BMI Percentile Calculator

The BMI percentile calculator compares a child or teen with the CDC reference data for the same sex and age (2 to 20 years). Enter age, sex, height and weight to see the BMI-for-age percentile, z-score, CDC weight category and the healthy BMI and weight range for that age. It is a screening tool, not a diagnosis.

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Sex

Age: years

Age: extra months

Height (or feet if ft + in)

Height unit

Extra inches (feet + inches only)

Weight

Weight unit

How to use the BMI Percentile Calculator

Choose the child's sex as used on CDC growth charts (boy or girl).
Enter age in whole years plus extra months (for example 10 years and 6 months). Valid ages are 2 to 20 years.
Enter height and weight and pick the units. For feet and inches, put the feet in the height box and the inches in the extra inches box.
Press Calculate to see the BMI, percentile, z-score, CDC category and the healthy weight range for that height and age.

Formula and method

Child and teen BMI uses the same arithmetic as adult BMI, BMI = weight (kg) / height (m)², but the number is then compared with the CDC reference distribution for the same sex and exact age instead of with fixed cutoffs. The CDC publishes three smoothing parameters for each age in months: L (skewness), M (median) and S (coefficient of variation). The calculator interpolates those values to the child's exact age, then computes the z-score z = ((BMI / M)^L − 1) / (L × S) (or ln(BMI / M) / S when L is 0). The percentile is the normal distribution function of z, multiplied by 100. A z-score of 0 is the 50th percentile; −1.64 is the 5th; +1.04 is the 85th; +1.64 is the 95th.

As a check, the CDC's own example of a 12-year-old girl who is 5 feet tall and weighs 136 pounds (BMI 26.6) lands at the 96th percentile with this method.

CDC BMI-for-age categories

CDC category (ages 2 to 19)	BMI-for-age percentile
Underweight	Below the 5th percentile
Healthy weight	5th percentile to below the 85th
Overweight	85th percentile to below the 95th
Obesity, class 1	95th percentile up to below 120% of the 95th percentile
Obesity, class 2 (severe)	120% to below 140% of the 95th percentile, or BMI 35 to below 40
Obesity, class 3 (severe)	140% of the 95th percentile or more, or BMI 40 or more

These are the categories the CDC uses for children and teens. The class 2 and class 3 obesity rows follow the expanded definitions used by the American Academy of Pediatrics, which the CDC lists on its BMI categories page.

Worked example: 12-year-old girl, 5 feet, 136 pounds

Weight 136 lb is 61.7 kg and height 5 ft is 152.4 cm (1.524 m). BMI = 61.7 / 1.524² = 26.6. For a girl exactly 12 years old the CDC tables give L = −1.98, M = 18.07 and S = 0.148, so z = ((26.6 / 18.07)^L − 1) / (L × S) is about +1.82. That is 96.6 percent of the reference distribution, which the CDC states as the 96th percentile (a percentile is the share of children the child equals or exceeds, so it is stated rounded down). It is above the 95th percentile, so the category is obesity (class 1). This is the CDC's own published example, and the calculator reproduces it.

BMI percentile cutoffs for boys by age

BMI values (kg/m²) at key percentiles, calculated from the CDC LMS tables. *Severe cutoff is the lower of 120% of the 95th percentile or 35.

Age	5th pct	50th (median)	85th pct	95th pct	Severe cutoff*
2 years	14.7	16.6	18.2	19.3	23.2
3 years	14.3	16.0	17.4	18.3	21.9
4 years	14.0	15.6	16.9	17.8	21.4
5 years	13.8	15.4	16.8	17.9	21.5
6 years	13.7	15.4	17.0	18.4	22.1
7 years	13.7	15.5	17.4	19.1	22.9
8 years	13.8	15.8	17.9	20.0	24.0
9 years	14.0	16.1	18.6	21.0	25.3
10 years	14.2	16.6	19.4	22.1	26.5
11 years	14.5	17.2	20.2	23.2	27.8
12 years	15.0	17.8	21.0	24.2	29.0
13 years	15.4	18.4	21.8	25.1	30.2
14 years	16.0	19.1	22.6	26.0	31.2
15 years	16.5	19.8	23.4	26.8	32.2
16 years	17.1	20.5	24.2	27.5	33.0
17 years	17.7	21.2	24.9	28.2	33.9
18 years	18.2	21.9	25.6	28.9	34.7
19 years	18.7	22.5	26.3	29.7	35.0
20 years	19.1	23.0	27.0	30.6	35.0

BMI percentile cutoffs for girls by age

The same cutoffs for girls, from the CDC's separate girls' tables. Boys and girls are never compared with each other because their growth and body composition differ, especially from puberty onward.

