BMI Calculator
Body mass index (BMI) is a simple measure of weight in relation to height. It is one of the criteria used when considering bariatric surgery: surgery is generally considered for a BMI of 40 or higher, or 35 or higher with obesity-related health conditions.
Standard BMI categories
- Below 18.5 — underweight
- 18.5–24.9 — healthy weight
- 25.0–29.9 — overweight
- 30.0–34.9 — obesity (class I)
- 35.0–39.9 — obesity (class II)
- 40.0 and above — severe obesity (class III)
BMI is a screening measure, not a diagnosis. It does not account for muscle mass, body composition, or individual health factors — discuss your results with a healthcare professional.
Frequently asked questions
How is BMI calculated?
BMI is body weight divided by the square of height: kilograms divided by meters squared. In US units, multiply weight in pounds by 703 and divide by height in inches squared. The calculator above does this for you in either unit system.
What BMI is needed to qualify for bariatric surgery?
As a general rule, surgery is considered for a BMI of 40 or higher, or 35 or higher with a serious obesity-related condition such as type 2 diabetes, hypertension, or sleep apnea. Final eligibility depends on a full evaluation by a bariatric team — read more in the overview of bariatric surgery on our homepage.
Is BMI accurate for everyone?
No. BMI is a population-level screening measure. It does not distinguish muscle from fat, does not reflect fat distribution, and can misclassify very muscular people, older adults, and some ethnic groups. It should be interpreted alongside other measures and a clinical assessment.
What do the BMI categories mean?
Below 18.5 is underweight, 18.5–24.9 healthy weight, 25.0–29.9 overweight, 30.0–34.9 class I obesity, 35.0–39.9 class II obesity, and 40.0 or above class III (severe) obesity. These are the standard categories used in research, including the LABS study of bariatric surgery outcomes.