. Both can be perfectly normal.
The Birth Weight Doubling Milestone
You may have heard that babies double their birth weight by about 5 or 6 months. The actual average is closer to 3.8 months, or about 119 days, meaning many 4-month-olds have already hit this milestone. Boys tend to get there a bit faster than girls (around 111 days versus 129 days), and formula-fed babies reach it slightly earlier than breastfed babies (113 days versus 124 days).
If your baby hasn’t doubled their birth weight by 4 months, that’s not automatically a concern. A baby born at 8.5 pounds has more ground to cover than one born at 6 pounds. Your pediatrician will look at the overall growth trajectory rather than this single benchmark.
How Fast 4-Month-Olds Gain Weight
Between 4 and 6 months, babies typically gain about 1 to 1.25 pounds per month. That works out to roughly 4 to 5 ounces per week. This rate is actually slower than the first three months of life, when weight gain is more rapid. It’s normal for the pace to ease up as your baby gets older, and it will continue to slow through the rest of the first year.
Breastfed vs. Formula-Fed Differences
Growth patterns between breastfed and formula-fed babies start to diverge right around this age. After about 3 months, formula-fed babies typically gain weight more quickly than breastfed babies. This doesn’t mean breastfed babies are falling behind. The CDC notes that healthy breastfed infants simply put on weight more slowly, and this difference persists even after solid foods are introduced. Length growth, by contrast, is similar regardless of feeding method.
This matters because many standard growth charts were originally based on formula-fed populations. The WHO growth charts, which your pediatrician likely uses for children under 2, are based on breastfed infants and give a more accurate picture if your baby is exclusively breastfed. A breastfed baby who looks like they’re “dropping” on a formula-based chart may be growing exactly as expected.
What Normal Feeding Looks Like at 4 Months
To support this growth, most 4-month-olds consume about 24 to 30 ounces of breast milk or formula per day, spread across multiple feedings of 3 to 4 ounces each. Breastfed babies may feed more frequently since breast milk is digested faster. The total daily volume stays relatively stable from about 1 month through 6 months, even as the baby grows. Individual feedings may get slightly larger while spacing out more.
When Weight Gain Is a Concern
Pediatricians watch for a few specific red flags rather than focusing on a single weigh-in. The most important warning signs include:
- Crossing two or more major percentile lines downward on the growth chart (for example, dropping from the 50th to below the 10th)
- Falling below the 5th percentile for weight when the baby wasn’t previously tracking that low
- Actual weight loss between visits, which healthy infants past the newborn stage should not experience
Context matters here. A baby who has always tracked along the 8th percentile and is growing steadily is far less concerning than a baby who was at the 60th percentile two months ago and has dropped to the 15th. The decline itself is the signal, not the number. Babies who are consistently small but gaining at a steady rate are often just following their own genetic blueprint.
Premature Babies and Adjusted Age
If your baby was born prematurely, the numbers above won’t apply to their calendar age. Growth should be plotted using corrected age, which adjusts for how early the baby arrived. A baby born 6 weeks early and now 4 months old would be compared to growth standards for a 2.5-month-old. This adjustment is recommended until age 2. Premature infants may also be plotted on specialized preterm growth charts until they reach their original due date, at which point standard charts apply.
So a 4-month-old preemie weighing 10 or 11 pounds might be right on track once adjusted age is factored in, even though that number looks low compared to the averages above.