WiseCalcs

BMI calculator for children

Enter the child's weight, height, age, and sex. The calculator gives the BMI and a rough healthy weight range. For children, BMI is interpreted against age- and sex-specific charts, so use this alongside official growth charts.

Use it to find a child's BMI — then read it against a percentile chart, not adult categories.

kg
cm

BMI

17.8

Rough healthy weight
23.7–32.1 kg

The result updates as you type. The headline is the BMI; the second figure is a rough healthy weight range for the height, not a diagnosis.

How does it work?

For children, BMI is read against age- and sex-specific percentile charts, not the fixed adult categories. The healthy range shown is a rough guide only — use official growth charts and a clinician for assessment.

Child BMI formula

BMI=weight (kg)height (m)2\text{BMI} = \frac{\text{weight (kg)}}{\text{height (m)}^2}
BMI
Body mass index in kg/m².
weight
Child's weight in kilograms.
height
Child's height in metres.

A 30 kg child who is 130 cm tall has a BMI of 30 / 1.3² ≈ 17.8.

Method & sources

BMI is weight in kilograms divided by height in metres squared. For children, BMI is interpreted with age- and sex-specific percentile charts. The healthy weight range uses a rough school-age BMI band (14–19).

Sources

Where this method comes from — use these references to understand the formula, assumptions, and limits.

How we calculate

  • BMI is weight in kilograms divided by height in metres squared.
  • For children, BMI is interpreted with age- and sex-specific percentile charts.
  • The healthy weight range uses a rough school-age BMI band (14–19).
  • It is a guide only — use official growth charts and a clinician.

Rounding

BMI and weights are shown to one decimal. The calculation uses full precision.

What this calculator does

BMI for a child is calculated the same way as for an adult — weight divided by height squared — but interpreted very differently. Children's healthy BMI changes with age and differs by sex, so the result is read against percentile charts rather than fixed categories.

How to use it

  1. Enter the child's weight in kilograms.
  2. Enter the child's height in centimetres.
  3. Enter the age and sex.
  4. Read the BMI and take it to a growth chart.

A worked example

A 30 kg child who is 130 cm tall has a BMI of 30 / 1.3² ≈ 17.8. Whether that is healthy depends on the child's age and sex on a percentile chart.

Why percentiles matter

A BMI that is healthy at age 12 may be high at age 6. Charts place a child's BMI as a percentile against peers of the same age and sex — for example, above the 85th is often flagged as overweight. The fixed adult cut-offs do not apply.

Common mistakes

  • Applying adult BMI categories to a child.
  • Entering height in metres instead of centimetres.
  • Treating the rough range here as a clinical assessment.

When it's useful

As a quick BMI figure to take to a growth chart or a check-up — never as a standalone diagnosis.

FAQ

How is a child's BMI calculated?
The same as for adults: weight in kilograms divided by height in metres squared. The interpretation is what differs.
Why do age and sex matter?
Children's body composition changes as they grow and differs by sex, so a healthy BMI is defined by percentile charts for each age and sex.
Can I use adult BMI categories?
No. Adult cut-offs like 25 for overweight don't apply to children. Use age- and sex-specific percentile charts.
What does the healthy range here mean?
It's a rough guide from a generic child BMI band, not a percentile result. Confirm with an official growth chart.
What age is this for?
Roughly ages 2 to 19. From 18 onward you can also use the adult BMI calculator.
Can I share a calculation?
Yes. Use Share to copy a link that reopens the calculator with the same details.

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<iframe src="https://wisecalcs.com/embed/en/bmi-calculator-for-children" width="100%" height="520" style="border:0" loading="lazy"></iframe> <p>Calculator from <a href="https://wisecalcs.com/en/health/bmi-calculator-for-children">WiseCalcs</a></p>