BMI for Children and Teens: Why Percentiles Replace Cutoffs

Why adult cut-offs do not work for children

Body proportions change constantly during growth. BMI typically rises during the first year of life, falls to a minimum at around five or six years of age, and then climbs again through childhood and adolescence. The upturn in early childhood is sometimes called the adiposity rebound.

Boys and girls also grow at different rates and reach puberty at different ages, so their typical BMI at the same age is not the same. A single threshold for everyone would misclassify many healthy children at some ages and miss real concerns at others.

How a BMI percentile works

A percentile shows where a child's BMI sits among a reference group of children of the same age and sex. If a child is at the 60th percentile, their BMI is higher than that of 60 out of 100 reference children, and lower than that of the other 40. The 50th percentile is the middle of the group.

A percentile is a ranking, not a grade. Being at the 30th percentile is not worse than being at the 70th; both are within the wide range that includes most healthy children.

You cannot work out a percentile with a short formula. It is read from sex-specific growth charts, or calculated by software from published reference tables, using the child's exact age in months. That is why a clinic or an online tool is needed for the last step.

The standard bands

For ages 2 to 19, the US Centers for Disease Control and Prevention (CDC) uses the following BMI-for-age categories. Other countries and organisations use similar ideas with their own reference data, such as z-scores based on World Health Organization growth references, so the exact lines can differ.

These categories are screening groups. They flag children whose BMI is unusual for their age so that a health professional can look further. They do not by themselves diagnose a problem. For children under 2, doctors use weight-for-length charts rather than BMI.

Worked example

A child weighs 30 kg and is 1.35 m tall. Height squared is 1.35 × 1.35 = 1.8225, and 30 ÷ 1.8225 = 16.46, so the BMI is 16.5.

To judge that number you also need the child's age and sex. The same BMI of 16.5 can sit at a higher percentile for a younger child than for an older one, because the reference values generally rise with age after about the age of six. The calculation is the easy part; the percentile lookup is what carries the meaning.

BMI = weight (kg) ÷ height (m)²The same formula as for adults. Only the interpretation differs.

The BMI calculator will do the division, but the categories it shows are the adult ones, so do not apply them to a child.

Reading one result sensibly

• Look at the trend, not one point. A child who stays near the same percentile line over several years is following their own growth curve. A sharp climb or drop across lines is more informative than any single value.
• Height matters too. A growth spurt can temporarily change BMI, and children of the same age can be at very different stages of puberty. Height can be checked against peers with the height percentile calculator.
• Measure carefully. Small errors in height have a large effect, because height is squared. Measure without shoes, standing straight against a wall.
• Avoid diets without guidance. Children are still growing, so changes to eating habits for a child should be guided by a doctor or dietitian rather than by an adult calorie target.

This article is general education and is not medical advice. If you are worried about a child's weight, growth or eating, speak with their doctor or a paediatric health professional.