How to Calculate BMI: Formula, Ranges & What It Really Means
BMI (Body Mass Index) is the world's most widely used screening tool for weight categories. Understanding how to calculate it — and what it does and doesn't tell you — helps you take a more informed approach to your health.
The BMI Formula
BMI is your weight divided by the square of your height:
Imperial version: BMI = (Weight in lbs × 703) ÷ Height² (inches²)
Example: 5'9" (69 inches), 170 lbs
BMI = (170 × 703) ÷ (69²) = 119,510 ÷ 4,761 = 25.1 (Overweight range)
BMI Categories (Adults)
| BMI Range | Category | Health Risk |
|---|---|---|
| Below 18.5 | Underweight | Increased risk |
| 18.5 – 24.9 | Normal weight | Lowest 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 and above | Obese Class III | Extremely high risk |
For Asian populations, overweight is defined at BMI ≥ 23 and obesity at ≥ 27.5 due to different body composition patterns.
What BMI Does and Doesn't Tell You
BMI tells you: whether your weight-to-height ratio falls in a statistically higher-risk range. It's useful as a population screening tool.
BMI doesn't tell you:
- Body fat percentage (a muscular athlete can have "obese" BMI)
- Where fat is distributed (belly fat is more dangerous than hip/thigh fat)
- Muscle mass, bone density, or overall fitness
- Metabolic health markers like blood pressure or blood sugar
Better measures include waist circumference, waist-to-hip ratio, DEXA scan body fat %, and metabolic blood panels.
BMI for Children and Teens
BMI for those under 20 is calculated the same way but interpreted differently using age- and sex-specific percentiles:
- Below 5th percentile: Underweight
- 5th–84th percentile: Healthy weight
- 85th–94th percentile: Overweight
- 95th percentile and above: Obese
Children's BMI naturally changes with age and puberty, so percentile charts are essential for accurate interpretation.