Health & Body Composition
BMI-for-Age Calculator
Adult BMI categories lie about children. This calculator uses the official CDC growth-chart data to give kids the percentile that actually applies.
BMI-for-age compares a child’s BMI with the CDC growth-chart distribution for their exact age and sex, expressed as a percentile. For ages 2–20: below the 5th is underweight, 85th–95th overweight, and at or above the 95th obesity. Results are screening estimates, not diagnoses — confirm with a clinician before acting on them.
Official CDC LMS data, ages 2–20.
Percentile, not adult categories.
85th+ = overweight · 95th+ = obesity.
Calculate Your BMI-for-Age
BMI Percentile
What Is BMI-for-Age?
Children's bodies change composition as they grow, so adult BMI cut-offs do not apply. The CDC instead plots each child's BMI against the national distribution for their exact age and sex — the BMI-for-age percentile.
This calculator implements the official CDC LMS method using the published growth-chart parameters (bmiagerev data, ages 2–20), converting BMI to a z-score and then to a percentile with the normal cumulative distribution.
How It Is Calculated
Z = ((BMI ÷ M)^L − 1) ÷ (L × S)
Percentile = Φ(Z) — normal cumulative distribution
Example: Example: 10-year-old boy, 32 kg, 138 cm → BMI 16.8 → ≈ 58th percentile (healthy)
CDC weight categories (ages 2–20)
| Percentile | Category |
|---|---|
| < 5th | Underweight |
| 5th – 84th | Healthy weight |
| 85th – 94th | Overweight |
| ≥ 95th | Obesity |
Limitations
- The LMS parameters are the official CDC values; clinical judgment still decides individual cases.
- Growth charts differ by country — WHO standards apply under age 2.
- One measurement is a point, not a trend; pediatricians track percentiles across visits.
Sources & Review
References used for this calculator’s formulas and thresholds:
BMI-for-Age FAQ
What is a healthy BMI percentile for a child?
Between the 5th and 84th percentiles for age and sex. Above the 85th is overweight; at or above the 95th is obesity.
Why can't children use adult BMI categories?
Because body fat naturally rises and falls with growth — the same BMI is normal at 8 and underweight at 15. Age-and-sex percentiles account for that.
What does the 50th percentile mean?
Exactly the median: half of children the same age and sex have a lower BMI, half higher. It is a population comparison, not a health score by itself.
Which data does this calculator use?
The official CDC LMS parameters (L, M, S per month of age, both sexes) published with the growth charts — the same method clinical software uses.
Important disclaimer: Results are educational estimates, not a substitute for professional medical advice. Discuss your results with a qualified professional before making decisions.