Children’s BMI naturally falls in the preschool years and rises again later in childhood, so the adult categories do not apply. Instead, a child is compared with children of the same age and sex: below the 5th percentile is underweight, 85th–95th is overweight and the 95th or above is obesity.
How to use the BMI Calculator for Kids
- 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: 6 years; Weight: 30 kg; Height: 135 cm.
Results — BMI = 16.5 kg/m²; BMI Prime = 0.66; BMI-for-age z-score = 0.73; BMI-for-age percentile = 77 %; Percentile group = 75th–85th; Weight status (CDC) = Healthy weight; BMI at the 50th percentile = 15.4; Healthy weight from (5th percentile) = 25 kg; Healthy weight to (85th percentile) = 31 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
What is a healthy BMI for a child?
Any BMI between the 5th and 85th percentile for age and sex.
Why does my child's BMI percentile differ from their weight percentile?
BMI accounts for height, so a tall child can be heavy for age but healthy for height.
Which growth charts are used?
The CDC 2000 BMI-for-age charts for ages 2 to 20.