How do I calculate a child’s BMI?

BMI = weight (kg) ÷ height² (m), as in adults. But a child’s result can’t be judged on the adult 25/30 scale — it’s compared with the norm for age and sex via WHO percentile and z-score. The calculator does this automatically.

What BMI is normal for a child?

Normal is a BMI-for-age roughly between the 3rd and 85th percentile (z from −2 to +1). Above is overweight, below is underweight. The exact BMI value depends on age and sex.

How does a child’s BMI differ from an adult’s?

In adults thresholds are fixed (normal 18.5–24.9). In children normal fatness changes with age, so it’s the percentile relative to same-sex peers that’s assessed, not the absolute BMI.

From what age can I calculate a child’s BMI?

The WHO reference covers ages from birth to 19 years. The calculator accepts age in years and months and picks the right table automatically.

Is a high child BMI obesity?

Not necessarily. Above the 85th percentile is overweight; obesity is diagnosed above the 97th (under 5 — above the 99.9th) and only with a doctor, considering height, trend and pubertal development.

Child BMI calculator by age

Enter the child’s sex, age (years and months), height and weight — the calculator computes BMI and compares it to the norm for that exact age and sex using WHO standards. In children and teens, BMI is judged not by the adult 25/30 but by percentiles and z-scores, because normal body fatness changes as they grow.

Calculate child BMI by age

Enter the child’s sex, age, height and weight — the result appears instantly.

Child BMI categories (WHO)

Assessment by BMI-for-age (z-score / percentile), not adult thresholds. For children under 5 and 5–19 years the overweight and obesity cut-offs differ slightly; the calculator handles this automatically.

Category	BMI-for-age
Severe underweight	below −3 SD
Underweight (thinness)	−3 to −2 SD
Normal	−2 SD to +1 SD (≈3rd–85th percentile)
Overweight	above +1 SD (under 5 years — above +2 SD)
Obesity	above +2 SD (under 5 years — above +3 SD)

Why children’s BMI is judged by age, not the adult scale

In adults the normal BMI is fixed (18.5–24.9), but in children normal fatness changes with age: an infant’s BMI is higher, it falls by 5–6 years, then rises again. So the same BMI means different things at ages 3 and 13.

WHO compares a child’s BMI with peers of the same sex and age — via percentile and z-score. The calculator uses WHO reference tables (LMS method) and shows where your child falls.

How to read percentile and z-score

The percentile shows how many peers have a lower BMI than your child: the 50th is exactly the middle, the 85th is higher than 85% of children. Normal is roughly between the 3rd and 85th percentile (z from −2 to +1).

Above the 85th percentile (z > +1) is overweight, above the 97th (z > +2) is obesity; below the 3rd (z < −2) is underweight. For children under 5 the overweight and obesity cut-offs are higher (+2 and +3 SD).

When to see a doctor

A single measurement isn’t a diagnosis: children’s weight and height come in waves, and the trend along the growth curve matters more than one point. Rapid crossing of percentile bands, persistent obesity or marked underweight should prompt attention.

A pediatrician makes the assessment, looking at BMI together with height, puberty stage, diet and family history. The calculator is a guide for that conversation, not a replacement for an exam.

Frequently asked questions

BMI = weight (kg) ÷ height² (m), as in adults. But a child’s result can’t be judged on the adult 25/30 scale — it’s compared with the norm for age and sex via WHO percentile and z-score. The calculator does this automatically.
Normal is a BMI-for-age roughly between the 3rd and 85th percentile (z from −2 to +1). Above is overweight, below is underweight. The exact BMI value depends on age and sex.
In adults thresholds are fixed (normal 18.5–24.9). In children normal fatness changes with age, so it’s the percentile relative to same-sex peers that’s assessed, not the absolute BMI.
The WHO reference covers ages from birth to 19 years. The calculator accepts age in years and months and picks the right table automatically.
Not necessarily. Above the 85th percentile is overweight; obesity is diagnosed above the 97th (under 5 — above the 99.9th) and only with a doctor, considering height, trend and pubertal development.

Decode your child’s tests

Upload your child’s blood test — AI checks the values against age-specific pediatric norms and explains any deviations clearly.

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This calculator is for reference and information only and is not a diagnosis. A child’s weight and development are assessed by a pediatrician from the growth trend, not a single measurement.