What Is BMI? The Number Explained
BMI stands for body mass index. This page explains, in plain words, what that number is, how it is figured, and what it cannot tell you.
General information only — not medical advice.
What BMI is
BMI is a number calculated from a person's weight and height.
It is a simple, reliable, and low-cost screening measure.
Screening means it is a first look. It is not the whole picture.
The CDC says it plainly:
“BMI is a screening measure and is not intended to diagnose disease or illness.”
BMI does not measure body fat directly.
The two formulas
There are two ways to figure BMI. They give the same kind of number.
Metric way: weight in kilograms ÷ [height in meters]2
That means: take the height in meters, multiply it by itself, then divide the weight in kilograms by that answer.
U.S. way: [weight in pounds ÷ [height in inches]2] × 703
That means: take the height in inches, multiply it by itself, divide the weight in pounds by that answer, then multiply by 703.
The adult category chart
For adults 20 and older, the CDC uses the same standard categories for everyone, regardless of age or sex.
| BMI | Category on the CDC chart |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy Weight |
| 25.0 – 29.9 | Overweight |
| 30.0 or greater | Obesity |
Inside the obesity range, the CDC chart lists three classes:
| BMI | Class |
|---|---|
| 30.0 – 34.9 | Class 1 |
| 35.0 – 39.9 | Class 2 |
| 40.0 or greater | Class 3 (severe) |
A note about children and teens. Children and teens ages 2 to 19 are read a different way. Their BMI is read by percentile, using the CDC's child and teen calculator. The chart on this page, and the calculator below, are for adults only. For a child or teen, talk with a parent or a doctor instead of using this page's chart.
An example from the CDC
This is the CDC's own example, from its BMI Frequently Asked Questions page.
An adult who is 5 feet 8 inches tall and weighs 200 pounds has a BMI of 30.4.
Here is the math, step by step:
Step 1: 5 feet is 60 inches. 60 + 8 = 68 inches tall in total.
Step 2: 68 × 68 = 4,624. That is the height in inches, multiplied by itself.
Step 3: 200 ÷ 4,624 = about 0.04325.
Step 4: 0.04325 × 703 = 30.4.
A BMI of 30.4 falls in the Obesity category on the CDC chart.
Try the calculator
This calculator is for adults only. It uses the same formulas shown above, and it shows each step of the math.
The number it gives is an EXAMPLE / screening number only. It is a first look, not a finding about anyone's health.
Nothing typed here is sent anywhere. Nothing is stored. The math runs on this page, in your browser.
Pick a way to measure:
EXAMPLE / screening only. A result here is a number on the CDC chart — nothing more. For what a number may mean for one person, talk with that person's healthcare provider.
What BMI cannot tell you
BMI is a screening measure. It is not intended to diagnose disease or illness.
BMI does not measure body fat directly.
BMI is just one measure of health. The CDC advises looking at it alongside other things: results from a physical exam, laboratory findings, and health behaviors.
The CDC advises discussing your BMI category with your healthcare provider. A provider can look at the whole picture for one person. This page cannot.
And one more time, plainly: children and teens ages 2 to 19 are read by percentile, not by this adult chart. This page's chart and calculator are for adults 20 and older only.
Sources
Source: Centers for Disease Control and Prevention (CDC) — Adult BMI pages and BMI Frequently Asked Questions.
Checked November 2026.
General information only — not medical advice.