How BMI Is Calculated: Adult Categories, Child Percentiles and the 2022 Extended Charts

BMI is one formula, but the category depends on rounding for adults and on age and sex for children. We checked the CDC’s tables and worked examples against the math.

The Formula

Body mass index is weight divided by height squared: kilograms over meters squared, or, in US units, pounds times 703 over inches squared, which is the version the CDC uses. At 5 ft 9 in (69 inches) and 160 lb that is 160 × 703 ÷ 69² = 23.6. Our BMI Calculator does both, for adults and for children and teens.

Adult Categories, and the Rounding at the Edges

For adults 20 and older the CDC uses one set of categories regardless of age, sex or race: underweight below 18.5, healthy weight from 18.5 to under 25, overweight from 25 to under 30, and obesity at 30 or more, divided into class 1 (30 to under 35), class 2 (35 to under 40) and class 3, also called severe obesity (40 or more).

The CDC’s own example for a 5 ft 9 in adult lists 125 to 168 lb as healthy weight and 169 to 202 lb as overweight. But 169 lb at that height is a BMI of 24.954, which is under 25. The table only works if the BMI is rounded to one decimal first: 24.954 shows as 25.0. We checked every pound from 80 to 400 at several heights; at 5 ft 9 in, rounding moves four weights into the next category up, and the CDC table agrees with the rounded version every time:

At 5 ft 6 in no weight in that span sits close enough to a cutoff to flip; at 6 ft 3 in five do (148, 200, 240, 280 and 320 lb). It is a small effect, but it is why two calculators can disagree by one category for the same person. Ours classifies on the rounded value so it matches the CDC’s table.

Children and Teens: Percentiles, Not Cutoffs

Children’s body proportions change as they grow, so the CDC compares a child’s BMI with others of the same sex and age, using the 2000 growth charts. The categories for ages 2 through 19 are percentiles: underweight below the 5th, healthy weight from the 5th to under the 85th, overweight from the 85th to under the 95th, obesity at the 95th or above, and severe obesity at 120% of the 95th percentile or a BMI of 35 or more. The same number means very different things at different ages. A BMI of 20 for a girl:

That is also why the adult categories should not be applied to a child: a BMI of 20 is “healthy weight” on the adult scale at every one of these ages.

How the Percentile Is Calculated

Each month of age has three published numbers for each sex: L (a power that removes skew), M (the median BMI) and S (a measure of spread). Up to the 95th percentile the z-score is ((BMI ÷ M)^L − 1) ÷ (L × S), and the percentile is the share of a normal distribution below that z-score. The CDC’s worked example: a girl of 9 years 6 months (114.5 months) with a BMI of 21.2 has L = −2.2578, M = 16.576 and S = 0.1328, giving z = 1.4215 and the 92.2nd percentile. Our calculator reproduces it exactly.

Above the 95th Percentile: the 2022 Extended Charts

The original 2000 curves were never meant for very high BMIs, and above the 95th percentile they squeeze very different children into nearly the same z-score. In December 2022 the CDC released extended BMI-for-age charts based on Wei and colleagues’ method: above the 95th percentile, the percentile is 90 + 10 × Φ((BMI − P95) ÷ σ), where P95 is the 95th-percentile BMI and σ a spread that grows with age. Below the 95th percentile nothing changes. For a boy of 10 years 6 months the difference is large:

On the old curves a BMI of 38 looks barely different from 30; on the extended charts the gap is clear. Our calculator uses the extended method above the 95th percentile and shows z-scores of 5 or more as “5 or more” because the extended charts stop there.

What BMI Does Not Tell You

The CDC calls BMI a screening measure. It does not measure body fat directly or tell muscle from fat, and it should be considered with other factors. A result in any category is a reason to talk with a health care provider, who can look at the whole picture, rather than a diagnosis.

How We Checked These Numbers

All the figures here come from the calculator’s own functions. The growth-chart values were accepted only after they matched the CDC’s published parameters to every digit, and the calculator matches the CDC’s worked examples, its 5 ft 9 in table, and an independent calculation on 3,000 adults and 4,003 children.

Frequently Asked Questions

Published Oct 14, 2026 by Derek Giordano