Age	5th pct	50th (median)	85th pct	95th pct	Severe cutoff*
2 years	14.4	16.4	18.0	19.1	22.9
3 years	14.0	15.7	17.2	18.3	21.9
4 years	13.7	15.3	16.8	18.0	21.6
5 years	13.5	15.2	16.8	18.2	21.9
6 years	13.4	15.2	17.1	18.8	22.6
7 years	13.4	15.4	17.6	19.6	23.6
8 years	13.5	15.8	18.3	20.7	24.8
9 years	13.7	16.3	19.1	21.8	26.1
10 years	14.0	16.8	19.9	22.9	27.5
11 years	14.4	17.4	20.8	24.1	28.9
12 years	14.8	18.1	21.7	25.2	30.3
13 years	15.3	18.7	22.5	26.3	31.5
14 years	15.8	19.3	23.3	27.2	32.7
15 years	16.3	19.9	24.0	28.1	33.7
16 years	16.8	20.4	24.6	28.9	34.7
17 years	17.2	20.9	25.2	29.6	35.0
18 years	17.5	21.3	25.7	30.3	35.0
19 years	17.8	21.5	26.1	31.0	35.0
20 years	17.8	21.7	26.5	31.8	35.0

What a BMI percentile means

A percentile shows where a child stands among children of the same sex and age in the reference population. A BMI at the 70th percentile means the child's BMI is the same as or higher than 70 out of 100 reference children. It does not mean the child is 70 percent of anything or that 70 is a score to aim to lower. Healthy children can sit anywhere from the 5th to below the 85th percentile, and a child who has tracked along the 90th percentile for years has a different story from one who jumped from the 40th to the 90th in a year.

Why children are not judged by adult BMI cutoffs

Adults are classed with fixed BMI numbers: below 18.5 underweight, 25 to 29.9 overweight and 30 or more obesity. Children's BMI changes naturally as they grow: it falls after infancy, reaches a low point around ages 5 to 6, then rises through adolescence. A BMI of 20 is at about the 32nd percentile for a 17-year-old boy but at about the 97th percentile for a 7-year-old boy. Using adult cutoffs would misclassify many children, so the CDC uses sex-specific, age-specific percentiles.

Limits of BMI in children and teens

BMI does not separate muscle, bone and fat, and children vary in how early puberty starts. A muscular athlete can be above the 85th percentile with low body fat, and a child with less muscle can have a healthy BMI but excess fat. CDC reference data are for the United States and are shown for all races and ethnic groups together. For these reasons clinicians treat BMI percentile as a screening step and follow up with growth history, family history, diet, activity and, when needed, other measurements.

When to talk with a pediatrician

Bring the result to a pediatrician if the percentile is below the 5th or at or above the 85th, if it has crossed two or more major percentile lines (for example from the 25th to the 75th), or if you are worried about a child's eating, energy, sleep or mood. A clinician can check height and weight history, family background, puberty stage and other health factors. This calculator cannot diagnose or treat any condition, and it is not a reason to put a child on a diet.

CDC charts or WHO charts?

For children age 2 to 20 the CDC recommends its 2000 growth charts, which this calculator uses. For infants and toddlers from birth to 24 months, CDC and the American Academy of Pediatrics recommend the WHO growth standards, which describe how breastfed babies grow in optimal conditions. For that younger group use the Baby Growth Percentile Calculator.

Frequently asked questions

What is a healthy BMI percentile for a child?

The CDC calls the 5th percentile up to (but not including) the 85th percentile a healthy weight range for children and teens. The 50th percentile is the median for the age and sex.

Is the 90th percentile of BMI bad?

A BMI between the 85th and 95th percentile is categorized as overweight by the CDC. It is a signal to look at growth history, diet, activity and family factors with a clinician, not a diagnosis by itself.

What BMI is the 95th percentile?

It depends on age and sex. For example, the 95th percentile for a 10-year-old boy is about 22.1 kg/m2 and for a 15-year-old girl about 28.1 kg/m2. The tables above list the values by age.

Why does my doctor's result differ slightly?

Different tools round age, height or weight differently or interpolate between months in different ways. Results should agree within about one percentile point when the inputs are the same.

Can I use this for a child under 2?

No. The CDC BMI-for-age charts begin at 2 years. For babies and toddlers use the WHO growth standards in the Baby Growth Percentile Calculator.

Does BMI percentile apply to adults?

No. After age 20 use the adult BMI calculator, which uses fixed cutoffs and does not depend on age or sex.

Sources

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