Free BMI Calculator for Children

Use our free BMI calculator for children to calculate your child’s body mass index, BMI percentile and estimated weight category.

Enter the child’s age, sex, height and weight to receive an instant result. The calculator may show whether the result falls within an underweight, healthy weight, overweight or obesity range according to the growth reference used.

Your result may include:

• Body mass index

• BMI-for-age percentile

• General weight category

• Estimated ideal body weight

• Estimated healthy weight range

• Height and weight information

• Age-specific growth guidance

A child’s BMI must be interpreted differently from an adult’s BMI. Children are still growing, so age and sex must be considered along with height and weight.

Important: This calculator is a general growth-monitoring tool. It cannot diagnose a medical condition or provide a complete child growth assessment.

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Calculate Your Child’s BMI

Enter the following information:

Select the child’s sex

• Girl

• Boy

Enter the child’s age

Enter the age in completed years and months when supported.

Enter the child’s height

Choose your preferred unit:

• Centimetres

• Feet and inches

Enter the child’s weight

Choose your preferred unit:

• Kilograms

• Pounds

• Stones and pounds

Select the calculation button to view the child’s BMI, BMI percentile and general weight category.

What Is a BMI Calculator for Children?

A BMI calculator for children estimates body mass index by comparing a child’s weight with the square of their height.

The basic BMI calculation is:

BMI = weight in kilograms ÷ height in metres²

Although the mathematical formula is the same for adults and children, the result is interpreted differently.

An adult BMI is usually compared with fixed ranges. A child’s BMI is compared with children of the same age and sex using a recognised growth reference.

This produces a BMI percentile or another age-specific measurement.

A children’s BMI calculator may help with:

• Pediatric health awareness

• Child growth assessment

• General weight screening

• BMI percentile calculation

• Growth monitoring

• Reviewing height and weight together

• Understanding age-specific BMI

• Supporting a conversation with a doctor

• Monitoring children’s health metrics

The result should always be considered alongside the child’s overall development.

How to Use the BMI Calculator for Kids

The calculator is simple to use.

1. Select Girl or Boy

Select the child’s sex. BMI percentiles are age-specific and sex-specific because typical growth patterns can differ between girls and boys.

2. Enter the Child’s Age

Enter the child’s current age accurately.

Age in months may provide a more precise percentile than age in completed years alone, especially for younger children.

3. Enter Height

Measure the child without shoes and enter the current height in centimetres or feet and inches.

4. Enter Weight

Use a reliable scale and enter the child’s current weight in kilograms, pounds or stones and pounds.

5. Calculate the Result

Select the calculation button to view the child’s BMI, percentile and general weight category.

6. Review the Result Carefully

Do not make major dietary changes based on one calculator result. Speak with a pediatric health professional when you have concerns about growth, weight or nutrition.

What Is a BMI Percentile Calculator for Children?

A BMI percentile calculator for children compares a child’s BMI with 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 that of approximately 60% of children of the same age and sex in the reference population.

A percentile is not a school score and a higher percentile is not automatically better.

The percentile helps show where the child’s BMI is positioned on an age-specific growth chart.

The CDC child and teen calculator produces BMI, BMI percentile and a BMI category for children and teenagers.

Understanding Children’s BMI Categories

BMI categories for children depend on the growth reference used by the calculator.

Under the CDC BMI-for-age reference, categories are generally interpreted as:

BMI-for-age percentile	General category
Below the 5th percentile	Underweight
5th percentile to below the 85th percentile	Healthy weight
85th percentile to below the 95th percentile	Overweight
95th percentile or above	Obesity

These categories are screening classifications and are not a complete medical diagnosis.

Other countries and health organisations may use different growth charts, percentile boundaries or terminology. Your calculator should clearly state whether it uses CDC, WHO, UK or another recognised reference.

Do not combine categories from one growth reference with percentiles calculated from another.

What Is a Healthy BMI Range for Kids?

There is no single fixed healthy BMI number for every child.

A healthy BMI range for kids changes with:

• Age

• Sex

• Height

• Weight

• Stage of growth

• Growth reference used

A BMI that may be typical for a young child may be interpreted differently for an older child.

This is why an age-specific BMI calculator is more useful than applying adult BMI limits to children.

The NHS child BMI calculator also considers age and sex as well as height and weight, and reports the result as a centile.

BMI Chart for Children by Age

A BMI chart for children by age shows how BMI values change throughout childhood.

