What is BMI? How to Calculate Body Mass Index and What It Means
Body Mass Index (BMI) is one of the most widely used health screening tools in the world. Doctors, insurance companies, and governments use it to assess population health. But what exactly is BMI, how is it calculated, and how useful is it?
The BMI Formula
BMI is calculated by dividing your weight (in kilograms) by the square of your height (in meters):
BMI = Weight (kg) / Height² (m²)
Example: A person who weighs 70kg and is 1.75m tall:
- BMI = 70 / (1.75 × 1.75) = 70 / 3.0625 = 22.9
In imperial units:
BMI = (Weight in pounds × 703) / Height in inches²
Use our BMI Calculator to get your result instantly without doing the math.
BMI Categories
| BMI Range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity (Class I) |
| 35.0 – 39.9 | Obesity (Class II) |
| 40.0 and above | Severe Obesity (Class III) |
These ranges apply to adults 20 years and older. For children and teens, age and sex-specific percentiles are used instead.
What BMI Tells You
BMI is a screening tool, not a diagnostic test. A high BMI suggests higher risk of:
- Type 2 diabetes
- Heart disease
- High blood pressure
- Certain cancers
- Sleep apnea
A low BMI can indicate malnutrition or other underlying conditions.
The Limitations of BMI
BMI has significant limitations that every person should understand:
1. It doesn't measure body fat directly. A muscular athlete may have a "overweight" BMI while having very low body fat.
2. It doesn't account for fat distribution. Belly fat (visceral fat) is more dangerous than fat stored in the hips and thighs — BMI can't distinguish these.
3. It varies by ethnicity. Asian populations have higher health risks at lower BMI values. Some health organizations use lower thresholds for Asian adults.
4. It ignores age. Older adults naturally lose muscle mass (sarcopenia). A "normal" BMI in an elderly person could mask muscle loss.
5. It ignores sex differences. Women typically have more body fat than men at the same BMI.
Better Metrics to Use Alongside BMI
- Waist circumference: Men > 40 inches, women > 35 inches = higher risk
- Waist-to-hip ratio: More predictive of heart disease than BMI alone
- Body fat percentage: Measured by DEXA scan, hydrostatic weighing, or body composition scales
- Blood markers: Cholesterol, blood sugar, blood pressure tell a fuller health story
How to Use the BMI Calculator
- Go to our BMI Calculator
- Select metric or imperial units
- Enter your height and weight
- See your BMI instantly along with what it means for your health
The calculator also shows where you fall on the BMI scale with a visual indicator.
Healthy Weight Management Tips
Regardless of your BMI category:
- Move daily: 150 minutes of moderate exercise per week (WHO recommendation)
- Eat whole foods: Focus on vegetables, lean proteins, and complex carbs
- Sleep 7-9 hours: Poor sleep is linked to weight gain and metabolic disruption
- Manage stress: Chronic stress elevates cortisol, which promotes fat storage
- Stay hydrated: Often mistaken for hunger
Check your BMI now with our free BMI Calculator — metric and imperial supported.