Teenagers grow and develop at very different rates, so BMI is interpreted as a percentile rather than with adult categories. Puberty changes body composition, and girls naturally gain more fat than boys.
How to use the BMI Calculator for Teens
- Sex
- Age
- Weight
- Height
Results update as you type, and every field accepts the units shown in its menu.
How it works
Children’s body fat changes with age and differs between boys and girls, so BMI is compared with children of the same age and sex. The calculator works out BMI, looks up the CDC 2000 reference values (L, M and S) for the exact age in months, converts BMI into a z-score and then into a percentile. It also shows the weights that correspond to the 5th and 85th percentiles at the child’s height.
BMI-for-age weight status
| BMI-for-age percentile | Weight status (CDC) |
|---|---|
| Below the 5th | Underweight |
| 5th to below 85th | Healthy weight |
| 85th to below 95th | Overweight |
| 95th and above | Obesity |
Inputs explained
| Field | What to enter |
|---|---|
| Sex | Choose one: Male, Female |
| Age | Units: years, months |
| Weight | Units: kg, lb, st, g |
| Height | Units: cm, in, m, ft, mm |
What each result means
- BMI = [Weight]÷([Height]÷100)^2
- BMI Prime = [BMI]÷25
- BMI-for-age percentile = ncdf([BMI-for-age z-score])×100
Health note: this calculator is for education and screening only and is not a diagnosis or treatment advice. Discuss your results with a qualified clinician.
Formula
BMI = weight (kg) ÷ height (m)²
z = ((BMI ÷ M)^L − 1) ÷ (L × S) (CDC LMS values for age and sex)
Percentile = Φ(z) × 100
Worked example
Inputs — Sex: Male; Age: 15 years; Weight: 55 kg; Height: 165 cm.
Results — BMI = 20.2 kg/m²; BMI Prime = 0.81; BMI-for-age z-score = 0.12; BMI-for-age percentile = 55 %; Percentile group = 50th–75th; Weight status (CDC) = Healthy weight; BMI at the 50th percentile = 19.9; Healthy weight from (5th percentile) = 45.1 kg; Healthy weight to (85th percentile) = 63.8 kg.
Key terms
- Percentile
- The percentage of children of the same age and sex with a lower BMI.
- LMS method
- A way of describing growth charts with three numbers: skewness (L), median (M) and spread (S).
- z-score
- Number of standard deviations from the median.
Frequently asked questions
When can teens use adult BMI categories?
From age 20; before that use BMI-for-age percentiles.
Is a high percentile always a problem?
Very muscular teens may have high BMI with normal body fat; a doctor can assess this.
How often should BMI be checked?
Usually once a year at routine check-ups.