What is BMI?

BMI (Body Mass Index) is a simple weight-for-height index used to classify underweight, normal weight, overweight, and obesity in adults. Because it uses only height and weight, BMI is quick to compute and helpful for population-level screening. That said, BMI does not capture body composition and can be misleading for muscular individuals, older adults with low muscle mass, or people with unusual body proportions. Use BMI alongside other measures such as waist circumference, body fat percentage, and blood markers.

BMI Formula

• Metric: BMI = weight (kg) / height (m)^2
• US/Imperial: BMI = 703 × weight (lb) / height (in)^2

Categories

• < 16: Severe Thinness
• 16 – 16.9: Moderate Thinness
• 17 – 18.4: Mild Thinness
• 18.5 – 24.9: Normal
• 25 – 29.9: Overweight
• 30 – 34.9: Obese Class I
• 35 – 39.9: Obese Class II
• ≥ 40: Obese Class III

Limitations

• Does not distinguish muscle vs fat
• Can overestimate risk in athletes with high lean mass
• Can underestimate risk in people with low muscle mass (e.g., elderly)
• Use alongside other measures (waist circumference, body fat %, metabolic markers)

Try Our BMI Calculator

Quickly compute BMI in metric or imperial units: BMI Calculator

After calculating BMI, consider tracking progress over time instead of focusing on a single number. Small sustained changes in nutrition and activity usually matter more than day-to-day fluctuations.