Why is child BMI shown as a percentile instead of a number?

Children's body composition changes with age and differs by sex, so a raw BMI is not meaningful on its own. The BMI-for-age percentile compares a child to others of the same age and sex using CDC reference data.

What growth-chart data does it use?

It uses the CDC 2000 growth-chart LMS parameters (L = power, M = median, S = coefficient of variation) for ages 2 to 20, interpolating between whole years.

How is the percentile actually calculated?

BMI is converted to a z-score with the LMS formula z = ((BMI/M)^L − 1) / (L·S), then the standard normal distribution converts that z-score to a percentile.

What do the categories mean?

CDC defines below the 5th percentile as underweight, 5th to under 85th as a healthy weight, 85th to under 95th as overweight, and 95th percentile or above as obesity.

How is BMI itself computed in imperial units?

Metric BMI is weight(kg) ÷ height(m)². Imperial BMI uses 703 × weight(lb) ÷ height(in)², which the tool applies automatically when you pick imperial units.

Is this suitable for adults?

No. This calculator uses pediatric reference data for ages 2 to 20. Adults should use a standard BMI calculator instead.

No. All calculation runs privately in your browser; nothing is sent anywhere.

BMI-for-Age Percentile Calculator

BMI-for-age percentile calculator

Enter a child’s sex, age, height and weight to calculate their BMI, BMI-for-age percentile and CDC weight category. The percentile compares the child to others of the same age and sex, which is the correct way to read BMI for anyone aged 2–20.

How it works

The tool computes BMI (metric: weight ÷ height²; imperial: 703 × lb ÷ in²), then looks up the CDC 2000 LMS parameters for the child’s sex and age (interpolating between whole years). It turns BMI into a z-score:

z = ((BMI / M)^L − 1) / (L × S)

and converts that z-score to a percentile using the standard normal distribution. The percentile maps to a category band.

Example

A 10-year-old boy, 138 cm and 32 kg:

BMI = 32 ÷ 1.38² ≈ 16.8 kg/m²
Using the boys’ age-10 LMS values, this lands near the 50th percentile
Category: Healthy weight

Percentile	CDC category
< 5th	Underweight
5th–84th	Healthy weight
85th–94th	Overweight
≥ 95th	Obesity

This is an estimate for general guidance, not a diagnosis — discuss concerns with a pediatrician. All calculation runs privately in your browser.