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 four screening categories most people have heard of (underweight below the 5th percentile, healthy weight 5th to 84th, overweight 85th to 94th, obese 95th and above) are defined by the CDC and the American Academy of Pediatrics on BMI-for-age percentile, not on weight-for-age. That distinction matters, because weight-for-age cannot tell a heavy child apart from a tall one. An 8-year-old boy weighing 37.5 kg sits at the 97th weight-for-age percentile, but if he is 145 cm tall his BMI is 17.8, which is the 84th BMI-for-age percentile: squarely in the CDC healthy weight range. This calculator therefore reports where a child sits on the weight-for-age curve and stops there. To assess weight status, pair it with a BMI-for-age percentile. What matters most to clinicians in either case 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. Percentiles should always be read 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.
Weight-for-age percentile bands (CDC growth charts, children 2-20)
| Percentile range | Weight-for-age band | What to do next |
|---|---|---|
| Below 5th | Low weight-for-age | Discuss with a pediatrician; check growth trend |
| 5th to 84th | Typical weight-for-age | Continue routine monitoring |
| 85th to 94th | High weight-for-age | Check BMI-for-age percentile before interpreting |
| 95th and above | Very high weight-for-age | Check BMI-for-age percentile and height percentile |
These bands describe position on the CDC weight-for-age curve. They are not weight-status categories: the CDC and the American Academy of Pediatrics define underweight (below 5th), healthy weight (5th to 84th), overweight (85th to 94th) and obesity (95th and above) on BMI-for-age percentile, which accounts for height. A tall child can sit above the 95th percentile for weight-for-age with a completely normal BMI-for-age.
Frequently asked questions
What percentile is considered healthy for a child?
For weight-for-age there is no official healthy range: most children fall between the 5th and 95th percentile, and any single point on the curve can be normal for that child. The familiar categories (underweight below the 5th, healthy weight 5th to 84th, overweight 85th to 94th, obesity 95th and above) are CDC and AAP thresholds for BMI-for-age percentile, which accounts for height. Use this calculator to see where your child sits on the weight curve, then check BMI-for-age before drawing conclusions about weight status. These are screening thresholds rather than 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 are worth a closer look. A very low percentile (below the 5th) may signal undernutrition or a growth problem, while a very high one (95th and above) may reflect either a large frame, a tall child, or excess weight, which is exactly why BMI-for-age is needed to tell those apart. 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.
Sources
- CDC Growth Charts - Percentile Data Files with LMS Values (2000)
- American Academy of Pediatrics - Clinical Practice Guideline for the Evaluation and Treatment of Children and Adolescents With Obesity (2023)
- Barlow SE and the Expert Committee. Expert committee recommendations regarding the prevention, assessment, and treatment of child and adolescent overweight and obesity. Pediatrics. 2007;120(Suppl 4):S164-92. Source of the BMI-for-age cut points (85th to 94th overweight, 95th and above obesity) that the CDC uses for weight status.