BMI Calculator for Kids
Enter your child's age, height and weight to get their BMI number. Because children grow, the number is read against growth charts, and we explain how.
This calculator gives general estimates for information only. It is not medical advice and does not replace a consultation with a doctor or other qualified health professional.
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The formula
BMI = weight (kg) / height (m)². The result is then compared with a growth chart for the child's age and sex.
Why adult categories do not apply to children
A healthy BMI changes a lot as a child grows. A BMI of 17 is perfectly normal for a 10 year old but low for an adult. So instead of fixed cut-offs, paediatricians place the child on a percentile chart for their age and sex.
How the percentile is read
The CDC and WHO growth charts are the usual references. In the CDC chart a child is generally considered to be at a healthy weight between the 5th and 85th percentile, overweight from the 85th, and having obesity from the 95th. A child below the 5th percentile is underweight.
We calculate the BMI for you here. To find the percentile, look the number up on the chart or ask your child's doctor, who can also look at the pattern over time, which matters more than any single reading.
Tips for parents
- One measurement tells you little. A steady growth curve is more reassuring than any number.
- Measure height without shoes, and weight in light clothes.
- Do not put children on diets. Speak to your doctor if you are concerned.
Examples to check
Type these into the calculator above to see how it behaves. Every result below was produced by the same calculator code that runs on this page.
| What you enter | What you get |
|---|---|
| Age (years): 8; Height (cm): 128; Weight (kg): 26 | BMI: 15.9 |
| Age (years): 12; Height (cm): 150; Weight (kg): 40 | BMI: 17.8 |
| Age (years): 16; Height (cm): 168; Weight (kg): 55 | BMI: 19.5 |
BMI Calculator for Kids: frequently asked questions
Why does the calculator not tell me if my child is overweight?
Because that depends on a percentile for their exact age and sex, which needs growth chart data. A wrong label would be worse than none, so we give the BMI and point you to the chart and your doctor.
Is BMI accurate for everyone?
No. BMI only compares weight with height. It cannot tell muscle from fat, so very muscular people can score as overweight, and older adults who have lost muscle can score as healthy while carrying extra fat. Use it as a starting point, not a verdict.
What is a healthy BMI range?
For adults, the WHO range for a healthy weight is 18.5 to 24.9. Some groups, including many people of South Asian descent, are advised to use lower cut-offs because health risks rise at lower BMI values.
Can I rely on this result as medical advice?
No. It is a general estimate for information only. It cannot replace a check-up, and your doctor can interpret the numbers for your own situation.
Is what I enter in this calculator saved or sent anywhere?
No. The calculation runs in your browser, so what you enter is not sent to our servers. We only count, anonymously, whether a tool ran successfully.
Is this calculator free to use?
Yes. It is free, needs no sign-up and has no usage limit.
Last reviewed: October 3, 2026