Kids BMI Calculator

Use standing height and weight from the same measurement date to calculate BMI, BMI-for-age percentile, and the CDC BMI screening category from the second birthday through the day before the 20th birthday. A high-BMI result may use the 2022 Extended BMI method.

BMI is a screening measure. It does not directly measure body fat or diagnose health. For a child under 2, use the WHO Baby Growth Percentile Calculator; from the 20th birthday, use adult BMI standards.

Before you calculate

Confirm the actual birth date and measurement date.
Use standing height and weight measured on the same date or during the same measurement session. Do not combine an old height with a new weight.
Select the CDC boys or girls chart used in the child's record.
Measure standing height without shoes, heavy clothing, hats, or interfering hair accessories, on firm flooring against a flat wall or stadiometer.
This page accepts standing height only. For a child age 2–3 who cannot stand reliably, a professional may use recumbent length; the CDC SAS contract subtracts 0.8 cm from recumbent length recorded at 24 months or older to approximate standing height. This calculator does not apply that conversion. Do not enter an unadjusted recumbent length or alter a clinical record yourself.
Use a digital scale on firm, level flooring with both feet centered, and record the displayed decimal.
The calculation uses CDC 2000 LMS for BMI at or below P95 and the 2022 Extended formula only above P95. Separate chart guidance recommends the Extended chart above P97 and allows either chart from P95 through P97.
Repeat either measurement if posture, equipment, or the reading was uncertain.

Review the measurement-pair workflow, four outputs, Extended BMI method, and age handoff. This result is a screening reference, not a diagnosis or treatment plan.

Have only weight? Use the Weight Percentile Calculator. Have only height? Use the Height Percentile Calculator. For height, weight, and BMI together, use the Child Growth Chart Calculator.

Sex used for the CDC chart

CDC provides separate BMI-for-age references for boys and girls. Choose the chart used in the child's medical record.

Units

Changing units clears height and weight values.

Date of birth

Date measured

Standing height (feet)

Standing height (inches)

Weight (lb)

These broad limits catch likely entry or unit errors. They are not biological normal ranges or diagnostic limits.

What this calculator returns

BMI: Uses same-date height and weight to describe weight relative to height. It does not include age or sex and cannot measure body fat, muscle, or overall health.
BMI-for-age percentile: Uses BMI, exact age, and the selected CDC sex chart to answer where this BMI falls in the reference. It is not a grade, health percentage, or body-fat percentage.
CDC screening category: Uses the official percentile thresholds, plus the severe-obesity definition. It is screening terminology, not an identity label, diagnosis, or complete health assessment.
Percentile method: Identifies whether the calculation used the CDC 2000 LMS reference or 2022 Extended BMI. The method is not a category or severity diagnosis.

Need a fuller explanation of the number? Read What Does a Child’s BMI Percentile Mean?

Measurement-pair workflow

Confirm the actual birth date and one measurement date.
Use standing height and weight recorded on that same date or during the same measurement session; do not mix an old height with a new weight.
Remove shoes, heavy clothing, hats, and hair accessories that interfere with the measurement. Use firm flooring and a flat wall or stadiometer.
Place feet flat and together, legs straight, arms at the sides, and shoulders level. Have the child look straight ahead and use a rigid right-angle headpiece.
Read standing height with the measurer's eyes level with the headpiece and record to the nearest 0.1 cm or 1/8 inch.
Use a digital scale on firm, level flooring with both feet centered and record the nearest decimal shown by the scale.
Keep conditions consistent and repeat either value when posture, equipment, or the reading was uncertain.

This calculator accepts standing height only. For a child age 2–3 who cannot stand reliably, a professional may measure recumbent length. The CDC SAS contract subtracts 0.8 cm from recumbent length recorded at 24 months or older to approximate standing height. This calculator does not perform that conversion. Do not enter an unadjusted recumbent length or alter a clinical record yourself; an appropriate healthcare professional should decide the measurement method, chart, and adjustment.

Standard and Extended BMI methods

The calculation uses the CDC 2000 BMI-for-age LMS reference when BMI is at or below P95 (BMI ≤ P95). It uses the 2022 Extended BMI formula only when BMI is above P95 (BMI > P95). Exact P95 therefore retains the original LMS z-score and an approximately 95th-percentile result, while its screening category remains obesity. The method does not create a diagnosis or severity conclusion.

CDC chart-use recommendations are a separate contract: the 2000 chart is usually used for BMI at or below P95, the Extended chart is recommended above P97, and either chart may be used from P95 through P97. Those recommendations do not move this calculator's official formula switch to P97. Method, category, and percent of P95 are different concepts. See the Methodology and official sources for technical detail.

BMI category is a screening result

Underweight: Below the 5th percentile.
Healthy weight: 5th to below the 85th percentile.
Overweight: 85th to below the 95th percentile.
Obesity: 95th percentile or higher.
Severe obesity: At least 120% of P95 or BMI at least 35 kg/m²; a subset of obesity.

These are official screening terms, not labels for a child and not a complete diagnosis. “Healthy weight” does not prove that overall health, nutrition, fitness, laboratory values, or body composition are healthy.

One result is not a trend

This page describes one measurement date. One BMI cannot show direction or velocity. When a later same-date height-and-weight pair is available, calculate a new single-point result. The Growth Velocity Calculator compares raw height and weight changes; it does not calculate BMI velocity or BMI category trends.

