BMI calculator
Body mass index is your weight divided by the square of your height. It is a rough screening number, widely used because it is easy to calculate - not because it is precise.
What the categories are
For adults aged 20 and over, the CDC groups BMI values into these ranges. They are the same for men and women.
| BMI | CDC category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25.0 to 29.9 | Overweight |
| 30.0 and above | Obesity |
What BMI does not tell you
BMI is a ratio of weight to height and nothing else. It cannot distinguish muscle from fat, it says nothing about where body fat sits, and it was never designed to assess an individual - it came out of population statistics.
Specifically, the number is a poor guide in these situations:
- Muscular people. A well-trained athlete can land in the "overweight" or "obesity" range on muscle mass alone.
- Older adults. Muscle is lost with age, so a stable BMI can hide a real change in body composition.
- Children and teenagers. The adult categories do not apply at all; growth charts and percentiles are used instead.
- Pregnancy. BMI is not interpreted the usual way during pregnancy.
- Different populations. Some health bodies use different thresholds for people of South Asian and some other ancestries, because metabolic risk appears at lower BMI values in those populations.
A related number that some health bodies now prefer as a first screen is the waist-to-height ratio, because it captures where weight sits rather than just how much there is.
Where this comes from
The formula and the adult categories are published by the CDC: cdc.gov/bmi.
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