How to Calculate BMI — Formula, Chart & What It Means
Body Mass Index (BMI) is a numerical value calculated from your height and weight that is widely used as a screening tool for weight categories. It was developed in the 1830s by Belgian mathematician Adolphe Quetelet and has been used in medical contexts since the 1970s. While BMI is useful as a quick population-level screening tool, it has significant limitations as an individual health indicator.
The BMI Formula
Metric: BMI = Weight (kg) ÷ Height (m)²
Imperial: BMI = (Weight (lbs) × 703) ÷ Height (inches)²
Example: 5'10" (70 inches), 170 lbs → BMI = (170 × 703) ÷ 70² = 119,510 ÷ 4,900 = 24.4
BMI Categories (Adult)
| BMI Range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obese (Class 1) |
| 35.0 – 39.9 | Obese (Class 2) |
| 40.0 and above | Severely Obese (Class 3) |
BMI for Children and Teens
For people under 20, BMI is calculated the same way but interpreted differently using age- and sex-specific percentile charts. A child's BMI is compared to other children of the same age and sex rather than to fixed cutoff numbers. This accounts for the normal changes in body composition during growth.
The Limitations of BMI
BMI is a blunt instrument that doesn't account for several important factors:
- Muscle mass: Highly muscular people (athletes, bodybuilders) frequently have "overweight" or "obese" BMIs despite having very low body fat percentages.
- Body composition: BMI can't distinguish between fat and muscle. Two people with the same BMI can have very different body fat percentages.
- Fat distribution: Where you carry fat matters for health risk. Abdominal (visceral) fat is associated with higher health risks than subcutaneous fat. BMI doesn't capture this.
- Age and sex: Older adults typically have more body fat at the same BMI as younger adults. Women typically have more body fat than men at the same BMI.
- Ethnicity: Research suggests that the same BMI values represent different health risks across ethnic groups. Some Asian populations face higher health risks at lower BMI values than the standard thresholds suggest.
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