Baby Percentile Calculator — Weight & Height for Age

Find your child's WHO/CDC growth percentile for weight-for-age or height/length-for-age using the official LMS method. Boys and girls, 0-20 years.

July 4, 2026

Sex

Metric

Age

Weight

Weight-for-Age Percentile

63th percentile

Heavier than 63% of same-age girls.

Z-Score

0.34

Age Used

6.0 mo

Calculated using the LMS (Box-Cox power-exponential) method: Z = ((measurement/M)^L − 1) / (L×S) when L≠0, or Z = ln(measurement/M)/S when L=0, then converted to a percentile with the standard normal cumulative distribution function. Ages under 24 months use WHO Child Growth Standards (as distributed by the CDC for US clinical use); ages 2-20 years use the CDC 2000 Growth Charts. L/M/S values are looked up from a monthly (0-24mo) or yearly (2-20yr) reference grid and linearly interpolated for ages that fall between grid points. This tool is for general reference only and does not replace a pediatrician's clinical growth assessment — premature infants should be tracked using corrected (adjusted) age, and any single percentile reading matters far less than the overall trend across multiple visits.

93% found this helpful

Worked Examples

3-Month-Old Girl, 5.5 kg

Sex: Girl
Age: 3 months
Metric: Weight-for-age
Measurement: 5.5 kg

47th percentile (Z = -0.07)

At 3 months the WHO LMS values for girls are L=0.818, M=5.545, S=0.125. Z = ((5.5/5.545)^0.818 - 1) / (0.818 x 0.125) = -0.07, which converts to the 47th percentile — squarely in the typical range, meaning her weight is almost exactly the median for 3-month-old girls.

24-Month-Old Girl, 15.0 kg

Sex: Girl
Age: 24 months
Metric: Weight-for-age
Measurement: 15.0 kg

97th percentile (Z = 1.88)

At 24 months the LMS values for girls are L=-0.735, M=12.055, S=0.107. Z = ((15.0/12.055)^-0.735 - 1) / (-0.735 x 0.107) = 1.88, which lands right at the 97th percentile — the upper edge of the typical range. Worth mentioning at the next well-child visit, but a single high reading isn't a diagnosis on its own.

9-Year-Old Boy, 135 cm Tall

Sex: Boy
Age: 9 years
Metric: Height-for-age
Measurement: 135 cm

60th percentile (Z = 0.24)

At 9 years the CDC LMS values for boys are L=0.413, M=133.51, S=0.046. Z = ((135/133.51)^0.413 - 1) / (0.413 x 0.046) = 0.24, which converts to the 60th percentile — taller than average but comfortably inside the typical range.

6-Month-Old Girl, 63 cm Long

Sex: Girl
Age: 6 months
Metric: Height/length-for-age
Measurement: 63 cm

19th percentile (Z = -0.87)

At 6 months the WHO LMS values for girls are L=1.331, M=65.26, S=0.040. Z = ((63/65.26)^1.331 - 1) / (1.331 x 0.040) = -0.87, which converts to the 19th percentile — on the shorter side but still well within the normal 3rd-97th percentile band.

2-Month-Old Boy, 3.5 kg

Sex: Boy
Age: 2 months
Metric: Weight-for-age
Measurement: 3.5 kg

0.5th percentile (Z = -2.59)

At 2 months the WHO LMS values for boys are L=0.882, M=5.276, S=0.133. Z = ((3.5/5.276)^0.882 - 1) / (0.882 x 0.133) = -2.59, which converts to the 0.5th percentile — below the 3rd percentile cutoff. This calls for a conversation with a pediatrician, especially to rule out prematurity (which needs corrected age) before assuming a growth concern.

How to Use This Calculator

1

Choose sex and metric

Select boy or girl, then weight-for-age or height/length-for-age — the two use separate growth curves.
2

Enter age

Type the child's exact age in months or years. Use corrected age instead of birth age for infants born premature.
3

Enter the measurement

Weight in kg or lb, or height/length in cm or inches. Use recumbent length for children under 2, standing height for 2 and up, matching how the reference data was measured.
4

Read the percentile and Z-score

The result shows the percentile, the underlying Z-score, and a plain-English comparison to same-age, same-sex children.

What Each Value Means

Percentile (percent): The percentage of same-age, same-sex children in the reference population with a lower measurement than the child being evaluated.
Z-score (standard deviations): The number of standard deviations a measurement sits from the reference median for that exact age and sex, calculated directly from the LMS parameters before being converted into a percentile.
LMS parameters (L / M / S): Three age-specific values — L (skewness/Box-Cox power), M (median), and S (coefficient of variation) — published by WHO and CDC that together describe the full growth distribution at a given age, allowing any raw measurement to be converted to a percentile.

Frequently Asked Questions

What's the difference between the WHO and CDC growth charts?

The WHO Child Growth Standards describe how children grow under optimal conditions (breastfed infants from six countries), and the CDC/AAP recommend using them for children under 24 months. The CDC 2000 Growth Charts describe how a large sample of US children actually grew between the 1960s-1990s, and are recommended for ages 2 and up. This calculator switches between the two data sets automatically at the 24-month mark, matching official US clinical practice.

What does the percentile actually mean?

A percentile shows how a measurement compares to same-age, same-sex children in the reference population. A weight-for-age percentile of 40 means the child weighs more than 40% of children the same age and sex, and less than the other 60%. It is not a grade or a health score — the 10th percentile and the 90th percentile are both considered typical, just different points on the same normal growth distribution.

What is a Z-score and why does the calculator show it?

A Z-score measures how many standard deviations a measurement is from the reference median, using the LMS method's L (skewness), M (median), and S (coefficient of variation) parameters for that exact age and sex. It's the number the percentile is actually calculated from, and clinicians often track raw Z-scores over time because they change more smoothly near the extreme ends of the distribution than percentiles do.

Should I be worried if my baby is below the 3rd or above the 97th percentile?

Not automatically. A single reading outside the 3rd-97th percentile range is common and often reflects normal variation, especially for breastfed infants (who tend to track lower on the WHO weight curve after about 6 months) or naturally larger/smaller-framed children. What pediatricians actually watch for is a percentile that changes sharply between visits — a baby who was tracking at the 50th percentile and drops to the 5th over a few months is a more meaningful signal than a single low reading in isolation.

How does prematurity affect the percentile result?

For babies born before 37 weeks, pediatricians typically use corrected (adjusted) age — the age counted from the original due date, not the birth date — when plotting growth through about 24 months. Entering a premature infant's actual chronological age into this or any growth calculator can make a perfectly on-track baby look artificially small. Use our chronological age calculator to work out the exact adjustment, then enter the corrected age here instead.