BMI-for-Age Percentile Calculator

Sex:

Choose the sex used by the child growth reference.

Age:

Current entry: {{ ageDisplay }}.

months

BMI entry:

Accurate source measurements matter most near a reference boundary.

BMI:

kg/m²

Height:

Weight:

Reference set:

Auto is the neutral default for the supported U.S.-oriented handoff.

Correct age for prematurity:

Off is neutral; enable only when corrected age is part of the child’s care workflow.

Gestational age at birth:

Forty weeks is neutral and applies no correction.

weeks

Measure	Value	Context	Copy
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How to use this screening result

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Children grow through changing proportions, so their body mass index (BMI) cannot be read with fixed adult categories. The same BMI may be ordinary at one age and unusual at another. BMI-for-age places the value on an age- and sex-specific reference curve to show how it compares with the reference population.

A percentile is a rank, not a percentage of an ideal BMI. The 80th percentile means the BMI is higher than about 80% of reference values for children of the same age and recorded sex. A z-score expresses the same position as a standardized distance from the reference median.

BMI: Weight in kilograms divided by height in metres squared.
Percentile: Relative rank on the chosen growth reference.
Z-score: Distance from the reference median in standard-deviation units after the curve's skew is accounted for.
P95: The 95th-percentile BMI at the same age and recorded sex; percent of P95 adds resolution above that point.

Reference choice matters. The World Health Organization (WHO) child growth standard covers birth through 60 months here. The Centers for Disease Control and Prevention (CDC) reference covers 24 through 240 months. Auto selection uses WHO below 24 months and CDC from 24 months onward.

One point on a curve is a screening clue. Measurement error, recent illness, growth spurts, pubertal timing, muscle mass, medical history, and a change of reference can all affect meaning. Trends measured consistently are usually more informative than a small move around one cutoff.

For children younger than 2 years, U.S. clinical practice commonly considers weight-for-length alongside growth history rather than treating BMI-for-age as the only measure. Results are informational and do not diagnose undernutrition, overweight, obesity, or any medical condition.

How to Use This Tool:

Use the child's recorded age, sex, height, and weight, then choose the reference source that applies.

Select the child's recorded Sex and enter age in months. Decimal months are accepted when the date-based age is known more precisely.
Choose to derive BMI from Height and Weight, or enter a known BMI. Measure without shoes and use consistent units.
Choose Auto, WHO, or CDC. WHO is limited to 0–60 months; CDC is limited to 24–240 months.
Enable Correct age for prematurity only when corrected age is part of the child's care workflow, then enter completed gestational weeks at birth.
Review the BMI, age used, percentile, z-score, category, and growth curve together. Recheck any surprising result against the original measurements.

Interpreting Results:

Read the category with the named reference and age used. WHO categories are based on z-score boundaries, while CDC categories use percentiles and add percent-of-P95 checks for severe-obesity ranges. The labels are therefore not interchangeable.

Values near a boundary can change category after a small measurement or age correction. Confirm height, weight, age, recorded sex, reference set, and prematurity handling before acting on a borderline result.

A high or low result, rapid movement across the curve, or a mismatch with the child's broader growth pattern deserves review with a qualified health professional. The percentile does not reveal body composition or the cause of a growth change.

Technical Details:

BMI is calculated first, then compared with interpolated LMS reference values for the selected age and sex. LMS uses three age-specific parameters: L for skewness, M for the median, and S for variation.

Formula Core

When height and weight are entered, BMI uses kilograms and metres.

BMI = \frac{W}{H^{2}}

The LMS z-score follows the power form below. When L is effectively zero, the logarithmic form z = ln(BMI ÷ M) ÷ S is used.

z = \frac{{(\frac{BMI}{M})}^{L} - 1}{L \times S}

The z-score is converted through the standard normal cumulative distribution to a percentile, displayed within 0.1–99.9. Reference rows are linearly interpolated for fractional ages.

Lookup and Rule Core

WHO and CDC BMI-for-age category boundaries used by the calculator
Reference	Condition	Category
WHO	z < −3	Severe wasting range
WHO	−3 ≤ z < −2	Wasting range
WHO	−2 ≤ z ≤ 1	Expected BMI-for-age range
WHO	1 < z ≤ 2	Risk of overweight
WHO	2 < z ≤ 3	Overweight range
WHO	z > 3	Obesity range
CDC	Percentile < 5	Underweight range
CDC	5 ≤ percentile < 85	Healthy weight range
CDC	85 ≤ percentile < 95	Overweight range
CDC	Percentile ≥ 95	Obesity range
CDC extended	P95 ≥ 120% or BMI ≥ 35	Severe obesity class 2 screening range
CDC extended	P95 ≥ 140% or BMI ≥ 40	Severe obesity class 3 screening range

Severe-obesity checks take priority over the ordinary CDC percentile category. Percent of P95 is BMI divided by the age- and sex-specific 95th-percentile BMI, multiplied by 100.

Corrected age subtracts the gap between 40 weeks and gestational age, converted at about 4.348 weeks per month. It is applied only when enabled, birth occurred before 40 weeks, and chronological age is at most 24 months.

Limitations and Safety Notes:

BMI is an indirect screening measure and does not measure body fat, nutrition, fitness, or health by itself.
WHO and CDC references describe different populations and age ranges; switching references can change the percentile and category.
Corrected age should follow the child's clinical care plan, not be enabled simply to obtain a different category.
Discuss concerning values or growth changes with a pediatric clinician rather than changing diet or treatment from this result alone.

Worked Examples:

Reference changes at 24 months

At 23.9 months, Auto uses the WHO curve; at 24.0 months, it uses the CDC curve. The raw BMI does not change, but the reference parameters and category rules do. If a result changes near this transition, compare the named reference and the child's longitudinal chart rather than treating the new label as a sudden physical change.

References:

Body mass index-for-age, World Health Organization.
What Growth Charts Are Recommended?, Centers for Disease Control and Prevention.
Child and Teen BMI Categories, Centers for Disease Control and Prevention.
CDC Extended BMI-for-Age Growth Charts, Centers for Disease Control and Prevention.