Almost every number on this site starts with one simple measurement: BMI, short for body mass index. It compares a person’s weight with their height, so that a tall person and a short person can be compared fairly.
The formula
BMI is a person’s weight in kilograms divided by the square of their height in metres. For example, someone who is 1.70 m tall and weighs 70 kg has a BMI of 70 ÷ (1.70 × 1.70) ≈ 24.2.
“BMI is a calculated measure of a person's body weight (in kilograms) divided by the square of their height (in meters).”
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BMI
24.2
Healthy weight
- Underweight: under 18.5
- Healthy weight: 18.5 to under 25
- Overweight: 25 to under 30
- Obesity class 1: 30 to under 35
- Obesity class 2: 35 to under 40
- Obesity class 3 (severe): 40 or more
For adults aged 20 and over. For children and teens, BMI is compared with growth charts for their age and sex instead. BMI is a screening number, not a diagnosis — only a health professional can tell you what it means for you. Nothing you type here leaves your device.
What the numbers mean for adults
For adults, WHO defines overweight as a BMI of 25 or more, and obesity as a BMI of 30 or more. CDC splits adult obesity into class 1 (BMI 30 to under 35), class 2 (35 to under 40) and class 3, also called severe obesity (40 or more).
“overweight is a BMI greater than or equal to 25; and obesity is a BMI greater than or equal to 30.”
Children and teens are different
Children are still growing, so a single cut-off would not work. Instead, a child’s BMI is compared with other children of the same age and sex. For ages 5–19, overweight means BMI-for-age more than 1 standard deviation above the WHO Growth Reference median; obesity means more than 2 standard deviations above it.
“overweight is BMI-for-age greater than 1 standard deviation above the WHO Growth Reference median; and obesity is greater than 2 standard deviations above the WHO Growth Reference median.”
In the United States, CDC growth charts are used instead: obesity means a BMI at or above the 95th percentile for the child’s sex and age.
A screening number, not a diagnosis
BMI is a quick screening measure of weight relative to height — for an individual it should be considered alongside medical history, habits, a physical exam and lab tests. Read more in the limits of BMI.
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Written by the ObesitySurvey team. Not medical advice.