What Is BMI?
Body Mass Index (BMI) is a numerical value calculated from a person's weight and height. It provides a simple screening tool to categorize individuals into weight categories such as underweight, normal weight, overweight, and obese. BMI is used worldwide by healthcare providers as a quick initial assessment, although it has important limitations.
The BMI Formula
Metric Formula
BMI = Weight (kg) ÷ Height (m)2
Imperial Formula
BMI = (Weight (lbs) × 703) ÷ Height (inches)2
Metric Example
Weight: 75 kg, Height: 1.75 m:
- Square the height: 1.75 × 1.75 = 3.0625
- Divide weight by result: 75 ÷ 3.0625 = 24.5 BMI
Imperial Example
Weight: 165 lbs, Height: 5'9" (69 inches):
- Square the height: 69 × 69 = 4,761
- Multiply weight by 703: 165 × 703 = 115,995
- Divide: 115,995 ÷ 4,761 = 24.4 BMI
BMI Categories
| BMI Range | Category | Health Risk |
|---|---|---|
| Below 18.5 | Underweight | Increased risk of nutritional deficiency |
| 18.5 - 24.9 | Normal weight | Lowest health risk |
| 25.0 - 29.9 | Overweight | Moderately increased 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 |
BMI Reference Table by Height
Here is what different BMI values correspond to at common heights:
| Height | Normal (BMI 22) | Overweight (BMI 27) | Obese (BMI 32) |
|---|---|---|---|
| 5'2" (157 cm) | 121 lbs / 55 kg | 148 lbs / 67 kg | 175 lbs / 80 kg |
| 5'6" (168 cm) | 136 lbs / 62 kg | 167 lbs / 76 kg | 198 lbs / 90 kg |
| 5'10" (178 cm) | 153 lbs / 69 kg | 188 lbs / 85 kg | 223 lbs / 101 kg |
| 6'0" (183 cm) | 162 lbs / 74 kg | 199 lbs / 90 kg | 236 lbs / 107 kg |
| 6'2" (188 cm) | 172 lbs / 78 kg | 211 lbs / 96 kg | 250 lbs / 113 kg |
Limitations of BMI
BMI is a screening tool, not a diagnostic tool. It does not directly measure body fat, muscle mass, or overall health.
- Muscle mass: Athletes and bodybuilders may have a high BMI but low body fat. A muscular person at 5'10" weighing 200 lbs has a BMI of 28.7 (overweight), even if they have low body fat.
- Age: Older adults tend to have more body fat than younger adults at the same BMI.
- Sex: Women generally have more body fat than men at the same BMI.
- Ethnicity: Health risks at specific BMI values vary among different ethnic groups.
- Fat distribution: BMI does not account for where fat is stored. Abdominal fat is more dangerous than fat stored in hips and thighs.
Better Alternatives to Consider
- Waist circumference: A waist over 40 inches (men) or 35 inches (women) indicates increased health risk.
- Waist-to-hip ratio: Provides insight into fat distribution.
- Body fat percentage: Measured with calipers, DEXA scan, or bioelectrical impedance.
- Consult a healthcare professional: BMI should be one of many factors considered in a health assessment.
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