Complete Guide to the BMI Calculator (Body Mass Index)
Learn how to calculate your BMI, understand the WHO classification and use our free online calculator to monitor your physical health.
What is the purpose?
This calculator aims to evaluate your body mass in relation to your height. It is a rapid screening method developed to identify states of thinness, normal weight, overweight and degrees of obesity.
Formula Used
The Body Mass Index (BMI) is calculated by dividing an individual's weight in kilograms by the square of their height in meters. The standard mathematical formula is:
IMC = Peso (kg)Altura (m)2
Example: For a weight of 70 kg and a height of 1.75 m:
1. Calculate the height squared: 1,75 × 1,75 = 3,0625
2. Divide the weight by the previous result: 70 ÷ 3,0625 = 22,86
3. The final BMI result is 22.9 kg/m².
How to interpret the result?
According to the guidelines of the World Health Organization (WHO), BMI is classified into:
- Underweight (BMI < 18.5): Indicates that the weight is below ideal, which may require nutritional attention.
- Normal weight (18.5 to 24.9): Indicates that the weight is adequate for the height, associated with a lower rate of diseases.
- Overweight (25.0 to 29.9): Indicates a pre-obesity range that signals the importance of healthier habits.
- Grade I Obesity (30.0 to 34.9): Beginning of excessive accumulation of body fat.
- Grade II Obesity (35.0 to 39.9): Moderate to severe health risk.
- Grade III or Morbid Obesity (≥ 40.0): Severe risk for the development of comorbidities.
Practical Examples
- If you weigh 80 kg and are 1.80 m tall: BMI =
80 ÷ (1,80 × 1,80) = 24,7(Classification: Normal Weight). - If you weigh 95 kg and are 1.70 m tall: BMI =
95 ÷ (1,70 × 1,70) = 32,9(Classification: Grade I Obesity).
Usage Tips
- Always measure your height barefoot and standing upright against a wall.
- Weigh in preferably in the morning, after going to the bathroom and before your first meal.
- Remember that BMI does not directly measure body fat percentage or muscle mass distribution.
Important Observations
BMI has limitations and may not be accurate for high-performance athletes (who have a lot of muscle mass), pregnant women, children, the elderly and people with loss of bone mass or fluid retention. This classification follows global health standards and guidelines established by the World Health Organization (WHO).
Frequently Asked Questions (FAQ)
Is the BMI valid for those who train bodybuilding?
For bodybuilders with a lot of hypertrophied mass, BMI can overestimate body fat, indicating overweight for healthy individuals.
What is the ideal BMI?
For most adults, the range considered ideal and healthy is between 18.5 and 24.9 kg/m².
How is the BMI of elderly people assessed?
The BMI for the elderly (over 60 years old) has different ranges. As there is a loss of lean and natural bone mass, the normal range is greater (generally between 22 and 27 kg/m²).
Is BMI different for adult men and women?
The BMI formula and WHO classification ranges are the same for adult men and women. However, average body composition varies between biological sexes.
What are the health risks of having a very high BMI?
A high BMI (overweight or obesity) increases the risk of developing chronic diseases such as hypertension, type 2 diabetes, cardiovascular problems and joint disorders.
Does BMI measure body fat directly?
No. BMI measures the relationship between total weight and height squared. It does not distinguish weight made up of muscle mass, fat, bones or retained fluids.
Can pregnant people use the traditional BMI calculation?
No. During pregnancy, there is weight gain in the fetus, placenta and amniotic fluid. Pregnant women should use specific gestational weight gain tables recommended by their obstetrician.
How does child and youth BMI work?
For children and adolescents (2 to 19 years), BMI is calculated in the same way, but the result is plotted on WHO age and biological sex percentile curves for classification.
What exams or measurements complement the BMI analysis?
Measurements of abdominal circumference, bioimpedance examination and measurement of skin folds are excellent tools to complement the diagnosis of body composition.