A Child BMI calculator calculates body mass index (BMI) from a child’s height and weight and then estimates where that BMI falls on an age- and sex-specific growth reference.

This is different from adult BMI interpretation. Children and teenagers are still growing, so the numerical BMI must be considered alongside:

age

and:

sex

to calculate a BMI-for-age percentile.

The CDC Child and Teen BMI Calculator is designed for children and teens ages 2 through 19 and reports BMI, BMI percentile, and BMI category.

Parent and caregiver note: BMI is a screening measure, not a diagnosis. A single percentile does not describe a child’s overall health, body composition, nutrition, development, or wellbeing. Discuss growth concerns with a pediatric healthcare professional rather than making restrictive diet or weight changes from one calculator result.

Measure height and weight carefully, enter age as accurately as possible, and interpret the BMI against the appropriate child growth reference rather than adult BMI cutoffs.

Child BMI is calculated in two steps

The calculation starts by finding the ordinary numerical BMI.

Then that value is compared with children of the same age and sex in the reference population.

So the process is:

Height + Weight → BMI

then:

BMI + Age + Sex → BMI-for-age percentile

This second step is what makes child BMI interpretation different from adult BMI.

How to use the Child BMI calculator

1. Enter the child’s age.
2. Enter sex as required by the growth reference.
3. Enter height.
4. Enter weight.
5. Check that the correct units are selected.
6. Calculate BMI.
7. Review the BMI-for-age percentile.
8. Review the corresponding screening category.
9. Keep the original age, height, and weight with the result.
10. Discuss persistent growth concerns with an appropriate pediatric healthcare professional.

Do not apply adult BMI categories to a child or teenager.

The BMI formula itself is the same

BMI is calculated from weight relative to height.

Using metric units:

BMI = Weight in kg ÷ Height in meters²

An equivalent formula using centimeters is:

BMI = Weight in kg ÷ Height in cm² × 10,000

Using U.S. customary units:

BMI = 703 × Weight in lb ÷ Height in inches²

These are the same formulas used to calculate adult BMI numerically. The difference comes afterward, when the child’s BMI is interpreted by age and sex.

Worked BMI example

Suppose a child weighs:

40 kg

and is:

1.48 m tall

BMI:

40 ÷ 1.48²

First:

1.48² = 2.1904

Then:

40 ÷ 2.1904 ≈ 18.26

So the calculated BMI is approximately:

18.3 kg/m²

That number alone does not determine the child BMI category.

The next step is to use the child’s:

exact age

and:

sex

to determine where:

BMI 18.3

falls on the appropriate BMI-for-age reference.

The same BMI can mean something different at another age

Consider two children who both have:

BMI = 18.2 kg/m²

If one is:

8 years old

and another is:

15 years old

they are compared with different age-specific BMI distributions.

Their BMI percentiles may therefore differ even though the numerical BMI is identical.

This is why adult-style fixed BMI cutoffs should not be used for children.

What a BMI percentile means

A BMI percentile describes a child’s position relative to a reference population of children of the same age and sex.

For example, a BMI at the:

60th percentile

means the child’s BMI is equal to or higher than approximately 60% of children of the same age and sex in the reference population.

It does not mean:

60% body fat

or:

60% of ideal weight

or:

60% of growth completed

A percentile is a ranking within the growth reference.

CDC BMI-for-age categories

For children and teens ages 2 through 19, CDC BMI-for-age categories are based on sex- and age-specific percentiles.

BMI category	BMI-for-age range
Underweight	Less than the 5th percentile
Healthy weight	5th percentile to less than the 85th percentile
Overweight	85th percentile to less than the 95th percentile
Obesity	95th percentile or greater
Severe obesity	120% of the 95th percentile or greater, or BMI 35 kg/m² or greater

The CDC notes that severe obesity is a subset of obesity and that extended BMI-for-age growth charts are available for very high BMI values.

These are screening categories, not judgments about a child’s character, appearance, effort, or worth.

