Weight Percentile Calculator for Children (Ages 2-20)
Enter your child's age, sex, and weight to find their weight percentile based on the CDC 2000 growth charts. The calculator uses the LMS method, the same approach clinicians use, to give an exact percentile and z-score for children ages 2 to 20. Switch between kilograms and pounds; results update instantly.
What is a weight percentile?
A weight percentile tells you what percentage of children of the same age and sex weigh less than a given child. A child at the 60th percentile weighs more than 60 percent of their peers and less than the remaining 40 percent. Percentiles do not say whether a child is "normal" in any diagnostic sense; they place a child on a population-level reference curve so clinicians and parents can track growth over time. The CDC 2000 growth charts used here were developed from a nationally representative U.S. sample and remain the standard reference for children aged 2 to 20 in clinical practice.
How the LMS method works
Raw weight data in children is skewed (a long tail toward heavier weights), so a simple z-score computed from mean and standard deviation would not map cleanly onto a normal distribution. The LMS method, introduced by Cole and Green, applies a Box-Cox power transformation controlled by three parameters: L (the power, which corrects the skewness), M (the median), and S (the coefficient of variation). For a child's weight X, the z-score is computed as z = [(X/M)^L - 1] / (L x S). That z-score follows a standard normal distribution, so normCDF(z) x 100 gives the exact percentile. The CDC publishes L, M, and S for each sex at each age in months, and this calculator interpolates between adjacent monthly values for fractional ages.
Interpreting the percentile - what it means and what it does not
The CDC and the American Academy of Pediatrics (AAP) define four screening categories: underweight (below the 5th percentile), healthy weight (5th to 84th), overweight (85th to 94th), and obese (95th and above). These are screening categories, not diagnoses. A child above the 95th percentile may have a large but healthy build, and a child below the 5th percentile may simply be constitutionally slight. What matters most to clinicians is the trend: a child who has tracked steadily at the 30th percentile for years is in a very different situation from one who has dropped from the 60th to the 10th over 12 months. Percentile categories should always be interpreted alongside height, growth velocity, dietary intake, activity level, and family history.
Metric and imperial conversions
The CDC growth charts publish all reference weights in kilograms. When you select imperial units, this calculator converts your input from pounds to kilograms (1 lb = 0.45359237 kg) before performing the LMS calculation, then converts the reference weights back to pounds for display. The conversion factor is exact by definition since the 1959 international agreement. Results shown in imperial units carry the same precision as their metric equivalents; the percentile and z-score are identical regardless of which unit system you choose.
CDC/AAP weight-for-age percentile categories (children 2-20)
| Percentile range | Category | Clinical action |
|---|---|---|
| Below 5th | Underweight | Medical evaluation recommended |
| 5th to 84th | Healthy weight | Continue routine monitoring |
| 85th to 94th | Overweight | Lifestyle assessment; monitor trend |
| 95th and above | Obese | Comprehensive evaluation recommended |
Thresholds used by the Centers for Disease Control and Prevention and the American Academy of Pediatrics to screen for weight problems in children and adolescents.
Frequently asked questions
What percentile is considered healthy for a child?
The CDC and AAP consider the 5th to 84th percentile range to indicate a healthy weight for children ages 2 to 20. A percentile below 5 is classified as underweight, 85 to 94 as overweight, and 95 or above as obese. These are screening thresholds rather than clinical diagnoses, and any concern should be discussed with a pediatrician who can assess the full growth picture.
Is a higher weight percentile better?
Not necessarily. Both extremes carry health implications. A very high percentile (95th and above) is associated with increased cardiovascular and metabolic risk, while a very low percentile (below 5th) may signal undernutrition or a growth problem. The healthiest range is roughly the middle of the curve (5th to 84th percentile), and staying on a consistent percentile over time is usually more reassuring than the absolute number.
My child is at the 95th percentile for weight but also very tall. Is that a problem?
It can be misleading to look at weight alone. A child who is both taller and heavier than average may have a healthy weight relative to their height. Clinicians often use BMI-for-age percentile alongside weight-for-age because it accounts for height. If your child is large-framed or taller than average, a pediatrician can interpret the weight percentile in the context of height and BMI percentile together.
How often should I check my child's weight percentile?
Weight and height are routinely measured at well-child visits, typically at ages 2, 3, 4, 5, 6, 8, 10, 12, 15, 18, and 21 years (schedules vary by provider and country). Tracking at these intervals gives a growth curve rather than a single data point, which is far more informative. Frequent home weigh-ins can cause unnecessary anxiety and are not recommended unless a clinician has requested close monitoring.
Why are there separate charts for boys and girls?
Boys and girls grow at different rates and have different body-composition patterns, especially during puberty. Using a single combined chart would systematically over- or under-estimate percentiles. The CDC maintains sex-specific LMS parameters for every age from 2 to 20 years, and this calculator applies the correct set based on the sex you select.
Does this calculator work for infants under 2?
No. The CDC 2000 growth charts used here cover ages 2 to 20 years. For children younger than 2, the WHO Child Growth Standards (which use weight-for-length rather than weight-for-age as the primary indicator) are recommended. Several dedicated infant growth calculators are available that use the WHO 0-24 month reference data.
What is a z-score and how does it differ from a percentile?
A z-score (also called a standard deviation score or SDS) measures how many standard deviations a child's weight falls above or below the median for their age and sex. A z-score of 0 corresponds to the 50th percentile, +1 to about the 84th percentile, -2 to about the 2nd percentile, and so on. Percentile is the more intuitive measure for parents, while z-score is preferred in research and for tracking children at the extremes of the distribution, where percentile differences are very small in absolute terms.