What BMI percentile is underweight for a teenager?

For children and teens aged 2 through 19, the CDC defines underweight as a BMI below the 5th percentile for their sex and age. The cutoff BMI changes with age, so the same BMI can be underweight at one age and healthy at another.

Is BMI calculated differently for kids?

The formula is the same as for adults: weight divided by height squared. What changes is how the result is read. Adults use fixed cutoffs, while children and teens are compared with others of the same sex and age using BMI-for-age percentiles.

Why did my child's percentile change when their BMI stayed the same?

The comparison group changes as a child gets older. In the CDC tables the median BMI shifts month by month through childhood, so an unchanged BMI can land on a different percentile a few months later.

Is this the same as the CDC child and teen BMI calculator?

It uses the same public CDC data file and the formulas the CDC publishes, including the extended method above the 95th percentile, and it reproduces the CDC worked examples. The CDC notes that rounding in age, weight and height can cause small differences between programs.

At what age should we switch to an adult BMI calculator?

At 20. The CDC child and teen calculator covers ages 2 through 19, and people 20 and older use the adult BMI categories.

What should I do if my child is underweight on the calculator?

Share the result with the child's doctor, ideally with earlier heights and weights. The doctor can look at the growth pattern over time and other health factors before deciding whether any change is needed.

BMI Calculator for Kids and Teens

Enter a child's sex, age, height and weight to get their BMI, their BMI-for-age percentile and the matching CDC weight category. The calculator uses the growth chart data the CDC publishes for children and teens aged 2 through 19, and it also shows the weight range that lines up with the healthy category at the child's current height. Nothing you type leaves your browser.

For parents and teens: BMI is a screening number, not a diagnosis. This page is educational and is not medical advice. If a child has lost weight without trying, is not growing as expected, or you are worried about their eating, talk with the child's doctor.

Result

BMI

BMI percentile

CDC category

Healthy weight range at this height

BMI z-score

The healthy weight range is the weight that would put this child between the 5th and 85th percentile at today's height and age. Children also grow taller, so the range moves as they grow. Percentiles above the 95th use the CDC extended method. Small differences from other calculators can come from rounding.

How to read a BMI percentile

Children are still growing, so their BMI is not judged with the adult cutoffs. The CDC compares a child's BMI with other children of the same sex and age, and that comparison is the BMI-for-age percentile. A child at the 30th percentile has a BMI that is the same as or higher than 30% of children of the same sex and age in the reference population used to build the CDC growth charts.

The CDC gives a worked example: a 12-year-old girl who is 5 feet tall and weighs 136 pounds has a BMI of 26.6. On the CDC charts that BMI is at the 96th percentile, so it falls in the obesity category. If you enter the same girl above (12 years, 0 months), the calculator returns the same result.

A single percentile is a snapshot. The CDC calls percentiles the most commonly used clinical indicator for judging growth patterns in children, which is why a doctor will usually compare today's number with earlier measurements on the child's own chart.

CDC BMI categories for children and teens

These are the categories the CDC uses for ages 2 through 19. The calculator applies them to the percentile it finds.

Category	BMI-for-age percentile
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 a BMI of 35 or greater

From age 20 the CDC switches to the adult categories, where underweight means a BMI below 18.5. For that age group, use our adult BMI calculator for weight gain.

If the result is below the 5th percentile

A BMI under the 5th percentile puts a child in the CDC underweight category. Treat it as a reason to check in with the child's doctor, not as a diagnosis. The CDC describes BMI as one possible health indicator and says it should be weighed alongside medical history, health behaviors, a physical exam and lab results.

The pattern over time matters more than one reading. Losing weight without trying, or not gaining as expected, is a different signal from being slim: the NHS lists "not growing or not putting on weight at the expected rate" as a sign of malnutrition in children and advises seeing a doctor if you are worried a child in your care may be malnourished.

Bring the height, weight and date of each measurement to the visit so the doctor can compare them with earlier ones. For food ideas that suit children, see our guide to healthy foods to help kids gain weight, and for older kids, how to gain weight as a teenager. Both start with the same advice: check with the child's doctor before starting a weight gain plan.

How to measure height and weight at home

The CDC calls accurate height and weight measurements essential for a correct BMI and BMI percentile. These steps follow its instructions.

