How to Calculate BMI and What It Means for Your Health
5 min read · Updated 2025
Body Mass Index (BMI) is a simple numerical measure derived from your weight and height. It is one of the most widely used screening tools to identify whether an adult is underweight, at a healthy weight, overweight, or obese. While it has limitations, BMI is a useful starting point for understanding your weight-related health risk.
The BMI Formula
BMI = Weight (kg) ÷ Height² (m²)
Imperial: BMI = 703 × Weight (lb) ÷ Height² (inches²)
Worked Example (Metric)
A person weighing 70 kg and 1.75 m tall:
- BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9
This falls in the Healthy Weight range.
BMI Categories (Adult)
| BMI Range | Category | Health Risk |
|---|---|---|
| Below 18.5 | Underweight | Nutritional deficiency, osteoporosis |
| 18.5 – 24.9 | Healthy Weight | Low risk |
| 25.0 – 29.9 | Overweight | Moderate risk |
| 30.0 – 34.9 | Obese (Class I) | High risk |
| 35.0 – 39.9 | Obese (Class II) | Very high risk |
| 40.0+ | Severely Obese | Extremely high risk |
Limitations of BMI
BMI is a screening tool, not a diagnosis. Its key limitations include:
- Doesn't distinguish muscle from fat — athletes and bodybuilders often have a high BMI despite having very low body fat.
- Ignores fat distribution — visceral fat (around the abdomen) is more dangerous than fat stored elsewhere, but BMI doesn't measure this.
- Varies by ethnicity — health risks at the same BMI can differ between ethnic groups. Some guidelines suggest lower thresholds for Asian populations.
- Not appropriate for children or the elderly — age-specific charts are used for children under 18.
Better Measures Alongside BMI
- Waist-to-Hip Ratio: A waist circumference above 94 cm (men) or 80 cm (women) indicates higher cardiovascular risk.
- Body Fat Percentage: Measured by DEXA scans or bioelectrical impedance — more accurate than BMI for body composition.
- Blood tests: Cholesterol, blood sugar, and blood pressure give a much more complete picture of metabolic health.
Use BrainBoost's free BMI Calculator to calculate your BMI instantly in both metric and imperial units. Also try the Calorie Calculator and Body Measurement Tracker for a more complete health picture.