Body Mass Index (BMI) is one of the most common metrics used to get a quick snapshot of your health. It’s a simple mathematical formula that assesses whether your weight is appropriate for your height. But what does that number actually mean? And more importantly, what are its limitations?

This guide will walk you through everything you need to know, from how to calculate your BMI to how you should—and shouldn’t—use it to make decisions about your health.

Key Takeaways

  • BMI is a screening tool, not a diagnostic one.
  • It estimates body fat based on height and weight but doesn't measure it directly.
  • BMI can be inaccurate for very muscular individuals, the elderly, and certain ethnic groups.
  • It should always be used alongside other health markers like waist circumference, blood pressure, and diet.

What is BMI and How is It Calculated?

BMI is a number calculated from a person's weight and height. It provides a simple, inexpensive way to categorize weight status and screen for potential health problems. The formula is:

BMI = weight (kg) / [height (m)]²

While you can do the math yourself, our calculator does it for you instantly and accurately.

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Get your BMI number and see which category you fall into with our free, world-class tool.

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Official BMI Categories Chart

According to the World Health Organization (WHO), adults can be categorized based on their BMI score. Here’s what the ranges mean:

Category BMI Range Health Implication
Underweight Below 18.5 Potential risk of nutritional deficiencies and osteoporosis.
Healthy Weight 18.5 – 24.9 Associated with the lowest risk of weight-related diseases.
Overweight 25.0 – 29.9 Increased risk of developing heart disease, high blood pressure, and type 2 diabetes.
Obese 30.0 and above Significantly higher risk of serious health conditions.

The Critical Limitations: What BMI Doesn't Tell You

While BMI is a useful starting point, it has major limitations because it doesn't account for body composition. It cannot distinguish between fat and muscle. An elite athlete with very high muscle mass could be classified as "obese" by BMI, even though they are incredibly fit. Conversely, an elderly person with low muscle mass could have a "healthy" BMI while still carrying excess body fat.

For this reason, BMI should never be the only metric you use. For a more complete picture, consider using our Body Fat Calculator and consulting with a healthcare professional.

Important Medical Disclaimer

This article is for informational purposes only and does not constitute medical advice. The BMI calculator is a screening tool and should not be used to diagnose any health condition. Always consult a qualified healthcare provider for personal medical advice.