What is BMI?
Body Mass Index (BMI) is a simple calculation — weight divided by height squared — used as a quick, population-level screening tool to flag whether someone's weight might be in a healthy range for their height. For adults, the same fixed categories apply regardless of age or sex. For children and teens, the same BMI number means something different at different ages, so this calculator switches to an age- and sex-specific percentile once you select Child & Teen above.
How to use it
- Choose Adult or Child & Teen depending on who this is for.
- For children and teens, also select sex and age in years and months.
- Choose Metric or Imperial units and enter height and weight — the result updates as you type.
- The badge (and, for children, the percentile) shows the category your result falls into.
Why children and teens are different
A BMI of 18 might be perfectly healthy for a 9-year-old and underweight for a 25-year-old — kids are still growing, and how much body fat is typical changes with age and differs between boys and girls. For ages 2–19, CDC uses BMI-for-age percentiles instead of fixed numbers: under the 5th percentile is underweight, 5th to under the 85th is a healthy weight, 85th to under the 95th is overweight, and the 95th percentile or above is in the obesity range.
The child/teen mode on this page calculates that percentile from the same reference growth data and method (the LMS method) used in CDC's own published growth charts. As with any growth measurement, a pediatrician can give you the fuller clinical picture, especially for a result near a category boundary or alongside other health factors.
Good to know — BMI's limitations
BMI doesn't distinguish between muscle and fat, so athletes and very muscular people can score as "overweight" despite low body fat, and it can under- or over-estimate risk in older adults, pregnant people, and some ethnic groups. It's a useful starting point, not a diagnosis — talk to a doctor or healthcare professional for a fuller picture of health, especially for a child or teen.
Worked example
Frequently asked questions
For adults 20 and over, a BMI from 18.5 up to 25 is generally classed as a healthy weight, under 18.5 as underweight, 25 up to 30 as overweight, and 30 or above as the obesity range. These are the standard WHO adult thresholds used worldwide.
Children are still growing, and how much body fat is typical changes constantly with age and differs between boys and girls — a BMI of 18 can be perfectly healthy for a 9-year-old but underweight for a 25-year-old. So for ages 2–19, this calculator switches to a percentile that compares a child's BMI to others of the same age and sex, using the same CDC growth-chart reference data and LMS method behind CDC's own published charts.
It's a useful, quick screening estimate, not a direct measurement of body fat. Because it only uses height and weight, it can't tell the difference between muscle and fat — a very muscular adult can register as "overweight" despite having low body fat. It's best treated as a starting point for a conversation with a healthcare professional, not a diagnosis on its own.
BMI Prime expresses someone's BMI as a ratio against the upper limit of the normal range (25), so a BMI Prime of 1.0 sits exactly at that boundary. This calculator shows the standard BMI value and category directly rather than BMI Prime, since the plain number and percentile are what most people are actually looking for.
BMI is known to be less reliable for pregnant people (due to pregnancy weight gain) and for older adults (where muscle loss can mask higher body fat), so results in those cases should be discussed with a doctor rather than read at face value.
About & privacy
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