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Child BMI Calculator

Child BMI percentile for ages 2-19, computed from the published CDC BMI-for-age LMS parameters rather than an approximation. Shows percentile, z-score and the healthy BMI range for that exact age in months.

Last revised 29 September 2026

By Team GlobalCalqulate · About our editorial standards

Child's Details

Age (Years)

Ages 2-19 years.

Plus Months

Extra months beyond the whole year (0-11). A child's reference BMI moves month to month, so this changes the percentile — an 8-year-old and an 8-year-11-month-old are not compared against the same curve.

Gender

Height

Child's height.

Height Unit

Weight

Child's weight.

Weight Unit

Medical Disclaimer: This calculator provides a general educational estimate only. It is not a diagnosis, prescription, or substitute for professional medical advice. Always consult a qualified doctor or healthcare provider to confirm your results and get guidance suited to your specific health situation before making any decisions based on this output. Full disclaimer.

Sources & methodology

Formula:: Step 1 — body mass index
Reference:: Centers for Disease Control and Prevention
Content last updated:: September 2026
Scheduled re-verification:: September 2027

Calculation type: Published clinical formula. Figures are taken from Centers for Disease Control and Prevention and National Center for Health Statistics and American Academy of Pediatrics and World Health Organization — see sources below.

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Formula & methodology

Formula used

Step 1 — body mass index

BMI = weight (kg) / height (m)²

Identical to the adult calculation. Nothing about the arithmetic changes for a child; only the interpretation does.

Step 2 — z-score from the CDC LMS parameters

z = ((BMI / M)^L − 1) / (L × S)

L, M and S are looked up for the child's exact age in months and sex, and interpolated between the half-month rows CDC publishes.

Origin: The LMS method was introduced by Tim Cole in 1990 to summarise a skewed, age-varying distribution with three smooth curves. CDC adopted it for the 2000 growth charts. When L is zero the expression reduces to the lognormal case, z = ln(BMI/M) / S.

L is what handles skew. BMI distributions in children have a long right tail, so a symmetric z-score computed as (BMI − mean)/SD would misplace both ends of the range.

Step 3 — percentile from the z-score

percentile = Φ(z) × 100

Φ is the standard normal cumulative distribution. A z of 0 is the 50th percentile, +1.645 is the 95th.

Step 4 — percent of the 95th percentile

%BMIp95 = BMI / BMI₉₅ × 100

Used instead of the percentile once a child is at or above the 95th, where percentiles stop separating meaningfully different BMIs.

Variable definitions

Symbol	Meaning	Unit
BMI	Body mass index. Weight divided by height squared.	kg/m²
age	Age in months. Whole months since birth. The reference is tabulated every half month, so months matter — not just years.	months
L	Box-Cox power. Corrects the skew of the BMI distribution at that age and sex. Typically negative for BMI-for-age.	—
M	Median. The 50th-percentile BMI at that age and sex.	kg/m²
S	Coefficient of variation. Spread of the distribution relative to the median.	—
z	Standard deviation score. How far the child's BMI sits from the median, in standard deviations, after the skew correction.	SD

Step-by-step calculation

1. Convert age to months
An 8-year-old with no extra months is 96 months. The reference row for 96 months is interpolated from the tabulated 95.5 and 96.5 month values.
2. Compute BMI
28 kg / (1.30 m)² = 16.57 kg/m².
3. Look up L, M and S
For a boy at 96 months: L = -3.191, M = 15.769, S = 0.1017. M is the key number to notice — the median BMI for an 8-year-old boy is under 16.
4. Compute the z-score
((16.57 / 15.769)^-3.191 − 1) / (-3.191 × 0.1017) = 0.45.
5. Convert to a percentile
Φ(0.45) = 0.674, so the 67.4th percentile. About 67 of every 100 boys aged 8 have a lower BMI.
6. Read the category
67.4 falls between the 5th and 85th percentiles, so the CDC category is healthy weight.

Clinical interpretation

Result	What it means
Below the 5th percentile	Underweight. Often a normal slim build, but the band where a clinician would want to see growth plotted over several visits rather than judge a single reading.
5th to 84th percentile	Healthy weight for that age and sex.
85th to 94th percentile	Overweight. In a growing child this frequently resolves as height increases, which is why the trend matters more than the point.
95th percentile and above	Obesity. A screening result indicating that assessment is warranted — not a diagnosis, which requires a clinician.
120% of the 95th percentile, or BMI 35	Class 2 severe obesity, whichever threshold is reached first.
140% of the 95th percentile, or BMI 40	Class 3 severe obesity.

Thresholds: CDC child and teen BMI categories; severe obesity classes per CDC 2022 extended BMI metrics.

