What Is BMI?

Body Mass Index (BMI) is a simple calculation that uses your height and weight to estimate whether you fall into an underweight, healthy, overweight, or obese category. The formula is: BMI = weight (kg) ÷ height² (m²). For those using imperial units: BMI = (weight in lbs × 703) ÷ height² (in inches).

BMI Categories

The World Health Organization defines the following BMI ranges for adults:

  • Below 18.5: Underweight — may indicate malnutrition or other health concerns
  • 18.5 – 24.9: Normal/Healthy weight — associated with the lowest health risks
  • 25.0 – 29.9: Overweight — increased risk of cardiovascular disease and type 2 diabetes
  • 30.0 and above: Obese — significantly elevated risk of chronic diseases

The Limitations of BMI

BMI is a screening tool, not a diagnostic one. It has several well-documented limitations. First, it does not distinguish between muscle and fat — a muscular athlete may have a high BMI despite having very low body fat. Second, it doesn't account for fat distribution; visceral fat (around the abdomen) is far more dangerous than subcutaneous fat, yet BMI treats all weight the same. Third, BMI thresholds were originally developed using data from European populations and may not be equally accurate for people of Asian, African, or Hispanic descent.

What to Use Alongside BMI

For a more complete health picture, consider these additional metrics: waist circumference (men above 40 inches and women above 35 inches face higher cardiovascular risk), body fat percentage (use our Body Fat Calculator), waist-to-hip ratio, and blood pressure and cholesterol levels. Together, these give a far more accurate assessment than BMI alone.

How to Lower Your BMI Healthily

If your BMI is in the overweight or obese range, a combination of caloric deficit and increased physical activity is the most evidence-based approach. A deficit of 500 calories per day leads to approximately 1 pound of weight loss per week. Use our Calorie Calculator to find your daily calorie target, and our BMR Calculator to understand your baseline metabolic rate.

The Bottom Line

BMI is a useful starting point for assessing weight-related health risks, but it should never be used in isolation. Use it as one data point among many, and consult a healthcare provider for a comprehensive evaluation. Try our free BMI Calculator to find your number instantly.