BMI Calculator: Your Complete Guide
What is BMI?
Body Mass Index (BMI) is an international measure used to assess whether a person has a healthy weight relative to their height. It was developed by Belgian mathematician Adolphe Quetelet in the 19th century and is currently the most widely used screening tool by healthcare professionals worldwide.
BMI Formula
BMI = Weight (kg) ÷ Height² (m²)
Formula variables:
- Weight = Your body weight in kilograms
- Height = Your height in meters (squared)
For the imperial system (pounds and inches), the formula is:
BMI = (Weight in pounds × 703) ÷ Height² (inches²)
BMI Categories According to WHO
- Underweight: BMI less than 18.5
- Normal weight: BMI between 18.5 and 24.9
- Overweight: BMI between 25 and 29.9
- Obesity class I: BMI between 30 and 34.9
- Obesity class II: BMI between 35 and 39.9
- Obesity class III: BMI of 40 or more
Practical Example
If you weigh 154 lbs (70 kg) and are 5'9" (1.75 m) tall:
BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9
This value falls within the normal weight range.
BMI Limitations
It's important to understand that BMI has some limitations:
- Doesn't distinguish between muscle mass and fat: Athletes may have a high BMI due to their muscle mass.
- Doesn't consider fat distribution: Abdominal fat is more dangerous than fat in other areas.
- Doesn't apply equally for all ages: Children and adolescents require specific percentile tables.
- Ethnic variations: Some populations may have different cut-off points for health risk.
When to Consult a Professional?
BMI is just a screening tool. If you have concerns about your weight or health, consult a doctor or nutritionist for a complete evaluation that includes other indicators such as body fat percentage, waist circumference, and blood tests.
Frequently Asked Questions
BMI (Body Mass Index) is a measure that relates a person's weight to their height. It is calculated by dividing weight in kilograms by height in meters squared: BMI = weight (kg) / height² (m²). It is the most widely used tool to assess whether a person's weight is in a healthy range.
According to the World Health Organization (WHO), a BMI between 18.5 and 24.9 is considered normal weight. Below 18.5 indicates underweight, between 25 and 29.9 is overweight, and 30 or more is classified as obesity in different degrees.
BMI is a useful screening tool but has limitations. It doesn't distinguish between muscle mass and fat, nor does it consider body fat distribution. Therefore, it should be used alongside other health assessments such as waist circumference measurement and clinical analyses.
In athletes and very muscular people, BMI may overestimate body fat level since muscle weighs more than fat. In these cases, additional measurements like body fat percentage using calipers or bioimpedance are recommended.
No. For children and adolescents (ages 2 to 18), BMI is interpreted using age and sex-specific percentiles. A pediatrician can help you correctly interpret a child's BMI by comparing it to WHO or CDC growth charts.
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