50th Percentile Baby: Average Weight and Length by Month, WHO Chart (2026)
50th percentile baby is exactly average for age and sex on the WHO chart. Monthly weight and length tables for boys and girls, and when tracking away from average matters.
Most parents treat the 50th percentile as a target. If their baby is at the 50th, they feel relieved. If the baby drops to the 30th, they feel like something is slipping. Neither reaction is quite right.
The 50th percentile is the median measurement for a given age and sex on the WHO growth chart. It is what exactly half of healthy babies measure above and half measure below. It is not a goal, and moving away from it is not a warning. What your pediatrician actually monitors is whether your baby tracks its own curve consistently over time, regardless of which percentile that curve sits on. The Baby Percentile Calculator shows where your baby falls on the WHO chart for any age and measurement.
What the 50th Percentile Actually Measures
The WHO Multicentre Growth Reference Study tracked breastfed children across six countries in optimal conditions to set a universal reference for healthy infant growth. The 50th percentile on those charts represents the median value: a weight, length, or head circumference that exactly half the study population fell above and half below.
That median is not the average in a mathematical sense, though for normally distributed measurements like infant weight, the two are very close. The key point is that the 50th percentile describes the midpoint of a wide healthy range that runs from the 3rd to the 97th percentile.
A baby at the 25th percentile is not below average in a clinical sense. They are simply lighter than 75% of same-age infants and heavier than 25%. That is a normal position. A baby at the 75th percentile is not overweight. They are heavier than 75% of same-age infants and lighter than 25%. Also normal.
The chart is a reference distribution, not a scorecard.
50th Percentile Baby Weight by Month: WHO Reference Tables
The WHO 50th percentile weight values differ by sex. Boys trend slightly heavier than girls at every age, with the gap growing through the first year.
Boys: 50th percentile weight by month
| Age | 50th Percentile Weight |
|---|---|
| Birth | 3.35 kg (7.4 lb) |
| 1 month | 4.47 kg (9.9 lb) |
| 2 months | 5.57 kg (12.3 lb) |
| 3 months | 6.37 kg (14.0 lb) |
| 4 months | 7.00 kg (15.4 lb) |
| 5 months | 7.51 kg (16.6 lb) |
| 6 months | 7.93 kg (17.5 lb) |
| 9 months | 9.18 kg (20.2 lb) |
| 12 months | 9.66 kg (21.3 lb) |
| 18 months | 10.94 kg (24.1 lb) |
| 24 months | 12.19 kg (26.9 lb) |
Girls: 50th percentile weight by month
| Age | 50th Percentile Weight |
|---|---|
| Birth | 3.23 kg (7.1 lb) |
| 1 month | 4.18 kg (9.2 lb) |
| 2 months | 5.12 kg (11.3 lb) |
| 3 months | 5.82 kg (12.8 lb) |
| 4 months | 6.42 kg (14.1 lb) |
| 5 months | 6.90 kg (15.2 lb) |
| 6 months | 7.28 kg (16.1 lb) |
| 9 months | 8.24 kg (18.2 lb) |
| 12 months | 8.95 kg (19.7 lb) |
| 18 months | 10.20 kg (22.5 lb) |
| 24 months | 11.50 kg (25.4 lb) |
These values are from the WHO Multicentre Growth Reference Study standards. Slight rounding has been applied for readability. The Baby Weight Percentile Chart by Age has the full reference table across all major percentile lines from the 3rd to the 97th.
50th Percentile Baby Length by Month: WHO Reference Tables
Length follows a similar sex-based split, with boys slightly longer on average throughout the first 2 years.
Boys: 50th percentile length/height by month
| Age | 50th Percentile Length |
|---|---|
| Birth | 49.9 cm (19.6 in) |
| 1 month | 54.7 cm (21.5 in) |
| 2 months | 58.4 cm (23.0 in) |
| 3 months | 61.4 cm (24.2 in) |
| 4 months | 63.9 cm (25.2 in) |
| 6 months | 67.6 cm (26.6 in) |
| 9 months | 72.0 cm (28.3 in) |
| 12 months | 75.7 cm (29.8 in) |
| 18 months | 82.3 cm (32.4 in) |
| 24 months | 87.8 cm (34.6 in) |
Girls: 50th percentile length/height by month
| Age | 50th Percentile Length |
|---|---|
| Birth | 49.1 cm (19.3 in) |
| 1 month | 53.7 cm (21.1 in) |
| 2 months | 57.1 cm (22.5 in) |
| 3 months | 59.8 cm (23.5 in) |
| 4 months | 62.1 cm (24.4 in) |
| 6 months | 65.7 cm (25.9 in) |
| 9 months | 70.1 cm (27.6 in) |
| 12 months | 74.0 cm (29.1 in) |
| 18 months | 80.7 cm (31.8 in) |
| 24 months | 86.4 cm (34.0 in) |
How Formula vs Breastfeeding Affects Where Babies Track
The WHO growth charts were built from a study population that was exclusively or predominantly breastfed. This matters when interpreting where formula-fed babies fall.
Formula-fed babies tend to gain weight faster in the first 3-4 months than breastfed babies. They commonly track above the 50th percentile during that window on WHO charts, not because they are growing abnormally, but because the reference population was breastfed and formula delivers calories at a slightly different rate.