Height

Take off shoes, bulky clothing, hats and hair ornaments, and undo hairstyles that get in the way.
Stand the child on a firm floor such as tile or wood, not carpet, with their back against a flat wall with no molding.
Feet flat and together against the wall, legs straight, arms at the sides, shoulders level, eyes looking straight ahead.
Lower a flat, rigid ruler or book until it touches the top of the head at a right angle to the wall, lightly mark the wall, then measure from the floor to the mark with a metal tape.
Record the height to the nearest 1/8 inch or 0.1 centimeter.

Weight

Use a digital scale on firm flooring, and avoid spring-loaded bathroom scales.
Have the child take off shoes and heavy clothing such as sweaters, and stand with both feet in the center of the scale.
Record the weight to the nearest decimal, for example 55.5 pounds or 25.1 kilograms.

How this calculator works

BMI. BMI is weight divided by height squared. In US units that is weight in pounds divided by height in inches squared, times 703. In metric units it is weight in kilograms divided by height in meters squared. These are the CDC formulas.

Age. Age is counted in completed months, and each month uses the CDC table row for the middle of that month. For example, a child aged 9 years and 6 months (114 months) uses the 114.5-month row, which is how the CDC data files list age.

Percentile. For a BMI at or below the 95th percentile, the calculator uses the L, M and S values from the CDC growth chart data file: z equals (BMI divided by M, raised to the power L, minus 1) divided by (L times S), and the percentile is the share of a normal distribution below that z-score. Above the 95th percentile it uses the CDC extended BMI-for-age method, which the CDC added because the original 2000 charts do not extend beyond the 97th percentile. The healthy weight range works the same formulas backward at the 5th and 85th percentiles.

Checked against the CDC. We tested the code with the worked examples on the CDC data file page. A girl aged 9 years 6 months with a BMI of 21.2 comes out at a z-score of 1.4215 and a percentile of 92.2, and a boy aged 4 years 2 months with a BMI of 22.6 comes out at a percentile of 99.7683, matching the CDC answers. The CDC notes that results can differ slightly between programs because of rounding in age, weight and height.

What BMI can and cannot tell you

The CDC calls BMI a simple, reliable and low-cost screening measure, but it is not a direct measure of body fat and it cannot tell fat from muscle and bone. Someone with a lot of muscle can have a BMI that looks high for their body fat. That is why the number works best as a starting point for a conversation with the doctor, together with how the child is growing, eating and feeling.

Educational, not medical advice. This calculator is a screening aid based on public CDC data. It does not diagnose any condition and it is not a substitute for a doctor's assessment. See the child's doctor about any weight loss that was not planned, a change in growth, or worries about eating. Operator: Mustafa Bilgic.

Frequently asked questions

What BMI percentile is underweight for a teenager?: For children and teens aged 2 through 19, the CDC defines underweight as a BMI below the 5th percentile for their sex and age. The cutoff BMI changes with age, so the same BMI can be underweight at one age and healthy at another.
Is BMI calculated differently for kids?: The formula is the same as for adults: weight divided by height squared. What changes is how the result is read. Adults use fixed cutoffs, while children and teens are compared with others of the same sex and age using BMI-for-age percentiles.
Why did my child's percentile change when their BMI stayed the same?: The comparison group changes as a child gets older. In the CDC tables the median BMI shifts month by month through childhood, so an unchanged BMI can land on a different percentile a few months later.
Is this the same as the CDC child and teen BMI calculator?: It uses the same public CDC data file and the formulas the CDC publishes, including the extended method above the 95th percentile, and it reproduces the CDC worked examples. The CDC notes that rounding in age, weight and height can cause small differences between programs.
At what age should we switch to an adult BMI calculator?: At 20. The CDC child and teen calculator covers ages 2 through 19, and people 20 and older use the adult BMI categories.
What should I do if my child is underweight on the calculator?: Share the result with the child's doctor, ideally with earlier heights and weights. The doctor can look at the growth pattern over time and other health factors before deciding whether any change is needed.

Keep reading

Sources: CDC: Child and Teen BMI Categories · CDC: BMI Calculator for Child and Teen · CDC: Body Mass Index FAQs · CDC: Measuring Children's Height and Weight · CDC: Growth Charts Data Files (LMS) · CDC: Extended BMI-for-Age Data File · NHS: Malnutrition.