BMI Percentile Calculator (Ages 2-20)
Enter your child's age, sex, height, and weight to find their BMI percentile. The calculator uses the Centers for Disease Control and Prevention (CDC) BMI-for-age growth charts and the LMS (Box-Cox) method, the same standard used by pediatricians across the United States. You get the raw BMI, the BMI-for-age percentile, a z-score, and the weight category. Switch between metric and imperial units and the result updates instantly.
Formula
Worked example
A 10-year-old boy weighing 70 lb at 4 ft 6 in: BMI = 31.75 kg / (1.3716 m)^2 = 16.88 kg/m^2. CDC LMS for a 10.5-year-old male: L = -0.088, M = 16.949, S = 0.0872. z = ((16.88/16.949)^-0.088 - 1) / (-0.088 * 0.0872) = 0.055 / 0.00767 = -0.47. Percentile = normCDF(-0.47) = 32nd - healthy weight.
Why children use percentiles instead of fixed BMI cut-offs
For adults aged 20 and over, a single set of BMI thresholds applies regardless of age. Children and teenagers are still growing, and the normal amount of body fat changes considerably with age and differs between boys and girls. A BMI of 18 in a 4-year-old would signal very different health status than the same BMI in a 14-year-old. The CDC solved this by producing BMI-for-age percentile charts based on a nationally representative sample, showing how any child's BMI compares to peers of the same age and sex. A percentile tells you what fraction of the reference population had a lower BMI: a child at the 60th percentile is heavier than 60 percent of same-age, same-sex children in the reference group.
The CDC LMS method and what it means
The CDC growth charts use the LMS (Lambda-Mu-Sigma) statistical method devised by Cole and Green. Three parameters describe the distribution of BMI at each exact age and sex: L (lambda) corrects for skewness using a Box-Cox power transformation, M (mu) is the median BMI at that age and sex, and S (sigma) is the coefficient of variation (a normalized measure of spread). The z-score formula is z = [(BMI / M)^L - 1] / (L x S). Once the z-score is known, it converts to a percentile via the standard normal cumulative distribution function. This approach handles the fact that BMI distributions are not perfectly symmetric, especially at the tails, so a z-score of +1.645 corresponds precisely to the 95th percentile rather than being an approximation.
How to interpret the results
The CDC defines four weight-status categories based on percentile ranges: below the 5th percentile is underweight, 5th to below 85th is healthy weight, 85th to below 95th is overweight, and 95th or above is obesity. A further category of severe obesity applies when a child's BMI is at or above 120 percent of the 95th percentile for their age and sex, or at or above 35 kg/m^2, whichever is lower. These categories are starting points for clinical conversation. A single measurement is less meaningful than the trend across multiple well-child visits; a child consistently in the healthy range who starts crossing percentile lines upward deserves attention even if they remain below the 85th percentile cut-off.
Limitations and when to seek clinical advice
BMI-for-age does not distinguish muscle from fat. A muscular athlete may have a high percentile without excess fat, and an older child who has lost muscle through illness may appear in the normal range despite elevated body fat. The CDC reference charts were developed from data collected between 1963 and 1994, and population BMI distributions have shifted since then. Height and weight measurement errors, especially in shoes or heavy clothes, can shift results noticeably. For children close to a category boundary, clinical judgment about growth trajectory, dietary patterns, physical activity, family history, and additional measurements such as waist circumference is far more informative than the number alone. Always share these results with a pediatrician or family physician rather than acting on them independently.
CDC BMI-for-age weight status categories (ages 2-20)
| Percentile range | Weight category | Clinical note |
|---|---|---|
| Below 5th | Underweight | Evaluation recommended |
| 5th to below 85th | Healthy weight | Within normal range |
| 85th to below 95th | Overweight | Monitoring and counseling suggested |
| 95th or above | Obesity | Clinical assessment recommended |
| 120% of 95th or BMI >= 35 | Severe obesity | Intensive management recommended |
Percentile cut-offs used by the Centers for Disease Control and Prevention for children and teens.
Frequently asked questions
What is a healthy BMI percentile for a child?
The CDC considers a BMI-for-age percentile between the 5th and the 85th as the healthy-weight range for children and teens. A percentile of 85 or above but below 95 indicates overweight, and 95 or above indicates obesity. Below the 5th percentile is classified as underweight. These are screening categories, not diagnoses, and a healthcare provider interprets them alongside other health information.
Why does this calculator ask for sex but not gender?
The CDC BMI-for-age reference charts are based on biological sex assigned at birth, because growth trajectories and body composition differ between males and females in ways that are captured in the LMS parameters. For transgender or intersex adolescents, a clinician will advise which reference charts are most appropriate.
My child is 20. Should I use this calculator or an adult BMI calculator?
The CDC percentile charts apply through age 20. At age 20 and beyond, the standard fixed adult cut-offs (underweight below 18.5, healthy weight 18.5-24.9, overweight 25-29.9, obesity 30 or above) are used instead of percentile comparison. If your child just turned 20 or is close to that birthday, either approach can be reasonable, but an adult calculator is the conventional tool from that birthday onward.
What is the difference between BMI percentile and z-score?
Both measure the same thing in different units. A z-score of 0 means exactly at the median (50th percentile), +1 is one standard deviation above it (around the 84th percentile), and -1 is one standard deviation below (around the 16th percentile). Percentile is easier to explain to families; z-score is preferred in clinical research and for tracking children in the very high or very low tails, where percentile values compress and become hard to distinguish.
Can I use this for a toddler under 2 years old?
No. The CDC BMI-for-age charts begin at age 2. For children under 2, the World Health Organization provides weight-for-length charts, which are the recommended reference for that age range. Enter ages 2 and above only.
How accurate is the percentile calculation?
This calculator implements the CDC's LMS method and uses the official CDC LMS parameter table with linear interpolation between monthly anchor points. The standard normal CDF approximation used here has a maximum error smaller than 0.0001 percentile points across all practically observed z-scores, which is negligible for clinical screening purposes. The main sources of real-world error are measurement imprecision in height and weight.
Does the 95th percentile threshold change with age?
Yes. Because the LMS parameters change every month, the BMI value that corresponds to the 95th percentile varies with age and sex. For an 8-year-old boy, the 95th percentile BMI is roughly 20 kg/m^2, while for a 15-year-old boy it is closer to 28 kg/m^2. That is why children cannot be assessed with the single fixed adult thresholds, and why this calculator reports the BMI values at key percentile cut-offs for the child's specific age and sex.