BMI Calculator
BMI Categories
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Understanding Body Mass Index (BMI)
Body Mass Index (BMI) is a simple calculation using a person's height and weight. The formula is BMI = kg/m² where kg is a person's weight in kilograms and m² is their height in meters squared.
The BMI Formula
Metric Formula
BMI = weight (kg) / height² (m²)
Example: 70 kg ÷ (1.75 m)² = 22.9
Imperial Formula
BMI = 703 × weight (lbs) / height² (in²)
Example: 703 × 154 lbs ÷ (69 in)² = 22.7
What Does Your BMI Mean?
BMI is a useful screening tool but doesn't directly measure body fat or account for factors like:
- Muscle mass: Athletes may have high BMI but low body fat
- Age: Older adults tend to have more body fat than younger adults at the same BMI
- Sex: Women typically have more body fat than men at the same BMI
- Ethnicity: BMI cutoffs may vary for different populations
- Fat distribution: Where fat is stored matters (abdominal fat poses higher health risks)
Health Risks by BMI Category
| Category | BMI Range | Health Risks |
|---|---|---|
| Underweight | < 18.5 | Malnutrition, weakened immune system, bone loss |
| Normal | 18.5 - 24.9 | Lowest risk of weight-related health problems |
| Overweight | 25 - 29.9 | Increased risk of heart disease, diabetes, high blood pressure |
| Obese | ≥ 30 | High risk of chronic diseases, reduced life expectancy |
Important Note
BMI is a screening tool, not a diagnostic tool. A high or low BMI doesn't necessarily mean you're unhealthy. For a complete picture of your health, consult with a healthcare provider who can assess other factors like blood pressure, cholesterol, blood sugar, family history, and lifestyle.
History of BMI
BMI was invented in the 1830s by Belgian mathematician Adolphe Quetelet. It was originally called the Quetelet Index and was designed for population-level statistics, not individual health assessment. The term "Body Mass Index" was coined in 1972, and it gained widespread use as a simple screening tool for weight categories.
Frequently Asked Questions
What is a healthy BMI range?
A healthy BMI range is typically between 18.5 and 24.9. This range is associated with the lowest risk of weight-related health problems. However, BMI is just one indicator and doesn't account for muscle mass, bone density, or fat distribution.
How is BMI calculated?
BMI is calculated by dividing your weight in kilograms by your height in meters squared (kg/m²). For imperial units, the formula is: BMI = 703 × weight (lbs) / height² (inches²). Our calculator handles both unit systems automatically.
Why is BMI not always accurate?
BMI doesn't distinguish between muscle and fat mass. Athletes with high muscle mass may have a high BMI but low body fat. Similarly, older adults may have a normal BMI but higher body fat percentage. It's best used as a screening tool alongside other health measurements.
What should I do if my BMI is outside the normal range?
If your BMI is below 18.5 or above 24.9, consult with a healthcare provider. They can assess additional factors like body composition, waist circumference, blood pressure, and cholesterol to give you a complete picture of your health and personalized recommendations.