How is the baby weight percentile calculated?

This calculator uses the WHO LMS method. The formula is: z = ((x/M)^L - 1) / (L × S), where x is the child's weight, and L, M, S are age- and sex-specific parameters from the WHO Child Growth Standards (2006). The z-score is then converted to a percentile using the standard normal distribution.

WHO vs CDC — which growth chart should I use for my baby?

For children under 2 years, WHO charts are recommended globally (including by the American Academy of Pediatrics). The WHO charts were built from optimally-fed, healthy children worldwide. This calculator uses WHO standards. See our article on WHO vs CDC growth charts for the full comparison.

How often should I track my baby's weight?

For healthy babies, weight tracking at routine pediatrician visits (well-child checks) is sufficient — typically at birth, 2 weeks, 2, 4, 6, 9, 12, 18, and 24 months. Between visits, you can track at home if you have a calibrated scale.

Baby Weight Percentile Calculator — WHO Standards

Q: What is a normal baby weight percentile?

Any weight percentile between the 3rd and 97th (z-score -2 to +2) is considered normal by WHO standards. The 50th percentile is the median. Your child's growth trend over time is more important than a single percentile reading.

How to Use This Calculator

Enter your child's current weight, their age in whole months, and their sex. The calculator uses the official WHO Child Growth Standards LMS method — the same data used by pediatricians globally — to compute:

Percentile rank — where your child falls among peers the same age and sex
Z-score — a precise measure of distance from the WHO median
Growth classification — WHO-standard interpretation (normal, monitor, concern)

What Is a Weight Percentile?

A percentile ranks your child's weight among 100 children of the same age and sex. The 50th percentile is the median — exactly half of children weigh more, half weigh less. Any value from the 3rd to the 97th percentile is considered within the normal range by WHO.

Importantly, the trend matters more than the number. A child consistently at the 15th percentile, following their own curve, is perfectly healthy. A child who drops from the 60th to the 10th percentile in 3 months warrants attention — even though both values are "normal."

Key principle: WHO growth charts are designed to be used over time. A single percentile is a snapshot; a series of measurements tells the real story. Track regularly and look for crossing of major percentile lines (3rd, 15th, 50th, 85th, 97th).

Understanding the Z-Score

The z-score (standard deviation score) is the raw number behind the percentile. It measures how many standard deviations your child's weight is from the WHO median for their age and sex:

Z-Score	Percentile	WHO Classification
< −3	< 0.1st	Severely underweight (action needed)
−3 to −2	0.1–2.3rd	Underweight (monitor closely)
−2 to +2	2.3–97.7th	Normal range
+2 to +3	97.7–99.9th	Overweight (monitor)
> +3	> 99.9th	Obese (action needed)

WHO vs CDC Growth Charts

This calculator uses WHO Child Growth Standards (2006). For children under 2 years, WHO charts are the internationally recommended standard — including by the American Academy of Pediatrics (AAP). The WHO charts are based on optimally-fed, healthy children from 6 countries, representing how children should grow under ideal conditions.

The CDC charts (2000) describe how US children actually grew in the late 1990s, including formula-fed and overweight children, which inflates expected weights in early infancy. For babies under 2, WHO is the correct choice.

Tips for Accurate Measurement

Weigh your baby without clothing or nappy/diaper for the most accurate result
Use a digital scale calibrated in 10g increments or better
Weigh at the same time of day (morning, before a feed) for consistency
For premature babies, use corrected age, not chronological age, until 2 years
Single measurements are less useful than a series — track at every pediatrician visit

Frequently Asked Questions

What is a normal baby weight percentile?

Any weight percentile between the 3rd and 97th (z-score −2 to +2) is considered normal by WHO standards. The 50th percentile is the median. Your child's growth trend over time is more important than a single percentile reading.

What does the 25th percentile mean for baby weight?

The 25th percentile means your baby weighs more than 25% of babies the same age and sex, and less than 75%. This is completely normal. A healthy baby can be at any percentile between the 3rd and 97th.

How is the percentile calculated?

This calculator uses the WHO LMS method: z = ((x/M)^L − 1) / (L × S), where x is your child's weight, and L, M, S are age- and sex-specific parameters from the WHO 2006 tables. The z-score is then converted to a percentile using the standard normal distribution. The same formula is used by WHO's own ANTHRO software.

Should I be worried if my baby is below the 10th percentile?

Not necessarily. The 10th percentile is within the normal range. More important is whether your baby is following their own growth curve consistently. If your baby drops two or more major percentile lines between visits, or falls below the 3rd percentile, discuss this with your pediatrician.

My baby is premature — do I use their actual age or corrected age?

Use corrected age (also called adjusted age) until your child is 2 years old. Corrected age = chronological age − weeks premature. For example, a 4-month-old born 8 weeks early has a corrected age of 2 months. Use our Corrected Age Calculator to compute this automatically.

Does weight percentile change over time?

Yes. It is completely normal for babies to shift percentiles, particularly in the first 12–18 months of life. Breastfed babies often track lower than formula-fed babies on old CDC charts (but not on WHO charts). Most children settle into a consistent channel by 18–24 months.

Track every measurement over time

A single percentile is just a snapshot. GrowChart stores your child's full growth history, plots WHO percentile curves, and alerts you to meaningful changes.

Start Tracking Free → Height Calculator

Data Sources

WHO Child Growth Standards: Weight-for-age tables and LMS values — who.int
de Onis M et al. (2006). WHO Child Growth Standards. Acta Paediatrica, Supplement 450.
WHO Multicentre Growth Reference Study Group (2006). WHO Child Growth Standards. Geneva: WHO Press.

Baby Weight Percentile Calculator