Child BMI Percentile Calculator: BMI-for-Age from WHO

How to use it

Choose the Sex, then enter the Date of birth and the Measurement date, or leave the date empty for today.
Enter the Weight in kilograms and the Height in centimetres.
Choose how the height was measured in How was it measured?: standing, or lying down for a length.
Read the BMI, the BMI-for-age z-score and the WHO category, then find the band in the WHO classes table.

How the BMI-for-age percentile is calculated

The body mass index is the weight in kilograms divided by the height in metres squared. For a child it is compared with the BMI of other children of the same age and sex, using the WHO LMS tables: the z-score is the number of standard deviations from the WHO median, and the percentile is the share of children below it.

The WHO Child Growth Standards of 2006 give the BMI-for-age for 2 to under 5 years, read by each day of age. The WHO Growth Reference of 2007 gives it for 5 to 19 years, read by month. WHO gives percentiles only between −3 and +3 SD.

This is not medical advice. A BMI percentile does not show body fat or muscle, and a single measurement says less than the change over several visits. Ask your child's doctor or paediatrician whether a result needs a check.

BMI = weight (kg) ÷ height (m)²
z = ((BMI ÷ M)^L − 1) ÷ (L × S), with L, M and S from the WHO table for the age and sex

The WHO classes from 5 to 19 years

The WHO page for BMI-for-age from 5 to 19 years states its cut-offs: thinness below −2 SD, severe thinness below −3 SD, overweight above +1 SD and obesity above +2 SD. WHO gives these equal to a BMI of about 25 kg/m² for overweight and 30 kg/m² for obesity at 19 years.

Between −2 SD and +1 SD the tool shows the normal range. From 2 to under 5 years the WHO standards give z-score bands only, so this page shows the bands there and names no class.

What this calculator does not include

Body composition and muscle: BMI cannot tell fat from muscle, so an athletic child can have a high BMI for the age.
Height growth, puberty and family build, which change the expected BMI at a given age.
Ages below 2 years or above 19 years; use the child growth percentile calculator for a younger child.

Worked example: boy aged 10 years, 34.0 kg, 140.0 cm standing

From 10 November 2016 to 10 November 2026 is 3,652 days: 10 years. The boy is 140.0 cm standing, so the height is read as it is.

BMI = 34.0 ÷ (1.40 × 1.40) = 34.0 ÷ 1.96 = 17.3. Against the WHO reference for boys of this age, the BMI-for-age z-score is 0.48 and the percentile 68.6. That is in the normal range, between −2 and +1 SD.

Step	Result
BMI	17.3 kg/m²
BMI-for-age z-score	0.48
BMI-for-age percentile	68.6
WHO class	Normal

Frequently asked questions

Does a BMI percentile show whether my child has too much fat?

Not on its own. BMI compares weight with height for the age and sex, and it cannot tell fat from muscle. A high or low result is a reason to ask a doctor to look at growth over time.

What is a z-score?

It is the number of standard deviations from the WHO median for the same age and sex. Zero is the median, and WHO uses minus 2 and plus 2 as lines for a closer look, not as a diagnosis.

Why are there no named classes under 5 years?

The WHO standards page for BMI-for-age from birth to 5 years states no cut-offs with names, so this tool shows the z-score bands there. From 5 years the WHO reference page does name its classes, and the tool uses them.

Why does the tool ask whether my child was measured lying down?

WHO uses length, lying down, up to 2 years and height, standing, from 2 years. If the measure was taken the other way, WHO adds or takes off 0.7 cm before it works out the BMI, and the tool does the same.

My child is under 2 years old. Can I use this page?

No. This page covers 2 to 19 years. For a younger child, the child growth percentile calculator gives the BMI-for-age with the other WHO measures.

Are the figures I enter sent to a server?

No. The calculation runs in your browser and nothing you enter is sent to us.

Sources

Figures and text last checked on Tuesday, November 10, 2026.

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