Kids BMI Calculator

A child’s BMI percentile shows how their BMI compares with other children of the same age and sex. A child’s healthy weight range changes constantly as they grow, which is why children are never assessed against the same fixed BMI cutoffs used for adults. Instead, a child’s BMI is compared to a percentile curve built from thousands of other children the same age and sex¹.

What This Calculator Measures

This calculates a child’s BMI, then converts it to a percentile against CDC reference data for their age in months and sex, placing them into one of the CDC’s weight categories, including severe obesity, rather than giving a raw number with no context.

The Formula

BMI = weight (kg) ÷ height (m)²

Up to the 95th percentile, that BMI is converted to a percentile with the CDC’s 2000 growth chart L, M and S values for the child’s age and sex¹:

z = ((BMI ÷ M)^L − 1) ÷ (L × S), and the percentile is the share of a normal distribution below z

Above the 95th percentile, the calculator uses the CDC’s extended BMI-for-age method from December 2022³, where P95 is the BMI at the 95th percentile and σ is a spread value the CDC publishes for each age⁵:

Percentile = 90 + 10 × Φ((BMI − P95) ÷ σ)

Why the CDC Extended the Charts in 2022

The 2000 growth charts were smoothed from data that stopped at the 97th percentile, so they can’t tell very high BMIs apart and squeeze them into a narrow band just below the 100th percentile. The extended method adds data on 8,777 children and teens with obesity, mostly from national surveys between 1988 and 2016, to model BMI above the 95th percentile. Every percentile up to the 95th stays exactly as it was³. Just above the 95th, extended percentiles run slightly lower: a 4-year-old boy with a BMI of 18.2 is at the 96.9th percentile on the original charts and the 95.6th on the extended ones³.

How This Calculator Computes Your Result

Enter your child’s sex, age in years and months, height, and weight. We calculate BMI, look up the CDC values for that month of age, and return the percentile, z-score and category. At or above the 95th percentile, the result also shows BMI as a percent of the 95th percentile, the measure the CDC and the American Academy of Pediatrics use to grade severe obesity⁴.

What the Categories Mean

The American Academy of Pediatrics splits obesity further. Class 2 is 120% to under 140% of the 95th percentile or a BMI of 35 to under 40, and class 3 is 140% of the 95th percentile or more, or a BMI of 40 or more⁴. Class 1 is everything from the 95th percentile up to 120% of it.

BMI Cut-Offs by Age

These are the BMI values at each birthday from the CDC data file⁵. Between birthdays the values change month by month, which is why the calculator asks for months as well.

Boys

Girls

Why Childhood BMI Percentile Is Tracked at All, Not Just a Snapshot Measure

Childhood BMI percentile isn’t just descriptive, it carries real predictive weight for adult outcomes.

A long-term study following children in the Fels Longitudinal Study from childhood into adulthood found that BMI percentile in childhood and adolescence meaningfully predicted the probability of adult overweight and obesity, the higher the childhood percentile and the older the child, the greater the predicted risk of being overweight decades later².

That’s part of why pediatric BMI screening exists at all, it isn’t just a snapshot of current size, it’s tracking a pattern that tends to carry forward, which is also exactly why a single reading matters less than the trend over multiple visits.