Why children's BMI is a percentile, not a number with cut-offs
For adults, BMI is read against fixed cut-offs — 18.5, 25, 30 — because adult bodies have finished growing. Children have not. A healthy seven-year-old and a healthy fifteen-year-old have completely different typical BMIs, and the typical value changes month by month and differs between boys and girls, so a child's BMI only means something compared with other children of the same age and sex. That comparison is the BMI-for-age percentile: if a child is at the 70th percentile, their BMI is higher than about 70 percent of the reference population of the same age and sex. Adult BMI cut-offs do not apply to children, and using them would mislabel most healthy children at some age or other. For the adult calculation itself, see the BMI Calculator. This page is general information only and is not medical advice.
The CDC 2000 LMS method this calculator uses
The percentiles here use the CDC 2000 BMI-for-age LMS parameters — public data published by the U.S. CDC / NCHS — for ages 2 through 19. At each month of age, separately for boys and girls, the CDC publishes three numbers: L (the power that makes the distribution symmetric), M (the median BMI) and S (the spread). This calculator looks up the row for your child's age in completed months, linearly interpolating between the neighbouring month rows when needed, converts the BMI to a standard z-score with the formula above, and turns the z-score into a percentile with the standard normal curve. It is the same method behind the growth charts used in paediatric offices across the United States.
The four categories
The CDC categories are defined by percentile: below the 5th percentile is classed underweight, the 5th to below the 85th is healthy weight, the 85th to below the 95th is overweight, and the 95th and above is obesity. Two honesty notes belong with those labels. First, a percentile is a screening result, not a health verdict: a muscular young athlete, a child mid-growth-spurt, or an early or late developer can sit outside the healthy band while being perfectly well, which is why professionals track the trajectory across visits rather than reacting to one point. Second, percentiles at or above the 95th are approximate under the classic LMS method: the CDC's classic curves compress at the top end, so a child at the 97th and a child at the 99.9th can have very different BMIs that the percentile scale separates poorly — one reason the CDC later published extended charts for high BMIs. Near the top, the BMI value itself and its trend carry more information than the exact percentile.
Measuring children so the percentile is fair
Percentiles are sensitive to measurement error, because the reference children were measured carefully. Weigh on a calibrated scale in light clothing without shoes, and measure height standing straight against a wall or stadiometer, heels together, head level — a centimetre of slouch or hairstyle can shift a young child's BMI by several tenths and their percentile by several points. Age matters just as much: enter years and extra months, since at age four the typical BMI changes noticeably across a single year. If a result surprises you, re-measure before worrying, then raise it at the next routine visit — growth is a long game, and single readings, honestly taken, are starting points for a conversation with your child's doctor, never reasons for diets or pressure at home. This calculator is not medical advice and does not diagnose any condition.
Worked example
A boy, 10 years 0 months old, 35 kg and 140 cm: BMI = 35 ÷ 1.40² = 35 ÷ 1.96 = 17.86. Against the CDC 2000 reference for a boy of 120 months (median BMI about 16.65), that BMI gives a z-score of about 0.53, which is roughly the 70th percentile — squarely in the Healthy weight band (5th to below the 85th percentile). The same BMI in a six-year-old or a fourteen-year-old would land on a very different percentile, which is the whole point of the age adjustment.
How to use this calculator
- Select boy or girl — the CDC reference tables are separate for each.
- Enter the child's age as years plus any extra completed months, as accurately as you can.
- Enter weight in kilograms and height in centimetres, measured without shoes.
- Read the BMI percentile and category in the headline panel and on the gauge.
- If the result is near a band edge or surprises you, re-measure carefully, and discuss persistent results with your child's doctor.
What to do with a high or low percentile
Start with what not to do: do not put a growing child on an adult-style weight-loss diet on the strength of a calculator. For children outside the healthy band, paediatric guidance usually focuses on the growth trajectory — often the goal is for weight to grow more slowly while height catches up, so the percentile drifts down naturally — supported by family-level habits around sleep, activity, sugary drinks and regular meals rather than restriction aimed at the child. A low percentile has its own range of ordinary explanations, from family build to a recent illness to simply being measured just before a growth spurt, alongside causes a doctor should rule out if the trend keeps falling.
In both directions, the trend is the signal and the single reading is not. Clinicians plot BMI on the growth chart at every visit precisely so that a pattern — steady tracking along a curve, or crossing curves up or down over a year — can be seen at a glance. If you use this page at home, imitate that: record the date, age in months, BMI and percentile every few months, and bring the series, not a single number, to appointments. This is general information only, not medical advice.
Important note
For information only — not medical advice. This calculator uses standard published formulas to estimate values for general education and planning. It does not diagnose, treat, or replace the advice of a doctor or other qualified health professional. If you have a health concern, are pregnant, or are making decisions about treatment, medication, or a major diet or exercise change, please talk to a healthcare professional first.