The chart normally includes separate curves for girls and boys. It may show selected percentiles or standard deviation lines across different ages.

A BMI chart should not be replaced by a simple list of one “correct” BMI for each age. Children of the same age can have different heights, weights and body structures.

The World Health Organization publishes separate BMI-for-age references for boys and girls. It provides child growth standards for younger children and growth-reference information for school-age children and adolescents.

BMI Chart for Boys

A BMI chart for boys compares a boy’s BMI with age-specific growth information for boys.

To use the chart accurately, you need:

• Exact age

• Current height

• Current weight

• Calculated BMI

• The correct male growth reference

A boy may move slightly between percentile lines as he grows. One measurement does not always show whether there is a health concern.

A healthcare professional will usually look at the pattern across several measurements rather than focusing only on one result.

BMI Chart for Girls

A BMI chart for girls compares a girl’s BMI with age-specific growth information for girls.

Girls may grow at different rates and enter stages of development at different times. For this reason, direct comparison with another child is rarely helpful.

A girl’s growth assessment may consider:

• Changes in height

• Changes in weight

• BMI-for-age percentile

• Previous measurements

• Physical development

• Eating habits

• Physical activity

• Medical history

The calculator provides a useful screening estimate, but it cannot assess all of these factors.

Height and Weight Chart for Children

A height and weight chart can help parents and healthcare professionals monitor a child’s growth over time.

Different charts may measure:

• Height for age

• Weight for age

• BMI for age

• Weight for height

• Length for age in younger children

Weight alone does not show whether a child is within a suitable range. Height, age, sex and growth history must also be considered.

CDC growth charts use percentile curves to show the distribution of selected measurements in children. The CDC states that growth charts should not be used as the only diagnostic tool; they contribute to an overall clinical impression.

Ideal Body Weight for Children

The term “ideal body weight” should be used carefully for children.

There is no single perfect weight that applies to every boy or girl of a particular age.

A child’s estimated reference weight may depend on:

• Height

• Age

• Sex

• Body structure

• Growth stage

• Genetics

• Nutrition

• Physical activity

• General health

The calculator may provide an estimated ideal body weight or a BMI-based reference weight. This should be presented as a general estimate rather than a required target.

A child should not be encouraged to reach one exact number without advice from a qualified health professional.

Healthy Weight Range for Children

A healthy weight range is generally more useful than one ideal weight.

The calculator may estimate the lower and upper weights associated with a selected healthy BMI-for-age range at the child’s current height.

This range is still only an estimate.

It does not account fully for:

• Muscle development

• Body composition

• Bone structure

• Puberty

• Ethnic background

• Medical conditions

• Medication

• Individual growth history

The NHS notes that a child BMI calculator cannot distinguish between muscle and fat and does not account for physical activity or ethnic background.

What Is a Weight Percentile?

A weight percentile compares a child’s weight with that of children of the same age and sex in a reference population.

Weight percentile is not the same as BMI percentile.

A child may have:

• A weight percentile

• A height percentile

• A BMI percentile

These measurements provide different information.

Weight-for-age does not account directly for height. BMI-for-age considers both height and weight, which may make it more useful when screening weight relative to height.

No percentile should be interpreted without considering the child’s growth pattern over time.

Why Growth Monitoring Is Important

Growth monitoring involves recording measurements over time rather than relying on a single result.

A child may naturally follow a lower or higher percentile while continuing to grow steadily.

A healthcare professional may pay attention when a child:

• Crosses several percentile lines

• Stops gaining height

• Loses weight unexpectedly

• Gains weight rapidly

• Develops eating difficulties

• Shows delayed development

• Has persistent symptoms

Regular growth assessment can help identify when further evaluation may be needed.

Child Obesity and BMI

BMI can be used as a screening tool when assessing the possibility of excess weight in children.

However, a BMI result does not explain why a child is above a particular percentile.

Factors may include:

• Genetics

• Food availability

• Family eating patterns

• Physical activity

• Sleep

• Medication

• Stress

• Medical conditions

• Social and environmental factors

Discussions about weight should be respectful and focused on health, growth, energy and well-being rather than appearance.

Avoid blaming or embarrassing a child because of a BMI result.

A doctor, pediatrician or registered dietitian can provide a more complete assessment and recommend suitable family-based support when needed.

Pediatric Nutrition and Healthy Growth

Healthy growth is supported by a balanced and varied eating pattern.

