Pediatric BMI Percentile Calculator
Determines where a child's BMI falls on CDC growth charts for ages 2–20. Use it at well-child visits or when a pediatrician asks you to track healthy weight trends over time.
Last updated: September 2026
Formula below · 3 sources (CDC, aap.org, healthychildren.org) · Updated Sep 2026
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About this calculator
Body Mass Index (BMI) for children uses the same core formula as adults — BMI = (weight in lbs / height in inches²) × 703 — but the result is interpreted differently. Because children's body fatness changes with age and differs between boys and girls, a raw BMI number means little on its own. Instead, BMI is plotted against CDC age- and sex-specific growth charts to produce a percentile. The percentile tells you what share of same-age, same-sex peers have a lower BMI. A score below the 5th percentile is considered underweight; 5th–84th is healthy weight; 85th–94th is overweight; 95th and above is obese. This calculator uses the official CDC 2000 BMI-for-age LMS reference values for the child's sex, interpolated to the exact age: z = ((BMI ÷ M)^L − 1) ÷ (L × S), converted to a percentile with the normal distribution. Above the 95th percentile it uses the CDC 2022 extended method (percentile = 90 + 10 × Φ((BMI − P95) ÷ σ)), so severe obesity is graded instead of being squeezed toward 99.9. Ages outside 2–20 return a message: use WHO weight-for-length charts under 2 and adult BMI cut-offs over 20.
How to use
Suppose a 10-year-old boy weighs 80 lbs and stands 54 inches tall. Step 1 — Calculate BMI: (80 / 54²) × 703 = (80 / 2,916) × 703 ≈ 19.3. Step 2 — Look up the CDC reference for boys at 10.0 years: L ≈ −2.77, M ≈ 16.6, S ≈ 0.120. Step 3 — z = ((19.3 / 16.6)^−2.77 − 1) / (−2.77 × 0.120) ≈ 1.02. Step 4 — Convert to a percentile: about the 84.5th percentile. This is at the top of the healthy-weight range (5th–84th) and just under the 85th-percentile overweight cut-off, so trend over the next visits matters. A girl of the same age, weight and height would be at about the 80th percentile.
Frequently asked questions
What BMI percentile is considered healthy for a child?
For children and teens aged 2–20, a BMI percentile between the 5th and 84th is classified as a healthy weight by the CDC. Below the 5th percentile indicates underweight, while the 85th–94th range signals overweight and at or above the 95th percentile is classified as obese. Because children's bodies change rapidly, these cut-points are age- and sex-specific, unlike the fixed thresholds used for adults. Regular monitoring over time matters more than any single measurement.
How is BMI percentile different from adult BMI categories?
Adult BMI uses fixed thresholds — underweight below 18.5, healthy 18.5–24.9 — regardless of age or sex. For children, the same raw BMI number can be healthy at one age but overweight at another because normal body fat shifts significantly during growth. That is why pediatric BMI is always expressed as a percentile relative to a reference population of the same age and sex. A 10-year-old and a 16-year-old with identical BMI scores can fall in very different percentile categories.
Why does gender matter when calculating a child's BMI percentile?
Boys and girls develop at different rates and accumulate body fat differently, especially during puberty. The CDC maintains separate growth charts for each sex to account for these physiological differences. Using the wrong chart would systematically overestimate or underestimate the percentile, potentially flagging a healthy child as overweight or missing a genuine concern. Always enter the child's biological sex to obtain an accurate percentile reading.