BMI formula
US units: BMI = 703 × weight (lb) ÷ height² (in)
Age is not in either formula. A 70 kg adult who is 1.75 m tall has BMI 70 ÷ 1.75² = 22.86. The same height and weight produce the same BMI at every age. What changes is the reference used to interpret that number.
BMI for adults age 20 and older
The CDC uses standard adult categories beginning at age 20: below 18.5 is underweight, 18.5 to 24.9 is healthy weight, 25.0 to 29.9 is overweight, and 30.0 or higher is obesity. Obesity is commonly divided into classes 1, 2 and 3 at BMI 30, 35 and 40.
Age and sex do not change these standard adult cutoffs. That does not mean every adult with the same BMI has the same health profile. Muscle, body composition, fat distribution, health history and other measurements can change the clinical context.
BMI for children and teens ages 2–19
Children grow at different rates, so fixed adult cutoffs are not used. After calculating BMI, the value is compared with children of the same age and sex in the CDC reference population. The result is called a BMI-for-age percentile.
The CDC categories are below the 5th percentile for underweight, the 5th to below the 85th for healthy weight, the 85th to below the 95th for overweight, and the 95th percentile or higher for obesity. CDC also defines severe obesity using at least 120% of the age- and sex-specific 95th percentile or BMI of at least 35 kg/m².
What a BMI percentile means
A percentile ranks the BMI within a reference distribution. A result at the 75th percentile means the BMI is equal to or higher than the BMI of about 75% of children of the same age and sex in the reference population. It is not a body-fat percentage and not a score out of 100.
Age should be entered as precisely as possible. CDC reference values are provided by month, so a 10-year-old at 0 months and a 10-year-old at 11 months can have different reference values even with the same BMI.
How the child percentile is calculated
The CDC growth-chart data provide L, M and S parameters for each age and sex. M is the median, S describes spread and L adjusts the distribution’s shape. For BMI values at or below the 95th percentile, the published LMS transformation produces a z-score:
The z-score is converted through the standard normal distribution to estimate the percentile. The OfficeCalculator tool bundles the CDC data locally, uses age in whole months and avoids displaying a falsely precise ordinary percentile above the 95th; it reports that boundary and the percentage of the age-specific 95th-percentile BMI instead.
Why the same BMI changes meaning with age
Consider two growing children with identical BMI values. Their percentile may differ because the reference median and spread change month by month and differ between the CDC male and female curves. That is exactly why entering age and selecting the correct reference curve are necessary. An adult lookup table cannot answer the child or teen question accurately.
Measure height and weight carefully
Small errors matter because height is squared. Measure without shoes, use a firm level surface for the scale and record values as accurately as the equipment permits. For a child, a repeat measurement and the pattern across visits are often more useful than reacting to a small change near one cutoff.
What BMI cannot tell you
BMI does not directly measure body fat, muscle mass, bone density, fat distribution, blood pressure, blood sugar or fitness. It is a screening measure. It can be less informative for highly muscular adults, during pregnancy and in other individual circumstances. For a child or teen, a healthcare professional can consider the growth curve, puberty, medical history, family history and other findings.
Do not use an online percentile to diagnose, label or start a restrictive diet for a child. Discuss concerns with a qualified clinician.
Sources
- CDC Child and Teen BMI Calculator
- CDC Child and Teen BMI Categories
- CDC Growth Charts Data Files and LMS Method
Frequently asked questions
Does adult BMI change with age?
The calculated BMI and standard adult cutoffs do not. Age can still affect how a clinician interprets the result in context.
At what age should adult BMI be used?
CDC guidance directs people age 20 and older to adult BMI categories. Ages 2–19 use BMI-for-age.
Why does child BMI need sex?
The CDC reference curves are sex-specific because growth patterns differ in the reference data.
Is BMI percentile the same as body-fat percentage?
No. A percentile is a rank within an age- and sex-specific reference population.