Health

Child BMI Calculator

Calculate a child's Body Mass Index from height and weight. Learn why children's weight status is read from age- and sex-specific CDC BMI percentiles.

Child BMI Calculator

Age (years)

Sex

Height (cm)

Weight (kg)

Method

How this calculator works

BMI = weight (kg) ÷ height (m)². For children, this BMI number is interpreted using age- and sex-specific CDC percentiles rather than the fixed adult categories.

Enter the child's age in years and select their sex.
Enter height in centimetres and weight in kilograms.
The tool converts height to metres and computes BMI = weight ÷ height².
Take the BMI number and compare it against a CDC age- and sex-specific percentile chart, or ask your pediatrician.

Examples

Worked examples

Real numbers, end-to-end results.

Weight 30 kg, height 130 cm

BMI = 17.8

1.30 m squared is 1.69; 30 ÷ 1.69 ≈ 17.8. This number must still be plotted on a growth chart for the child's age and sex.

Weight 22 kg, height 118 cm

BMI = 15.8

A lower BMI is common in younger children; interpretation depends entirely on age and sex.

Weight 55 kg, height 150 cm

BMI = 24.4

Whether this is a healthy weight depends on the child's percentile, not the adult cut-off of 25.

Use cases

When to use it

Getting a quick BMI figure before a pediatric visit
Understanding the number your doctor plots on a growth chart
Tracking a child's BMI trend over time alongside a percentile chart
Teaching older children and teens about healthy growth

Disclaimer: This calculator provides estimates for informational and educational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider for decisions affecting your health. Results depend on the accuracy of inputs provided.

FAQ

Frequently asked questions

How is a child's BMI calculated?

The formula is the same as for adults: BMI = weight in kilograms ÷ (height in metres)². For example, a child weighing 30 kg and 1.3 m tall has a BMI of 30 ÷ (1.3 × 1.3) ≈ 17.8.

Why can't I use adult BMI categories for a child?

Children's body composition changes rapidly with age and differs by sex. A BMI that is healthy for a 5-year-old may be overweight for a 12-year-old, so pediatric weight status uses age- and sex-specific percentiles, not the fixed adult cut-offs of 18.5, 25, and 30.

What is a BMI percentile?

A BMI percentile compares a child's BMI to a reference population of children of the same age and sex. For example, a BMI in the 60th percentile means the child's BMI is higher than 60% of children of the same age and sex on the CDC growth charts.

What percentile ranges are used for children?

The CDC generally defines under the 5th percentile as underweight, the 5th to under 85th as a healthy weight, the 85th to under 95th as overweight, and the 95th percentile or above as obesity. Always confirm with your pediatrician.

Does this tool give my child's percentile?

No. This tool computes the raw BMI number only. Determining the percentile requires the official CDC (or WHO) growth reference for your child's exact age and sex, so plot the BMI on a chart or ask your pediatrician.