The age boundaries

Child BMI-for-age starts on the second birthday. Before then, use the WHO Baby Growth Percentile Calculator; infant weight-for-length is not BMI. Starting on the 20th birthday, use the CDC Adult BMI Calculator and adult standards, which must not be applied early.

This page accepts standing height only. For a child age 2–3 who cannot stand reliably, recumbent length may be appropriate. The CDC SAS contract subtracts 0.8 cm from recumbent length recorded at 24 months or older to approximate standing height. This calculator does not apply that conversion. Do not enter an unadjusted recumbent length or change a clinical record yourself; an appropriate healthcare professional should decide the measurement method, chart, and adjustment.

If the result surprises you

Check the birth date.
Check the measurement date.
Confirm height and weight were measured on the same date.
Check the CDC chart selection.
Check cm versus ft/in and kg versus lb.
Repeat standing height.
Repeat weight.
Compare with prior dated clinical records when available.
Discuss a persistent concern with an appropriate healthcare professional.

Worked examples

Standard example: for a girl at an exact reference age of 114.5 months, BMI 21.2 is about the 92.2nd percentile and falls in the Overweight range using the CDC 2000 LMS formula. This is a screening category, not a diagnosis.

Extended example: for a boy at an exact reference age of 50.5 months, BMI 22.6 is about the 99.8th percentile. Extended BMI preserves high-percentile detail; the method is not a treatment recommendation, and neither example is a target or an individual health conclusion.

These examples follow the calculator's formula boundary of BMI ≤ P95 for 2000 LMS and BMI > P95 for Extended BMI. The separate CDC chart recommendation above P97 does not change that calculation rule.

How the calculation works

Metric BMI is weight in kilograms divided by height in meters squared. The US formula is 703 times pounds divided by inches squared. Exact age and the selected sex-specific reference then place BMI in the CDC BMI-for-age distribution.

The calculation retains full intermediate precision, uses CDC 2000 LMS through exact P95 and the Extended formula only above P95, derives P5, P85, P95 and the severe screening threshold, and rounds only the parent-facing display. Technical z-scores, interpolation anchors, percent of P95, source, and formula version remain in the collapsed details.

Limitations

Results require same-date standing height and weight and depend on accurate dates, units, and technique.
BMI does not directly measure body fat, muscle, or body composition.
The category is screening terminology, not a diagnosis or individualized health assessment.
One result does not establish a trend or BMI velocity.
The age boundaries and CDC sex chart must match the intended reference.
The formula uses 2000 LMS for BMI ≤ P95 and Extended BMI only for BMI > P95. Separate chart-use guidance recommends the Extended chart above P97 and permits either chart from P95 through P97.
This page accepts standing height only. It does not automatically subtract 0.8 cm from recumbent length, and unadjusted recumbent length must not be entered as standing height.
Users should not alter clinical records themselves; a healthcare professional should decide the measurement method, chart, and any recumbent-to-standing adjustment.
The calculator does not provide diet, exercise, medication, target-weight, or treatment advice.
A healthcare professional can interpret the result with the child's growth history and other health information.

Sources and common questions

Official sources rechecked 2026-08-02.

Helpful context

Frequently asked questions

What information do I need?

You need the actual birth date, one measurement date, standing height and weight measured on that same date, the submitted units, and the CDC boys or girls chart used in the child’s record.

Why should height and weight be measured on the same date?

BMI relates weight to height at one point in time. Combining an old height with a newer weight creates a mixed-date value that does not represent either measurement session.

Why does child BMI use age and sex?

Children and teens are growing, so the same BMI can have a different reference position at different ages. CDC provides separate BMI-for-age references for boys and girls.

What are the CDC BMI categories?

CDC defines underweight as below the 5th percentile, healthy weight as the 5th to below the 85th, overweight as the 85th to below the 95th, and obesity as the 95th percentile or higher. These are screening categories, not complete diagnoses.

Does BMI directly measure body fat?

No. BMI uses weight relative to height and does not directly measure body fat, muscle, or overall health. A healthcare professional can interpret it with growth history and other information.

What is 2022 Extended BMI?

This calculator uses CDC 2000 LMS when BMI is at or below P95 and the 2022 Extended BMI formula only when BMI is above P95. CDC chart-use guidance is separate: the 2000 chart is usually used at or below P95, the Extended chart is recommended above P97, and either chart may be used from P95 through P97. The chart recommendation does not change this calculator’s formula boundary.

What does severe obesity mean in the CDC category?

It means the BMI is at least 120% of P95 or at least 35 kg/m². Severe obesity is a subset of the CDC obesity screening category; the calculator does not diagnose a condition.

What if the child cannot stand reliably?

This page accepts standing height only. For a child age 2–3 who cannot stand reliably, recumbent length may be needed. The CDC SAS contract subtracts 0.8 cm from recumbent length recorded at 24 months or older to approximate standing height, but this calculator does not apply that conversion. Do not enter unadjusted recumbent length or change a clinical record yourself; an appropriate healthcare professional should decide the measurement method, chart, and adjustment.

What should I use for a child under 2 or a person age 20 or older?

Use the WHO Baby Growth Percentile Calculator before the second birthday. People age 20 or older should use adult BMI standards rather than a child BMI-for-age percentile.

Does this site save the measurements?

Calculator inputs are processed in your browser and are not saved as a child health record or sent to an external calculation service.