Why age in months can matter

Children grow continuously rather than in one-year jumps.

A child who has just turned:

10 years old

and a child who is almost:

11

may both be casually described as age 10, but they are at different points on the growth reference.

CDC growth-chart data are organized by sex and age in months, which allows more precise percentile placement than simply rounding age to the nearest whole year.

If the calculator accepts date of birth or age in months, use the most accurate information available.

Do not round age casually

Suppose a child is:

10 years 10 months

Rounding down to:

10 years

or up to:

11 years

can shift the reference comparison.

This may matter especially when the BMI is close to a percentile boundary.

Use the child’s actual age rather than choosing whichever age produces a more reassuring category.

Measure height carefully

BMI contains:

height²

in the denominator.

That means even a relatively small height error can noticeably change the calculated BMI.

For example, measuring a child as:

145 cm

instead of:

148 cm

changes the BMI even if weight is unchanged.

Accurate measurement therefore matters before percentile interpretation.

Remove shoes for height measurement

For a standing height measurement:

• remove shoes;
• use a firm, level surface;
• stand upright;
• keep feet appropriately positioned;
• look straight ahead;
• and use a stable vertical measuring surface.

CDC measurement guidance emphasizes obtaining accurate height and weight because those values directly affect BMI and percentile calculations.

Measure weight under consistent conditions

Use a reliable scale placed on firm flooring.

Heavy outer clothing, shoes, backpacks, or objects in pockets can add unnecessary error.

For growth tracking over time, consistent measurement conditions make comparisons more meaningful.

Small errors matter near category boundaries

Suppose a child’s calculated percentile is very close to:

85th

or:

95th

A small difference in:

• height;
• weight;
• age;
• rounding;
• or calculation method

could shift the displayed category.

CDC notes that child BMI calculator results can differ slightly between programs because of rounding in age, height, and weight.

A borderline result should therefore not be interpreted as a sharp biological dividing line.

Percentiles are continuous

A child at:

84.9th percentile

and one at:

85.1st percentile

are extremely close numerically even though a category label may change at:

85th percentile

The category is useful for screening.

It does not mean there is a sudden biological change at the exact cutoff.

A pediatric professional considers the full growth and health context.

One BMI measurement is only one point

Children do not grow at a perfectly steady rate.

Growth can happen in spurts.

Height may increase quickly during one period while weight changes differently.

Puberty can also substantially alter:

• height;
• muscle mass;
• fat distribution;
• and body composition.

For this reason, a sequence of accurate measurements is generally more informative than one isolated percentile.

Growth trajectory matters

Suppose a child’s BMI-for-age percentile remains relatively stable over several measurements.

That tells a different story from a child whose percentile changes substantially over a relatively short period.

A pediatric clinician may consider:

• previous growth measurements;
• height trajectory;
• weight trajectory;
• puberty;
• medical history;
• nutrition;
• activity;
• sleep;
• and other factors.

The calculator cannot evaluate that entire pattern from one entry.

A percentile can change even when BMI barely changes

Because the reference distribution changes with age, a numerical BMI can remain similar while the percentile changes.

Likewise, BMI can change while percentile stays relatively stable if the change is consistent with the reference growth pattern.

This is why saving only the BMI number without age can make later interpretation difficult.

Save age, height, weight, BMI, and percentile together

A useful growth record might include:

Date	Age	Height	Weight	BMI	BMI percentile
Visit 1	10 y 2 m	—	—	—	—
Visit 2	10 y 8 m	—	—	—	—
Visit 3	11 y 3 m	—	—	—	—

This makes it easier to review the trajectory rather than focusing only on the latest category.

Adult BMI categories should not be applied

Adult BMI commonly uses fixed numerical categories.

Children require age- and sex-specific BMI-for-age interpretation because normal body composition changes during growth.

The CDC Child and Teen BMI Calculator covers ages:

2 through 19

while people age:

20 and older

are directed to the adult BMI calculator.

