What is BMI and how is it calculated?

What is BMI and How is it Calculated?

Body Mass Index (BMI) is a numerical measure that provides an indication of whether a person’s weight is appropriate for their height. It’s a widely used tool to assess potential health risks associated with weight status, helping individuals and healthcare professionals gauge whether someone is underweight, normal weight, overweight, or obese. Understanding BMI and how it is calculated is essential for evaluating overall health and making informed decisions about lifestyle and wellness. Here’s a comprehensive overview:

1. BMI Definition:

BMI is a calculation based on an individual’s weight and height. It provides a rough estimate of body fat and is used to screen for potential health problems related to weight.

2. BMI Calculation Formula:

BMI is calculated using the following formulas, depending on whether you use metric or imperial units:

  • Metric Units (kilograms and meters):BMI=weight in kilograms(height in meters)2BMI = \frac{{\text{weight in kilograms}}}{{(\text{height in meters})^2}}
  • Imperial Units (pounds and inches):BMI=weight in pounds(height in inches)2×703BMI = \frac{{\text{weight in pounds}}}{{(\text{height in inches})^2}} \times 703

3. Step-by-Step Calculation:

Using Metric Units:

  • Step 1: Measure your weight in kilograms (kg).
  • Step 2: Measure your height in meters (m). If your height is in centimeters (cm), divide by 100 to convert to meters.
  • Step 3: Square your height in meters (height ×\times height).
  • Step 4: Divide your weight by the result from Step 3 to get your BMI.

Example Calculation:

  • Weight = 70 kg
  • Height = 1.75 m
  • BMI = 70(1.75)2\frac{{70}}{{(1.75)^2}}
  • BMI ≈ 703.0625\frac{{70}}{{3.0625}}
  • BMI ≈ 22.86

Using Imperial Units:

  • Step 1: Measure your weight in pounds (lbs).
  • Step 2: Measure your height in inches (in).
  • Step 3: Square your height in inches (height ×\times height).
  • Step 4: Multiply the result from Step 3 by 703.
  • Step 5: Divide your weight by the result from Step 4 to get your BMI.

Example Calculation:

  • Weight = 154 lbs
  • Height = 68 inches
  • BMI = 154(68)2×703\frac{{154}}{{(68)^2}} \times 703
  • BMI ≈ 1544624×703\frac{{154}}{{4624}} \times 703
  • BMI ≈ 1543259792\frac{{154}}{{3259792}}
  • BMI ≈ 23.36

4. Interpreting BMI Results:

Once you have calculated your BMI, you can interpret the results using standard categories:

  • Underweight: BMI less than 18.5
  • Normal weight: BMI between 18.5 and 24.9
  • Overweight: BMI between 25.0 and 29.9
  • Obesity (Class 1): BMI between 30.0 and 34.9
  • Obesity (Class 2): BMI between 35.0 and 39.9
  • Severe Obesity (Class 3 or Morbid Obesity): BMI 40.0 and above

BMI categories provide a general guideline for assessing weight status and potential health risks associated with different ranges.

5. Considerations for BMI:

  • Muscle Mass vs. Fat Mass: BMI does not differentiate between muscle mass and fat mass. Athletes or individuals with higher muscle mass may have a higher BMI despite having a lower body fat percentage.
  • Individual Health Factors: Other factors such as age, sex, muscle mass, and ethnicity can influence BMI interpretation. It’s important to consider BMI alongside other health indicators for a comprehensive assessment.
  • Health Recommendations: Depending on your BMI category, healthcare professionals may recommend lifestyle changes, such as dietary modifications and increased physical activity, to achieve and maintain a healthy weight.

6. Using BMI as a Health Tool:

BMI serves as a valuable initial screening tool for assessing weight status and potential health risks related to weight. Regularly monitoring changes in BMI can help individuals make informed decisions about their health and well-being, guiding efforts towards maintaining a healthy weight and reducing associated health risks.


BMI is a simple yet effective tool for assessing whether an individual’s weight is within a healthy range relative to their height. By understanding how to calculate BMI using metric or imperial units and interpreting the results based on established categories, individuals can gain valuable insights into their weight status and potential health risks. Incorporating healthy lifestyle choices and consulting with healthcare professionals can further support efforts to achieve and maintain optimal health and well-being based on BMI assessments.