These bands are statistical positions in a reference population, not statements about an individual child's health. A child at the 90th percentile who has tracked there since age 3 is in a different situation from one who has moved from the 50th to the 90th in eighteen months, even though both get the same label today.

Who this applies to

Children and adolescents aged 2 through 19
Screening and growth monitoring, alongside a growth chart plotted over time
Children in the United States, or anywhere the CDC reference is the local standard

Who it should not be used for

Infants under 2 — weight-for-length on the WHO standard is the applicable measure, and this calculator declines to answer rather than extrapolating
Adults from age 20, who use the fixed adult cut-offs
Children being assessed against the WHO reference, which will give a different percentile for the same measurements
Children with conditions affecting stature or body composition — including Down syndrome, cerebral palsy, achondroplasia and Turner syndrome — for which condition-specific growth charts exist
Pregnant adolescents

How this is used in practice

Clinicians read the trajectory, not the point: A BMI percentile is plotted on a chart the child already has a history on. Crossing upward through two major percentile lines is the pattern that prompts action, and it can happen entirely inside the healthy band — a move from the 20th to the 75th percentile never triggers a category change but is a clearer signal than a stable 88th.
Why the z-score is shown alongside the percentile: Percentiles are what parents understand; z-scores are what track change. Between the 97th and 99.5th percentiles the percentile barely moves while z rises substantially, so paediatric research and severe-obesity follow-up use z or percent-of-the-95th rather than percentiles.
The adiposity rebound is a real signal: BMI normally reaches its lowest point around ages 5-6 before rising again. When that upturn happens unusually early — before about age 5 — it is associated with higher adult obesity risk, independent of the child's current percentile. It is one of the few things a growth chart reveals that a single BMI cannot.
Screening thresholds are set for population sensitivity: The 85th and 95th percentile cut-offs were chosen so that screening catches most children who would benefit from assessment, accepting that many children flagged will turn out to be healthy. That is a deliberate trade-off, and it means a result in the overweight band is a prompt to look more closely, not a finding in itself.

Common mistakes

Comparing a child's BMI to the adult categories
A 6-year-old boy at the perfectly healthy 50th percentile has a BMI of 15.4, which the adult scale calls severely underweight. The adult numbers are meaningless before age 20.
Entering age in whole years only
The reference median changes every month. Across a single year of a child's life the healthy BMI range shifts enough to move a borderline reading into or out of a category, so the extra months field is not decoration.
Treating the percentile as a school grade
The 60th percentile is not better than the 40th. Both are ordinary positions inside the healthy band. The percentile is a location in a distribution, not a score.
Reading a CDC percentile as if it were a WHO percentile
The two references disagree, particularly in the preschool years and the teens. A child assessed as overweight on one may be healthy weight on the other. Check which chart your clinician uses before comparing results.
Assuming a percentile above 99 conveys how far above the threshold a child is
It does not. Percentiles saturate in the tail. Percent of the 95th percentile is the measure that keeps separating, which is why it is shown once a child reaches the 85th.

Limitations

BMI is computed from height and weight only. It cannot distinguish muscle from fat, so an athletic adolescent can screen as overweight while carrying little body fat.
The CDC reference describes US children measured between 1963 and 1994. It is a record of how children did grow in that period, not a statement of how they should grow, and it is not representative of every population.
Percentiles lose resolution above roughly the 97th, where very different BMIs map to nearly identical values. Percent of the 95th percentile is reported for that reason but is itself a newer metric with a shorter evidence base.
Puberty is not in the model. Two 13-year-olds at different pubertal stages have genuinely different expected body composition, and age and sex alone do not capture that.
Self-measured height and weight carry more error than clinical measurement, and BMI is sensitive to height error because height is squared.
A percentile from a single visit cannot show a trajectory, which is the part clinicians weigh most heavily.

This page is educational. It explains how a published formula works and what its output means in general terms. It does not examine you, cannot account for your medical history, and is not a diagnosis or a treatment recommendation. Decisions about your health should be made with a qualified clinician who can assess your individual circumstances.