Use the BMI Calculator for adults when appropriate.

BMI does not directly measure body fat

BMI is calculated from:

height

and:

weight

It does not directly measure:

• body-fat percentage;
• muscle mass;
• bone mass;
• fat distribution;
• or metabolic health.

Children with the same BMI can have different body compositions.

That is one reason BMI is used as a screening measure rather than a standalone diagnosis.

A category does not explain why the result occurred

A BMI percentile cannot tell you whether a child’s growth pattern is influenced by:

• genetics;
• puberty timing;
• medical conditions;
• medications;
• food access;
• sleep;
• physical activity;
• development;
• or another factor.

Those questions require broader assessment.

Do not make restrictive diet changes from one percentile

A child should not be placed on a restrictive eating plan solely because a calculator displays a particular BMI category.

Children need sufficient energy and nutrients for:

• growth;
• brain development;
• bone development;
• puberty;
• physical activity;
• and general health.

If there is concern about growth or weight, use pediatric guidance rather than applying an adult weight-loss strategy.

Use neutral language around children

Health measurements can influence how children think about:

• food;
• exercise;
• appearance;
• and their bodies.

Instead of describing a child as:

“bad”

“too heavy”

or:

“successful”

because of a percentile, describe the result as:

one growth-screening measurement

that can be reviewed with a healthcare professional.

The goal is useful health information, not praise or blame.

Avoid turning percentile into a score

A:

50th percentile

is not automatically better than:

40th

or:

60th

Percentiles describe position in a reference distribution.

They are not exam scores.

Likewise, moving toward the 50th percentile is not automatically the correct goal for every child.

Individual growth patterns differ.

Siblings do not need matching percentiles

Two siblings can have different:

• body builds;
• growth timing;
• puberty timing;
• genetics;
• heights;
• and BMI percentiles.

A sibling’s result is not the appropriate growth reference.

Each child should be interpreted using their own age, sex, measurements, and growth history.

Puberty can change body composition quickly

Adolescence can involve rapid changes in:

• height;
• muscle;
• body fat;
• hormones;
• appetite;
• and development.

These changes do not occur at exactly the same age for every child.

A percentile should therefore be interpreted within developmental context rather than compared only with appearance.

Athletically active children can still need context

A child or teen involved in sports may have greater muscle mass.

Because BMI uses total weight rather than body composition, this can affect the numerical BMI.

The calculator cannot tell how much weight is:

muscle

versus:

fat

or another tissue.

A clinician can consider the child’s activity, development, growth pattern, and other information.

A low percentile also needs context

Attention should not focus only on higher BMI percentiles.

A BMI below the:

5th percentile

falls in the CDC underweight screening category.

That does not explain the cause.

Some children naturally track at lower percentiles, while others may have:

• growth concerns;
• nutritional issues;
• illness;
• or another explanation.

Persistent concerns should be reviewed professionally.

BMI percentile is not height percentile

A child can have:

height at one percentile

and:

BMI at another percentile

These are different growth measurements.

BMI-for-age combines:

weight relative to height

then compares that BMI with an age- and sex-specific reference.

Do not confuse BMI percentile with:

• height-for-age percentile;
• weight-for-age percentile;
• or weight-for-stature percentile.

BMI percentile is not weight percentile

A child could be at a relatively high weight percentile while also being tall.

BMI considers weight relative to height.

That means weight percentile alone cannot determine the BMI category.

The full set of measurements matters.

For children younger than 2, use different growth measures

The CDC child BMI calculator is intended for ages:

2 through 19.

For infants and younger children, clinicians generally focus on other growth measures such as:

• weight;
• recumbent length;
• weight-for-length;
• and head circumference

using age-appropriate growth references.

For a separate early-childhood growth estimate, use the Baby Growth Percentile Calculator where appropriate.

Different growth charts should not be treated as identical

A result depends on the reference chart and calculation method.