After 4-6 months, this difference often normalizes as solid foods are introduced and growth velocity slows across all feeding types.
The practical takeaway: a formula-fed baby consistently at the 65th or 70th percentile is not overweight. They may simply be feeding on a pattern that tracks higher on a chart calibrated to breastfed infants. Your pediatrician will consider feeding method when interpreting percentile position.
Breastfed babies, by contrast, should theoretically track the WHO chart precisely. If a breastfed baby is consistently below the 25th percentile, that warrants a closer look at feeding frequency, transfer volume, and whether supplementation is appropriate.
When Your Baby Tracks Away From the 50th Percentile
Babies do not always stay on the same percentile. Some natural shifts are normal. Others are worth discussing.
Normal shifts in percentile:
Newborns often drop 5-10% of their birth weight in the first week, which can temporarily shift their percentile downward. Most regain birth weight by 2 weeks. This early dip is expected and not a sign of a problem.
Between 6 and 12 months, many breastfed babies cross one percentile line downward as their growth rate naturally slows and solid food intake ramps up. A baby who was at the 50th percentile and drifts to the 35th over 4 months, while feeding and developing normally, is usually within the expected range of variation.
Shifts that warrant a conversation:
Crossing two major percentile lines downward in a short period is the pattern that prompts clinical attention. Dropping from the 50th to the 10th over 3-4 months, without a clear explanation like illness or a feeding disruption, is a different situation than a gradual drift.
Weight falling while length stays on the same curve is also worth raising with your pediatrician. That pattern (weight dropping relative to length) suggests the baby is lean for their height, which is different from constitutional smallness.
For context on what low percentiles mean and when they become clinical concerns, the 3rd Percentile Baby post covers what doctors assess when a baby falls below the reference range and how growth velocity changes the picture.
Growth Velocity and Why the Direction Matters More Than the Number
Growth velocity is the rate of gain between visits, not the percentile at any single point. Two babies at the 50th percentile can be in very different positions if one got there by gaining steadily and one got there by dropping from the 80th.
Expected weight gain velocity by age gives you a reference:
| Age Range | Expected Weekly Gain |
|---|---|
| Birth to 3 months | 150-200 g per week |
| 3 to 6 months | 100-150 g per week |
| 6 to 12 months | 70-90 g per week |
| 12 to 24 months | 40-60 g per week |
A baby gaining 160 g per week in the first 3 months while tracking the 50th percentile is growing well. A baby gaining only 80 g per week in that same window, even if still near the 50th percentile due to a high starting point, has a velocity concern.
The direction and rate of growth, plotted across multiple visits, gives your pediatrician more information than any single measurement. That is why missing well visits in the first year costs information more than it costs time.
For a full explanation of what all the percentile lines represent and how to read a growth chart at each visit, the What Does Baby Percentile Mean post walks through the WHO reference system from the 3rd to the 97th percentile.
The 50th percentile means the baby's weight or length is exactly at the midpoint of the WHO growth chart for their age and sex. Half of healthy babies the same age and sex measure above that value, and half measure below. It is the median, not a goal. Babies anywhere from the 3rd to the 97th percentile are within the normal reference range, and tracking any consistent curve over time is what matters, not hitting the 50th.
The 50th percentile birth weight on the WHO chart is 3.35 kg (7.4 lb) for boys and 3.23 kg (7.1 lb) for girls. These values represent the median among healthy full-term newborns in the WHO Multicentre Growth Reference Study. Babies born between roughly 2.5 kg and 4.0 kg at full term are within the typical range. Birth weight below 2.5 kg is classified as low birth weight and warrants closer monitoring in the early weeks.
At 6 months, the 50th percentile weight on the WHO chart is 7.93 kg (17.5 lb) for boys and 7.28 kg (16.1 lb) for girls. These are median values; healthy babies range from roughly 6.5 kg to 9.5 kg at this age across the full 3rd to 97th percentile span. A baby who weighed 3.5 kg at birth and 7.5 kg at 6 months has roughly doubled their birth weight, which is a common and healthy milestone.
A gradual drop of one percentile line over several months is common and usually not concerning, particularly in the transition from 4 to 8 months as growth velocity naturally slows and solid food is introduced. A drop across two major percentile lines (for example from the 50th to the 10th) over a short period, or a sudden drop in weight while length stays the same, is worth raising with your pediatrician. Consistent tracking of any curve, not staying at the 50th, is the goal.
Yes. The WHO growth charts were built from a study population that was predominantly breastfed. Formula-fed babies often gain weight faster in the first 3-4 months and may track above the 50th percentile during that window. This does not mean they are overweight. After 4-6 months, the difference typically narrows as solid foods are introduced. Pediatricians factor in feeding method when interpreting percentile position in the first year.
At 12 months, the 50th percentile length on the WHO chart is 75.7 cm (29.8 inches) for boys and 74.0 cm (29.1 inches) for girls. Most babies triple their birth length by around 3 years, and the 12-month measurement falls roughly at the midpoint of the rapid early growth phase. Length measurement accuracy matters more at this age because babies wiggle; a recumbent length measurement with two people positioning the baby is more reliable than standing height.
Written by
Hassaan Rasheed
Web Developer & Content Researcher
Hassaan builds calculators and writes research-backed guides on finance, math, payroll, and construction topics. Every number in his articles is sourced from official data and worked through by hand.
View LinkedIn Profile