Sources & references

Reference dataBMI-for-age LMS parameters, percentile data files — Centers for Disease Control and Prevention, 2000
The L, M and S values this calculator uses are taken verbatim from this file. Its published percentile columns are what the implementation is checked against.
MethodCDC growth charts for the United States: methods and development (Series 11, No. 246) — National Center for Health Statistics, 2002
Documents how the charts were constructed and why the LMS method is used, including the decision to exclude post-1980 weight data above age 6 so the reference would not encode the rising obesity prevalence.
GuidelineDefining child BMI categories and severe obesity — Centers for Disease Control and Prevention
Source of the 5th / 85th / 95th percentile category boundaries and of the percent-of-the-95th-percentile thresholds used here for class 2 and class 3 severe obesity.
GuidelineClinical practice guideline for the evaluation and treatment of children and adolescents with obesity — American Academy of Pediatrics, 2023 doi:10.1542/peds.2022-060640
The current paediatric guideline. It sets out how BMI percentile is meant to be used — as one screening input alongside history and examination — and is the basis for the wording that a result at or above the 95th percentile indicates a conversation, not a diagnosis.
ReferenceWHO Growth reference data for 5-19 years — World Health Organization, 2007
The alternative reference used outside the United States. Cited because the choice of chart changes the percentile, and readers outside the US should know which one their clinician will use.

Help & FAQs

Frequently Asked Questions

Clear answers to common questions to help you use this calculator confidently.

Why can't I just use the adult BMI categories for my child?

Tap to view the answer

Because the adult cut-offs (18.5, 25, 30) are fixed numbers, and a child's healthy BMI changes with age and sex. The median BMI for a healthy 6-year-old boy is around 15.4 -- on the adult scale that reads as severely underweight, which it isn't. That's why this calculator compares your child's BMI to the CDC reference for children of the same age in months and sex, and gives you a percentile instead of a fixed category.

Why does the calculator ask for months, not just years?

Tap to view the answer

Because the CDC reference curve moves every month, not just every year. An 8-year-old and an 8-year-11-month-old are compared against different points on the growth curve, and for a child near a percentile boundary, that difference can change the category. Entering only whole years is one of the more common ways parents get a slightly off result.

What do the percentile bands actually mean?

Tap to view the answer

Below the 5th percentile is classed underweight, 5th to 84th is healthy weight, 85th to 94th is overweight, and 95th and above is the obesity band -- these are the CDC's own category boundaries. A percentile is a position in a reference population, not a grade: the 60th percentile isn't 'better' than the 40th, both are ordinary positions inside the healthy range.

My child's result was 99th percentile or the calculator showed a 'percent of the 95th percentile' number -- why?

Tap to view the answer

Percentiles compress badly at the extreme top of the scale -- a BMI of 30 and a BMI of 38 in the same child can both round to '99th percentile,' which hides how far above the threshold they actually are. Once a child reaches the 85th percentile, this calculator also shows BMI as a percentage of the 95th percentile, which keeps distinguishing severity where the percentile itself stops being useful. This is a screening flag for a conversation with a pediatrician, not a diagnosis on its own.

Does this use the same growth chart my child's doctor uses?

Tap to view the answer

It uses the CDC 2000 BMI-for-age reference, which is the standard in the United States for ages 2 and up. Outside the US -- the UK and much of Europe, for instance -- clinicians more often use the WHO growth reference, which can classify the same child differently, especially in the preschool and teen years. If your provider uses a different chart, expect the percentile to differ somewhat, and ask which reference they're using before comparing numbers.

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Calculator inputs

Child's Details

Your results

Sources & methodology

Formula & methodology

Formula used

Step 1 — body mass index

Step 2 — z-score from the CDC LMS parameters

Step 3 — percentile from the z-score

Step 4 — percent of the 95th percentile

Variable definitions

Step-by-step calculation

Clinical interpretation

Who this applies to

Who it should not be used for

How this is used in practice

Common mistakes

Limitations

Sources & references

Why can't I just use the adult BMI categories for my child?

Why does the calculator ask for months, not just years?

What do the percentile bands actually mean?

My child's result was 99th percentile or the calculator showed a 'percent of the 95th percentile' number -- why?

Does this use the same growth chart my child's doctor uses?

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Calculator inputs

Child's Details

Your results

Sources & methodology

How to Use This Calculator

Worked example: 8-year-old boy, 130 cm, 28 kg

Understanding Your Results

Key Tips

Child BMI and paediatric growth

Why children cannot use adult BMI categories

How the CDC reference actually works

CDC or WHO — which chart applies

Why the 95th percentile is not the end of the scale

What a percentile does not tell you

Formula & methodology

Formula used

Step 1 — body mass index

Step 2 — z-score from the CDC LMS parameters

Step 3 — percentile from the z-score

Step 4 — percent of the 95th percentile

Variable definitions

Step-by-step calculation

Clinical interpretation

Who this applies to

Who it should not be used for

How this is used in practice

Common mistakes

Limitations

Sources & references

How to Use This Calculator

Worked example: 8-year-old boy, 130 cm, 28 kg

Understanding Your Results

Key Tips

Child BMI and paediatric growth

Why children cannot use adult BMI categories

How the CDC reference actually works

CDC or WHO — which chart applies

Why the 95th percentile is not the end of the scale

What a percentile does not tell you