CDC growth charts, extended CDC BMI-for-age charts, and other national or international references may not produce identical percentile values.

When tracking changes over time, use the same reference system unless a healthcare professional intentionally changes it.

Extended BMI-for-age charts cover very high values

CDC introduced extended BMI-for-age growth charts to improve monitoring for children and adolescents with very high BMI values.

The extended charts provide additional percentiles above the traditional upper range and are intended to improve tracking of BMI values above the 95th percentile.

This allows changes at higher BMI levels to be monitored more meaningfully than simply grouping all high values together.

Severe obesity has a specific screening definition

CDC defines severe obesity for children and teens as:

120% of the 95th percentile or greater

or:

BMI of 35 kg/m² or greater

according to its child BMI category guidance.

This is still a screening classification.

It does not replace medical evaluation or determine treatment by itself.

Percentage of the 95th percentile is not the same as percentile rank

This distinction is important.

A value described as:

120% of the 95th percentile

does not mean:

120th percentile

A 120th percentile does not exist in the ordinary percentile ranking system.

Instead, it compares the child’s BMI numerically with the BMI value located at the age- and sex-specific 95th percentile.

Example of BMI versus percentile interpretation

Suppose a calculator reports:

BMI = 18.2 kg/m²

The correct next question is not:

“Is 18.2 good or bad?”

Instead ask:

“Where does BMI 18.2 fall for this child’s exact age and sex?”

The same 18.2 can correspond to different percentile positions for different children.

That two-step interpretation is essential.

Measurement errors can compound

Because BMI divides weight by:

height squared

height errors can have a noticeable effect.

Suppose height is underestimated.

The denominator becomes smaller.

That can make the calculated BMI appear higher.

This is why accurate standing height is especially important.

Unit mistakes can produce extreme errors

Common input mistakes include:

• entering pounds as kilograms;
• entering centimeters as meters;
• entering feet in an inches field;
• forgetting to convert feet and inches;
• or using the wrong age format.

If a result looks implausible, check the units before interpreting the percentile.

Feet and inches must be combined correctly

Suppose height is:

4 ft 10 in

Convert feet to inches:

4 × 12 = 48 in

Then add:

48 + 10 = 58 in

Use:

58 inches

if the calculator expects total inches.

Entering:

4.10

inches or:

4.10 feet

would represent a different height.

Do not overinterpret small percentile changes

A change from:

62nd percentile

to:

64th percentile

may reflect:

• ordinary growth;
• measurement differences;
• age calculation;
• or rounding.

It should not automatically be described as a meaningful health change.

Look at the broader trajectory and clinical context.

A large shift deserves context too

A larger percentile shift can be worth discussing, but the calculator cannot determine why it happened.

Possible explanations may include:

• normal growth spurts;
• puberty;
• changes in activity;
• illness;
• medication;
• measurement error;
• or nutritional changes.

A professional can interpret the pattern alongside other health information.

The result should support questions, not labels

Useful questions after calculating might include:

Has the growth pattern changed over time?

Were height and weight measured accurately?

Does puberty or growth stage explain part of the change?

Are there nutritional or medical concerns?

Does the pediatric clinician want follow-up measurements?

These are more useful than treating one percentile as a verdict.

Common mistakes

Applying adult BMI categories

Children require BMI-for-age interpretation.

Rounding age to the nearest year

Age in months can improve percentile placement.

Measuring height with shoes on

Height error affects BMI.

Entering the wrong units

Check pounds, kilograms, inches, and centimeters carefully.

Treating percentile as body-fat percentage

They are completely different quantities.

Comparing siblings directly

Growth patterns are individual.

Using one measurement as a diagnosis

BMI is a screening measure.

Making restrictive diet changes from the calculator alone

Children’s growth and nutrition require broader assessment.

Comparing results from different charts as though they were identical

Keep the reference system consistent.

Before interpreting the result

Check the inputs first.

Age

Is the child’s age accurate, preferably including months when supported?

Sex

Was the correct growth-reference input selected?

