Full answer
The BMI formula is straightforward. Imperial: multiply your weight in pounds by 703, then divide by your height in inches squared. Example: 180 lbs, 5'10" (70 inches): 180 × 703 = 126,540 ÷ 70² (4,900) = BMI 25.8. Metric: weight in kilograms divided by height in meters squared. A 70kg person who is 1.75m tall: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = BMI 22.9.
The four BMI categories: Underweight = below 18.5. Normal weight = 18.5–24.9. Overweight = 25–29.9. Obese = 30 and above. For children and teens, BMI is interpreted differently using age- and sex-specific percentile charts rather than fixed ranges.
BMI has significant known limitations. It doesn't distinguish between muscle and fat — many athletes and bodybuilders are classified as "obese" by BMI despite having very low body fat. It also doesn't account for where fat is stored, which matters enormously for health risk. Visceral fat (around organs) is far more dangerous than subcutaneous fat (under skin).
Better metrics for health risk exist. Waist circumference is more predictive of metabolic disease than BMI: men should be under 40 inches, women under 35 inches. Waist-to-height ratio (waist ÷ height; aim under 0.5) accounts for body size. DEXA scan body composition testing is the gold standard for measuring actual body fat percentage, though it requires a clinic visit.
Despite its limitations, BMI remains widely used because it's free, fast, and correlates reasonably well with health outcomes at the population level — even if it's imprecise for individuals. It's a starting screen, not a diagnosis. If your BMI falls outside the normal range, talk to your doctor about more comprehensive assessments.