Child BMI Percentile Calculator

Why children get a percentile instead of a BMI number

For adults, BMI has fixed cutoffs: under 18.5 is underweight, 18.5 to 24.9 is normal, 25 to 29.9 is overweight and 30 and above is obesity. Those numbers do not change with age, because adults have stopped growing.

Children are different. Body shape changes constantly through childhood: fat rises in the first year, falls through the preschool years, then rises again through puberty. A BMI that is perfectly ordinary at 15 can be well above the healthy range at 5. So instead of comparing a child to a fixed number, you compare them to other children of the same age and the same sex.

That comparison produces a percentile. The 50th percentile is the middle of the range. The 85th percentile means the child's BMI is higher than 85 out of every 100 children the same age and sex. The 95th means higher than 95 out of 100.

This tool uses the CDC's own growth chart parameters for children aged 2 to 20. It is a reference estimate for screening, not medical advice. Everything runs in your browser, nothing you type is uploaded or stored.

How the percentile is calculated

The CDC publishes a table of three numbers for every combination of sex and single month of age from 24 to 240 months. They are called LMS parameters:

• L, the power used to make the distribution behave like a normal curve
• M, the median BMI for that age and sex
• S, the coefficient of variation, which describes the spread

From those three numbers, a z-score is calculated:

z = ((BMI ÷ M)^L − 1) ÷ (L × S)

The z-score is then converted to a percentile using the standard normal distribution. A z-score of 1.645 is the 95th percentile, 1.036 is the 85th, and 0 is the 50th. This tool interpolates between the published monthly rows so that a child aged 7 years 4 months gets an answer built from the correct parameters, not from a rounded-down age.

One honest caveat. In 2022 the CDC introduced extended BMI-for-age charts, which use a different formula for children whose BMI is at or above the 95th percentile. Above that point the extended method produces a somewhat lower z-score than the classic LMS method. This tool reports the classic LMS value and flags any result at or above the 95th percentile, and it also shows what percentage of the 95th percentile value your child's BMI represents, which is how the severe obesity category is defined.

Worked example: a 10-year-old boy, 55 inches, 100 pounds

Start with BMI using the imperial formula: BMI = 703 × pounds ÷ inches².

703 × 100 = 70,300. Then 55 × 55 = 3,025. So 70,300 ÷ 3,025 = 23.24.

On an adult chart, 23.24 sits comfortably in the normal range. For a 10-year-old boy it does not. At 120.5 months, the CDC parameters give a 50th percentile BMI of about 16.6, an 85th percentile of about 19.4 and a 95th percentile of about 22.2.

His BMI of 23.24 is above 22.2, so he is above the 95th percentile, the obesity range, and his BMI is about 105% of the 95th percentile value. The classic LMS calculation gives a z-score of about +1.81 and a percentile near the 96.5th.

Now the practical part. If his height stays at 55 inches, the healthy weight range for that height is BMI 14.2 to 19.4, which converts to about 61 to 84 pounds. He is 100 pounds. That is the number a pediatrician would discuss: not "lose 16 pounds", but "the range that keeps BMI in the healthy band for this height".

Worked example: a 5-year-old girl, 42 inches, 38 pounds

Same formula. 703 × 38 = 26,714. And 42 × 42 = 1,764. So 26,714 ÷ 1,764 = 15.14.

At 60.5 months, the CDC parameters for a girl give a 5th percentile BMI of about 13.5, a 50th of about 15.2, an 85th of about 16.8 and a 95th of about 18.3.

A BMI of 15.14 is right at the 50th percentile, z-score of about −0.01, percentile 49.8. Squarely in the middle of the healthy range. Her healthy weight range at 42 inches runs from about 34 pounds (BMI 13.5) to about 42 pounds (BMI 16.8).

Compare the two children and the point becomes obvious. The 5-year-old girl has a BMI of 15.1 and is at the 50th percentile. The 10-year-old boy has a BMI of 23.2 and is above the 95th. If you swapped their BMIs, both would be wrong for their age. That is exactly why the percentile exists.

What each band means

These are the CDC's weight status categories for children and teenagers aged 2 to 20. They are screening bands, not diagnoses.

Two things worth remembering about these bands. First, the boundaries are population statistics, not magic lines, a child at the 86th percentile is not in a different world from one at the 84th. Second, percentile bands describe size, not health. Fitness, blood pressure, sleep, diet and family history all matter independently.

Reference BMI values by age and sex

These are the CDC values this calculator uses, rounded to one decimal place. Find your child's age and sex, then see where their BMI lands.

Read the 95th percentile column from top to bottom and you can see the point of the whole exercise: it climbs from 19.3 at age 2 to 30.6 at age 20 for boys. A fixed adult cutoff would misclassify thousands of children at both ends.

Note also the dip between ages 5 and 6, when BMI naturally reaches its lowest point before the rise through puberty. Pediatricians sometimes call this the adiposity rebound, and the age at which it happens is itself a growth milestone.

What a percentile cannot tell you

A percentile is one measurement on one day. It is genuinely useful, and it has real limits.

• It does not measure body fat. BMI has no idea whether the weight is muscle, bone or fat. A muscular teenage athlete can sit at the 90th percentile with low body fat, and a child who has lost muscle can sit lower than their fat level suggests.
• It does not describe the trend. A child steady at the 90th percentile for four years is a different situation from one who jumped from the 50th to the 90th in a year. Pediatricians plot growth over time for exactly this reason.
• It does not account for height velocity or puberty timing. A child who has not started their growth spurt yet can look heavier than they will in 18 months.
• It does not cover under-2s. For babies and toddlers the CDC recommends the World Health Organization growth standards, which are built on a different reference population.
• It is not a diagnosis. A percentile outside the healthy band is a prompt for a conversation, not a conclusion.

Where to read more: the CDC growth charts site has the full percentile data files and the LMS parameters this tool uses, the WHO child growth standards cover birth to 5 years, and the American Academy of Pediatrics publishes parent-facing guidance at HealthyChildren.org.

And the disclaimer that matters: this is a reference estimate, not medical advice. It applies published formulas to the numbers you enter. It does not know your child's history, their growth curve, their blood pressure or their family background. Do not start, stop or change anything based on a number from a web page, bring the result to your pediatrician and let them interpret it.