How to Calculate BMI
Calculating BMI is simple and can be done in seconds. The formula was developed by the Belgian mathematician Adolphe Quetelet in the 19th century and is used worldwide as a health indicator.
BMI Formula:
BMI = Weight (kg) ÷ Height² (m²)
Practical Example: If you weigh 75kg and are 1.70m tall:
- 1. Square the height: 1.70 × 1.70 = 2.89
- 2. Divide the weight by the result: 75 ÷ 2.89 = 25.95
- 3. Result: BMI of 25.95 (slight overweight)
History of BMI
The Body Mass Index was created between 1830 and 1850 by the Belgian mathematician and statistician Adolphe Quetelet. Originally called the "Quetelet Index", the measure was developed for statistical studies on obesity in the general population, not for individual assessment.
It was only in the 1970s that the term "Body Mass Index" (BMI) began to be widely used in medical literature. In 1995, the World Health Organization formalized the classifications we use today, making BMI the world's most popular screening tool for assessing nutritional status.
Despite criticisms and limitations, BMI remains relevant as it is a quick, inexpensive, and non-invasive method for identifying health risks related to weight in large populations.
Complete BMI Classification Table
| BMI | Classification | Risk of Diseases |
|---|---|---|
| Below 18.5 | Underweight | Low (other risks) |
| 18.5 - 24.9 | Normal weight | Normal |
| 25.0 - 29.9 | Overweight | Elevated |
| 30.0 - 34.9 | Obesity Grade I | Moderate |
| 35.0 - 39.9 | Obesity Grade II | High |
| Above 40 | Obesity Grade III | Very high |
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Frequently Asked Questions
What is BMI and how is it calculated?
BMI (Body Mass Index) is an international measure used to assess whether a person is at their ideal weight. It is calculated by dividing the weight (in kg) by the height (in meters) squared: BMI = weight ÷ (height × height). For example, a person with 70kg and 1.75m has BMI = 70 ÷ (1.75 × 1.75) = 22.86.
What are the BMI classifications according to the WHO?
The World Health Organization establishes the following ranges: Below 18.5: Underweight; 18.5 to 24.9: Normal weight; 25.0 to 29.9: Overweight; 30.0 to 34.9: Obesity grade I; 35.0 to 39.9: Obesity grade II; Above 40: Obesity grade III (morbid).
- Below 18.5: Underweight
- 18.5 to 24.9: Normal weight
- 25.0 to 29.9: Overweight
- 30.0 to 34.9: Obesity grade I
- 35.0 to 39.9: Obesity grade II
- Above 40: Obesity grade III (morbid)
Is BMI accurate for all people?
No. BMI is a useful screening tool, but it has limitations. It does not distinguish muscle mass from fat, so muscular athletes may have a high BMI without being overweight. Children, the elderly, pregnant women, and very muscular people need additional assessments. For a complete analysis, consult a nutritionist or doctor.
What is the difference between male and female BMI?
The BMI formula is the same for men and women. However, women naturally have a higher percentage of body fat than men. Therefore, for a more accurate assessment of body composition, it is also recommended to use body fat percentage measurement by skinfolds or bioimpedance.
How to use BMI to lose or gain weight?
BMI serves as a starting point for establishing healthy weight goals. If your BMI indicates overweight, consult a nutritionist to create an appropriate eating plan. If you are underweight, it may be necessary to increase your calorie intake with professional guidance. The ideal BMI is between 18.5 and 24.9 for most adults.
Useful Comparisons
BMI vs Body Fat Percentage
BMI: Only measures the weight/height ratio. Does not differentiate muscle from fat. Useful for population screening.
Body Fat Percentage: More accurate measurement of body composition. Assesses how much of the total weight is fat. Recommended for athletes and detailed monitoring.
When to Use Each?
Use BMI when: You want a quick assessment, do not have access to specialized equipment, or for general health monitoring.
Use Body Fat Percentage when: You are an athlete, are in hypertrophy training, want to assess body recomposition, or have borderline BMI.