Calculating BMI for children involves three steps: calculating the BMI value, converting it to a z-score using CDC growth data, and then determining the percentile.
Step 1: Calculate BMI
First, calculate BMI using the standard formula:
For imperial units:
Step 2: Calculate Z-Score
Next, convert the BMI to a z-score using the LMS (Lambda-Mu-Sigma) method. The CDC provides age and sex-specific L, M, and S parameters for this calculation.
The z-score formula is:
Where:
- L = Box-Cox transformation power (skewness)
- M = Median BMI for the age and sex
- S = Coefficient of variation
Step 3: Convert to Percentile
Finally, the z-score is converted to a percentile using the standard normal distribution. A z-score of 0 equals the 50th percentile, meaning the child's BMI equals the median for their age and sex.
CDC Weight Status Categories
The CDC classifies children's weight status based on these percentile ranges:
| Percentile Range | Weight Status |
|---|---|
| Less than 5th percentile | Underweight |
| 5th to 84th percentile | Healthy Weight |
| 85th to 94th percentile | Overweight |
| 95th percentile or greater | Obese |
Example Calculation
A 10-year-old boy with a BMI of 18.5 would have approximately a 75th percentile ranking, meaning his BMI is higher than 75% of boys his age. This falls within the healthy weight range.
Ready to calculate? Try our BMI Calculator for Children.