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Why is BMI interpreted differently in children?
In adults, BMI (Body Mass Index) is assessed against fixed cut-offs: under 18.5 is underweight, over 25 is overweight, over 30 is obese. In children these cut-offs don't work, because the body keeps changing as it grows; a healthy 6-year-old naturally has a lower BMI than a healthy 16-year-old. That's why, in children and teens, BMI is compared with children of the same age and sex and converted into a percentile. The percentile tells you where the child ranks among 100 peers: the 50th percentile is exactly average, below the 5th is underweight, above the 85th is overweight, and above the 95th is considered obese (CDC classification).
How is the percentile calculated?
First, BMI is found with the classic formula: weight (kg) ÷ height² (m²). Then, for the child's age and sex, three reference values are pulled from the WHO/CDC table — M (the median, i.e. the middle BMI for children of that age), S (how spread out the values are) and L (a number that corrects the skew of the distribution). These three values are combined using the internationally accepted LMS method (Cole 1990 — the standard statistical method for growth-curve calculations) to produce first a z-score (a figure showing how far the child sits from the average), and from that the percentile:
z-score = ((BMI ÷ M)L − 1) ÷ (L × S)
percentile = the z-score's position on the bell curve × 100
(the M, S, L values come from the table for the child's age and sex)
Example: a 10-year-old boy (120 months), 138 cm, 32 kg. BMI = 32 ÷ 1.38² = 16.8 kg/m². On the WHO table for 10-year-old boys, this corresponds to roughly the 58th percentile (z ≈ +0.2) — a little above his peers but within the healthy range. If his weight were 31 kg, the BMI of 16.4 would land exactly on the 50th percentile (the median).
Data: World Health Organization (WHO) 2007 Growth Reference (ages 5–19) and U.S. Centers for Disease Control and Prevention (CDC) 2000 Growth Charts (ages 2–20) — the growth tables officially published by these organizations. The calculation has been validated against CDC's own published percentile columns and matches them exactly (difference < 0.0001) — so the results aren't approximations, they're identical to the official tables.
Should I choose WHO or CDC?
Both references are valid; knowing the differences makes the right choice easier:
- WHO — World Health Organization (ages 5–19): An international reference used in the official guidelines of many countries, including Turkey. The first choice in most cases.
- CDC — U.S. Centers for Disease Control and Prevention (ages 2–20): Based on U.S. data and also covers ages 2–5; it's the only option for children under 5.
- Stay consistent: When tracking a child over time, always use the same reference; results differ slightly between the two.
Frequently asked questions
The healthy range is between the 5th and 85th percentile (on the CDC reference). Below the 5th is underweight, 85–95 is overweight, and above the 95th is considered obese. On the WHO reference, classification is based on the z-score: between −2 and +1 is healthy. Rather than a single number, what matters is that the child follows their own curve steadily over time.
Both convey the same information in different ways. The <strong>z-score</strong> is a figure showing how far the child is from the average: 0 is exactly average, positive values are above average, negative values are below. The <strong>percentile</strong> turns this into a more intuitive 0–100 scale: a z-score of 0 → the 50th percentile, a z-score of about +1.6 → the 95th percentile. For everyday use the percentile is easier to grasp; at the extremes the z-score gives a more precise measure.
The WHO reference covers ages 5–19 and the CDC reference covers ages 2–20. For children aged 2–5, choose CDC. BMI isn't used for infants under 2; instead, weight-for-age and length/height-for-age curves are assessed — see your doctor for that.
Don't panic; a single measurement isn't a diagnosis. If the percentile is in the 85–95 range or above, talk to your family doctor or pediatrician. In most cases the answer isn't medication but a balanced diet, cutting back on sugary drinks and regular physical activity. Because strict diets can harm children's growth, always proceed under professional guidance.
Small fluctuations are normal; children don't grow in a straight line but in spurts, and measurement conditions (clothing, time of day, the scale) create small differences too. What matters isn't any single value but that the child follows their <b>own curve</b> steadily over time — that is, stays in roughly the same percentile band. The real thing to watch for is a <b>sudden, large jump or drop</b> in percentile (for example, going from 50 to 85 in a few months); even if the value is within the normal range, that kind of abrupt change should be reviewed with a doctor.
Most of the time no — adult-style restrictive diets are usually wrong for children and can harm growth and development. A growing child often "grows into" the healthy range not by losing weight, but by holding their weight steady while <b>getting taller</b>. The goal is usually habits rather than the number on the scale: cutting back on sugary drinks, adding more vegetables and fruit to the plate, reducing screen time and moving more, and doing it as a family. Any restriction or special program should always be set by a pediatrician or a dietitian experienced in child nutrition.