Height

Was height measured without shoes using appropriate technique?

Weight

Was a reliable scale used?

Units

Are kilograms, pounds, centimeters, or inches entered correctly?

Reference

Is the calculator using the intended child growth chart?

Trend

Are previous measurements available?

Context

Are there growth, nutrition, developmental, or medical concerns that require pediatric review?

These checks make the percentile more meaningful.

Related growth calculators

For children younger than two, use age-appropriate infant growth measures rather than child BMI categories. The Baby Growth Percentile Calculator can provide a separate growth reference where applicable.

For people age 20 and older, use the BMI Calculator rather than a child BMI-for-age calculator.

FAQ

How is child BMI calculated?

The mathematical BMI uses:

BMI = Weight in kg ÷ Height in meters²

or:

BMI = 703 × Weight in lb ÷ Height in inches²

Then age and sex are used to determine BMI-for-age percentile.

Why is child BMI different from adult BMI?

Children are still growing, so BMI changes with age and development.

The numerical BMI must therefore be interpreted using age- and sex-specific growth references.

What does a BMI percentile mean?

It describes where the child’s BMI falls relative to children of the same age and sex in the reference population.

Does the 60th percentile mean 60% body fat?

No.

Percentile is a ranking, not a body-fat percentage.

What are the CDC child BMI categories?

For ages 2 through 19, CDC categories use:

Below 5th percentile — Underweight

5th to below 85th — Healthy weight

85th to below 95th — Overweight

95th or greater — Obesity

CDC also defines severe obesity using 120% of the 95th percentile or BMI 35 kg/m² or greater.

Can BMI diagnose a health condition?

No.

It is one screening measure used alongside growth history, examination, medical history, health behaviors, and other information.

Why is age in months useful?

BMI reference values change continuously through childhood and adolescence.

Using age in months can improve placement compared with rounding to whole years.

Can the same BMI produce different percentiles?

Yes.

The percentile depends on age and sex as well as the BMI value.

Should siblings have similar BMI percentiles?

Not necessarily.

Children differ in genetics, growth timing, puberty, height, body composition, and development.

Does a high percentile mean the child definitely has too much body fat?

No.

BMI does not directly measure body composition.

A professional can interpret BMI alongside the child’s broader growth and health information.

Does a low percentile automatically mean something is wrong?

No.

It is a screening result that needs context.

Persistent growth or nutrition concerns should be discussed with a pediatric healthcare professional.

Should a child lose weight because of one calculator result?

Do not make restrictive diet or weight-management changes based only on one BMI percentile.

Children need nutrition appropriate for growth and development, and concerns should be reviewed professionally.

Can I use this calculator for a child younger than 2?

The CDC Child and Teen BMI Calculator is designed for ages 2 through 19.

Younger children are generally monitored with other age-appropriate growth measures.

When should an adult BMI calculator be used?

CDC directs people age:

20 and older

to its adult BMI calculator.

Why might two child BMI calculators give slightly different percentiles?

They may use different:

• age rounding;
• growth references;
• extended-chart methods;
• height or weight rounding;
• or implementation details.

Use the same validated reference when comparing trends over time.

Growth reference

For current U.S. child and teen BMI guidance, percentile categories, measurement information, and the CDC calculator, see:

CDC — Child and Teen BMI Calculator

CDC’s calculator is intended for children and teens ages 2 through 19 and reports BMI, BMI percentile, and BMI category.

Follow the growth pattern, not one label

A Child BMI calculator is most useful when it is treated as one part of growth monitoring.

Height and weight produce the numerical BMI.

Age and sex place that BMI on the appropriate growth reference.

The percentile then describes the child’s position within that reference population.

The result becomes much more useful when it is compared with accurate previous measurements and interpreted alongside development, nutrition, health history, and the child’s overall wellbeing.

Keep the measurements, avoid turning the percentile into a judgment, and use pediatric guidance when the growth pattern raises questions.