What is BMI? How to Calculate It and Interpret the Results

What exactly is Body Mass Index?

The Body Mass Index (BMI) is a statistical indicator that relates a person's weight and height to provide a quick estimate of whether their weight is within a healthy range. It was developed in the 19th century by the Belgian mathematician Adolphe Quetelet, and since then it has become the most used screening tool worldwide by doctors, nutritionists and public health organizations such as the World Health Organization (WHO).

BMI does not directly measure body fat nor does it distinguish between muscle, bone, or fat mass. It is a population approach, not an individual diagnosis. That said, for most sedentary or moderately active people, it is a very useful initial indicator. You can calculate your BMI instantly with our BMI Calculator.

How BMI is calculated: formula and example

The formula is simple:

For example, a person who weighs 75 kg and is 1.72 m tall would have a BMI of 75 / (1.72 × 1.72) = 25.35. If your weight is in pounds or your height is in feet and inches, you can convert the units with our Weight Converter before doing the calculation.

Remember that the result is a dimensionless number. Its usefulness lies in comparing it with the standardized tables that we will see below.

BMI classification table according to WHO

BMI	Classification	Associated risk
< 18.5	Low weight	Moderate
18.5 – 24.9	Normal weight	Low
25.0 – 29.9	Overweight	Increased
30.0 – 34.9	Grade I obesity	High
35.0 – 39.9	Grade II obesity	Very high
≥ 40	Grade III obesity	Extreme

These categories apply to adults over 18 years of age. In children and adolescents, percentiles adjusted for age and sex are used. If you are interested in knowing your exact age precisely, our Age Calculator It allows you to know it in years, months and days.

Limitations of BMI that you should know

BMI has important blind spots that should be kept in mind:

• It does not distinguish muscle mass from fat. An athlete with a lot of muscle mass can have an "overweight" BMI while being in excellent physical shape.
• It does not measure fat distribution. Abdominal (visceral) fat is much more dangerous than subcutaneous fat, and BMI does not differentiate between the two.
• It varies by ethnicity. Asian populations have a higher cardiometabolic risk with lower BMI, while in people of African origin the threshold may be somewhat higher.
• It does not apply to pregnant women, elderly people or elite athletes. In these groups, complementary metrics are needed.

For all these reasons, the BMI should be interpreted as a first filter, not as a verdict. For a more complete evaluation, complement your BMI with a waist circumference measurement and, if possible, a body composition analysis. If you're working on your weight goals, also check out our guide on how many daily calories do you need.

What to do after calculating your BMI?

Once you know your result, these are the recommended next steps:

• Normal weight (18.5-24.9): maintain healthy habits. Periodically review your weight and body composition.
• Overweight (25-29.9): adjust your caloric intake and increase physical activity. The Calorie Calculator helps you establish a reasonable deficit.
• Obesity (≥30): consult with a health professional. In addition to diet and exercise, a comprehensive medical approach may be necessary.
• Underweight (<18.5): Evaluate possible underlying causes with your doctor. A sustained nutritional deficit can have serious consequences.

For detailed monitoring of your intake, don't miss our article on how many calories do you need according to your goal. By combining BMI with adequate calorie control, you will have a much clearer view of your health status.