Child & Teen BMI Calculator

Last updated: 2026-05-22

Child and Teen BMI

Child and teen BMI is the body mass index of a child aged 2–19, interpreted against the BMI distribution of other children of the same age and sex rather than against a fixed table. BMI itself is the same quantity used for adults — weight divided by height squared — but for a growing child the healthy range shifts with age, so the raw number is converted into a percentile or Z-score before it can be classified.

Adult BMI uses a single fixed table: under 18.5 is underweight, 25–30 is overweight, and so on. Children grow, and their healthy BMI range moves with them. A 2-year-old with BMI 18 is lean; a 12-year-old with the same BMI is roughly average; an adult at 18 sits below the underweight threshold. The same number means three different things at three ages, which is why a fixed cut-off cannot classify a child.

The clinical approach is to compare the child's BMI not to a fixed cut-off but to other children of the same age and sex. National and international bodies publish reference distributions — "growth references" — that give the median BMI and its spread for every age. The result is expressed two ways:

• Percentile — the share of same-age, same-sex children whose BMI is at or below this value, expressed as a percentage (median = 50%).
• Z-score — how many standard deviations from the median this BMI is. Z = 0 is exactly the median; Z = +2 is roughly the 97.7th percentile; Z = −2 is roughly the 2.3rd percentile.

Growth references

No single global reference exists, so the calculator offers the two in common use:

• WHO — the World Health Organization growth references (Child Growth Standards for 0–5 years, Growth Reference 5–19). Built from an international sample and used as the default in most of the world, and for under-2s everywhere.
• CDC — the 2000 CDC Growth Charts, built from US survey data and the clinical standard used by US pediatricians for ages 2–20.

The two disagree by a few percentile points for the same child: WHO tends to classify more children as overweight and fewer as underweight than CDC. The applicable reference depends on context — CDC in the US, WHO in most other countries.

How the score is calculated

BMI itself is computed the same way as adult BMI:

$\text{BMI} = \frac{\text{weight (kg)}}{\text{height (m)}^2}$

The score is then converted using the LMS method, which models the BMI distribution at each age as a skewed (Box-Cox transformed) distribution with three parameters:

• L — power of the Box-Cox transformation (controls skewness),
• M — the median BMI for that age and sex,
• S — the generalized coefficient of variation.

The Z-score follows from these three values:

$Z = \frac{(\text{BMI}/M)^L - 1}{L \cdot S}$

The percentile is then read from the standard normal cumulative distribution. Both WHO and CDC publish their own L, M, S tables, and the calculator reads the set that matches the selected reference.

Classification

The two references draw their category lines differently — WHO on the Z-score, CDC on the percentile:

CDC does not separate a "severely underweight" band; everything below the 5th percentile is simply underweight.

Worked example

Consider a 9-year-old girl with a BMI of 17, classified against the WHO reference. The WHO median BMI for girls at this age is about 15.5 with a modest positive skew, so a BMI of 17 sits above the median — around the 84th percentile, or a Z-score of roughly +1. That places her at the boundary between "healthy weight" and "overweight." The same BMI of 17 in a 6-year-old would land near the 90th percentile, and in a 14-year-old near the 25th — the identical number moves across categories purely because the reference median changes with age.

Z-score and percentile

Percentiles are easier to interpret at a glance. Roughly: Z = +1 ≈ 84th percentile, Z = +2 ≈ 98th, Z = −1 ≈ 16th, Z = −2 ≈ 2nd. Pediatric papers often use Z-scores because they remain interpretable above the 99th and below the 1st percentile, where percentile values compress toward 100% and 0% and lose resolution.

Trajectory over single readings

Pediatric weight assessment is about trajectory, not a single point. A growth chart plots BMI percentile across visits: a child consistently at the 75th percentile is healthy, while a child crossing two major percentile bands (for example 50th → 90th) over a year warrants attention even if the absolute number still lands in "healthy weight." Single readings are noisy — height changes by season, and recent meals affect weight by 1–2 kg. School screening programs reflect this by using BMI percentile as a first-line flag: crossing into "overweight" (above the 85th percentile in CDC, Z > +1 in WHO) typically triggers a follow-up letter, not a diagnosis.

Limits

• It is a screening tool, not a diagnosis. A high BMI in a muscular child athlete is as misleading as in an adult bodybuilder, and a normal BMI does not rule out under- or over-nutrition.
• Reference resolution varies. The CDC tables are monthly; the WHO 0–5 and 5–19 references are tabulated at coarser intervals here and interpolated. For a borderline result near a category line, the category is approximate.
• The references don't perfectly align. WHO and CDC come from different populations, and even the WHO 0–5 and 5–19 references show a small discontinuity at age 5. The reference a child's care is anchored to is the one to stay with.
• Single point versus trend. Pediatric weight assessment is fundamentally about trajectory. One reading is informational; a series across years is the clinical signal.

For any concern about a child's growth, the trajectory — not a single reading — should be discussed with a pediatrician or school health professional. A young person reviewing their own result should treat one number as a snapshot, not a verdict, and raise any surprise with a parent, school nurse, or doctor rather than drawing conclusions alone.