Depending on age and individual needs, this may include:

• Fruit and vegetables

• Wholegrain or higher-fibre foods

• Beans and pulses

• Suitable protein foods

• Dairy foods or alternatives

• Healthy sources of fat

• Water and other suitable drinks

Children need enough energy and nutrients to support growth, learning, activity and development.

Do not place a child on a restrictive weight-loss diet based only on a calculator result.

Children with food allergies, digestive conditions, feeding difficulties or other medical needs may require individual advice from a pediatric dietitian.

How to Measure a Child Accurately

Correct measurements help produce a more useful BMI result.

Measuring Height

1. Remove shoes and bulky clothing.

2. Ask the child to stand against a flat wall.

3. Keep the feet flat on the floor.

4. Make sure the legs are straight.

5. Ask the child to look directly ahead.

6. Place a flat object on top of the head.

7. Mark the wall and measure from the floor.

8. Record the height accurately.

Measuring Weight

1. Place the scale on a hard, level surface.

2. Ask the child to remove shoes and heavy clothing.

3. Check that the scale reads zero.

4. Ask the child to stand still in the centre.

5. Record the weight accurately.

6. Use the same scale when comparing measurements over time.

Avoid weighing a child repeatedly over short periods unless a healthcare professional has recommended it.

Benefits of This Free BMI Calculator for Kids

This free BMI calculator for kids provides:

• Instant BMI calculation

• BMI-for-age percentile

• General weight category

• Separate calculations for boys and girls

• Age-specific interpretation

• Estimated ideal body weight

• Estimated healthy weight range

• Metric and imperial units

• Simple and clear results

• Free online access

• Support for general growth monitoring

The tool is intended to support awareness and not to replace pediatric medical care.

Frequently Asked Questions

How is BMI calculated for children?

BMI is calculated by dividing weight in kilograms by height in metres squared. The result is then compared with an age-specific and sex-specific growth reference.

Is children’s BMI calculated differently from adult BMI?

The mathematical formula is the same, but the interpretation is different. A child’s result is assessed according to age and sex rather than fixed adult BMI ranges.

What is a BMI percentile?

A BMI percentile shows how a child’s BMI compares with children of the same age and sex in the selected reference population.

What is a healthy BMI percentile for children?

The answer depends on the growth reference used. Under the CDC reference, the healthy-weight category generally begins at the 5th percentile and extends to below the 85th percentile.

Can the calculator tell whether my child is underweight?

It may identify that the BMI falls within an underweight screening category. It cannot diagnose the cause or determine whether treatment is needed.

Can the calculator identify overweight or obesity?

It can show a general BMI category according to the selected growth chart. A healthcare professional should confirm and interpret any concerning result.

Does a high BMI always mean excess body fat?

No. BMI does not directly measure body fat and cannot distinguish fat from muscle.

Is ideal body weight accurate for children?

It is an estimate only. Children should generally be assessed using growth patterns, BMI-for-age and other pediatric health information rather than one ideal-weight number.

Why is my result different from another calculator?

The other calculator may use a different growth reference, age-rounding method, unit conversion or percentile category.

Should I compare siblings using BMI percentiles?

No. Each child has an individual growth pattern. Monitor each child separately and discuss concerns with a qualified professional.

Can I use an adult BMI calculator for a child?

No. Adult BMI categories are not appropriate for children because they do not account for age-specific and sex-specific growth.

Can this calculator replace a pediatrician?

No. It is a screening and educational tool only.

Calculate Your Child’s BMI Now

Enter the child’s age, sex, height and weight to calculate:

• Body mass index

• BMI-for-age percentile

• General weight category

• Estimated ideal body weight

• Estimated healthy weight range

Use the results as a starting point for understanding your child’s growth.

Calculate Child BMI

Medical Disclaimer

This BMI calculator for children and all information on this page are provided for general educational purposes only.

The calculator does not provide medical advice and is not intended to diagnose, prevent or treat underweight, overweight, obesity, poor growth or any other medical condition.

Results are estimates based on the information entered and the growth reference used. They may not reflect every difference in body composition, development, ethnicity, health, medication or growth history.

Do not place a child on a restrictive diet or weight-loss programme based only on an online calculator result.

Speak with a doctor, pediatrician, school nurse, health visitor or registered pediatric dietitian if you have concerns about a child’s weight, height, eating habits, growth or development.