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.
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 estimates the percentile by computing a z-score — z = (BMI − median) / standard deviation — and converting it to a percentile using the normal distribution.
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 / 2916) × 703 ≈ 19.3. Step 2 — Plug age (10) into the approximate median: 23.5 + 10 × 0.1 = 24.5, and SD: 5 + 10 × 0.05 = 5.5. Step 3 — Compute z-score: (19.3 − 24.5) / 5.5 ≈ −0.95. Step 4 — Convert to percentile: 50 + (−0.95 / 1.96) × 50 ≈ 26th percentile. This falls in the healthy weight range.
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.