A healthy five-month-old boy typically weighs around 16.5 pounds (7.5 kg), while a girl averages about 15 pounds (6.8 kg). These numbers come from the WHO growth standards, which the CDC recommends for all U.S. infants under two years old. But averages only tell part of the story. What matters more is whether your baby is growing consistently along their own curve.
Average Weight and Length at Five Months
The WHO growth charts place the 50th percentile (the statistical middle) at roughly 16.5 pounds for boys and 15 pounds for girls. A wide range is completely normal, though. A boy at the 15th percentile might weigh around 14 pounds, while one at the 85th percentile could be closer to 18.5 pounds. Girls follow a similar spread about a pound lighter at each marker.
Length at this age averages about 26 inches (66 cm) for boys and 25.2 inches (64 cm) for girls. Weight and length tend to track together: a baby who’s long for their age will often weigh more, and that’s perfectly proportionate. Your pediatrician looks at both measurements together, not weight in isolation.
Why Percentiles Matter More Than the Number
A baby who has been at the 25th percentile since birth and stays there is growing exactly as expected. A baby who was at the 75th percentile at two months and drops to the 25th by five months is a different situation, even if their actual weight looks “normal” on paper. The trajectory is the signal. Pediatricians track this at every well visit, plotting weight, length, and head circumference on a growth chart to watch for sudden jumps or drops across percentile lines.
The CDC specifically recommends using the WHO growth standards rather than the older CDC charts for babies under two. The WHO charts are based on how healthy, breastfed infants should grow, collected from a large international study that included U.S. babies. The older CDC charts had gaps in data for the first six months and were built on a mix of feeding types, making them less reliable for young infants.
How Fast Five-Month-Olds Gain Weight
Growth speed changes dramatically in the first year. In the early weeks, babies pack on about an ounce a day. By four months, that slows to roughly 20 grams (about two-thirds of an ounce) per day. By six months, many babies gain 10 grams or less daily. So at five months, your baby is right in the middle of this slowdown, and it’s completely normal for weight gain to feel like it’s tapering off compared to those rapid early months.
Most babies roughly double their birth weight by four to five months. If your baby was born at 7.5 pounds, landing somewhere around 15 pounds at five months is right on track. Premature babies or those with low birth weight follow adjusted growth curves, and their pediatrician will use a corrected age (based on their due date, not their birth date) when plotting growth.
Breastfed vs. Formula-Fed Growth Patterns
Breastfed and formula-fed babies don’t gain weight at the same pace, and knowing this can save you unnecessary worry. Breastfed infants typically put on weight more slowly than formula-fed infants during the first year. The difference becomes noticeable after about three months, when formula-fed babies start gaining weight more quickly. This gap continues even after solid foods enter the picture around six months.
Interestingly, length growth is similar regardless of feeding method. It’s specifically fat accumulation that differs. The WHO growth charts were designed with breastfeeding as the biological norm, so a breastfed baby tracking along the 30th percentile on that chart is growing exactly as expected. Problems arise when providers use older charts calibrated to formula-fed infants, which can make a healthy breastfed baby look like they’re falling behind.
Signs Your Baby’s Weight Needs Attention
Rather than fixating on a specific number, watch for patterns that suggest your baby isn’t getting enough nutrition. The clearest indicators are output-related: fewer wet and dirty diapers than usual, dry lips, or a sunken soft spot on the top of the head. Dark circles around the eyes and unusual sleepiness are also signs of dehydration that warrant a prompt call to your pediatrician.
Feeding frequency is another useful benchmark. At five months, a breastfed baby should still be nursing about 8 to 12 times in 24 hours. Some babies are naturally “sleepy” feeders who don’t wake or signal hunger often enough on their own. If your baby consistently goes long stretches without cueing for a feed, you may need to wake them every two hours during the day and every three to four hours at night to keep weight gain on track.
A single weigh-in that looks low isn’t cause for alarm. Babies fluctuate day to day based on when they last ate or had a diaper change. What raises concern is a pattern over two or more visits showing a drop across two major percentile lines, like falling from the 50th to below the 15th. Your pediatrician will distinguish between a naturally small baby who’s growing steadily and one who’s genuinely faltering.