What is BMI?
Body mass index (BMI) is a simple measure of weight relative to height, used worldwide to screen for weight categories that may be linked to health problems. It is calculated as weight in kilograms divided by height in meters squared: BMI = kg ÷ m². In U.S. units: BMI = 703 × lb ÷ in².
Adult BMI categories (CDC and WHO)
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity class 1 |
| 35.0 – 39.9 | Obesity class 2 |
| 40.0 and above | Obesity class 3 (severe) |
These categories apply to adults 20 and older (18+ in Canada). Health Canada uses the same cut-offs.
Limitations of BMI
BMI doesn't measure body fat directly. Very muscular people, such as athletes, may have a high BMI without excess fat, and older adults may have a normal BMI with low muscle mass. Some health organizations suggest lower cut-offs for people of Asian descent. Waist circumference is a useful addition: health risk rises with a waist over about 40 inches (102 cm) for men and 35 inches (88 cm) for women. Talk to a healthcare provider for a full assessment.
Healthy weight and next steps
The calculator shows the weight range that corresponds to a BMI of 18.5–24.9 for your height. If you want to change your weight, our calorie calculator estimates your daily energy needs. Gradual changes of about 0.5–2 pounds (0.25–1 kg) per week are generally recommended.
Frequently asked questions
What is a healthy BMI?
What is the BMI for 5'9" and 165 lbs?
Is BMI accurate for athletes?
Can children use this calculator?
Sources & methodology
This calculator was last reviewed on November 1, 2026. Figures are based on the following official and primary sources:
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