To calculate BMI, divide your weight in kilograms by your height in meters squared. If you weigh 70 kg and stand 1.75 m tall, your BMI is 70 ÷ (1.75 × 1.75) = 22.9. If you use pounds and inches, the formula is (weight in pounds ÷ height in inches squared) × 703. Both give you the same number, which falls into one of several standard categories.
The Metric Formula
The standard BMI equation is simple: weight (kg) ÷ height (m²). You square your height first, then divide your weight by that number. For someone who is 1.68 m tall and weighs 65 kg, you’d calculate 1.68 × 1.68 = 2.82, then 65 ÷ 2.82 = 23.0.
The Imperial Formula
If you measure in pounds and inches, the math needs one extra step. Divide your weight in pounds by your height in inches squared, then multiply the result by 703. That conversion factor bridges the gap between imperial and metric units. For a person who weighs 160 pounds and stands 5’7″ (67 inches): 160 ÷ (67 × 67) = 160 ÷ 4,489 = 0.03563, then 0.03563 × 703 = 25.1.
What the Number Means
Once you have your BMI, it falls into one of these categories for adults:
- Under 18.5: Underweight
- 18.5 to 24.9: Healthy weight
- 25 to 29.9: Overweight
- 30 to 34.9: Class 1 obesity
- 35 to 39.9: Class 2 obesity
- 40 or higher: Class 3 (severe) obesity
These thresholds were developed using data primarily from non-Hispanic white populations. For people of Asian descent, the World Health Organization recommends lower cut points: a BMI of 23 to 27.5 counts as overweight, and 27.5 or above as obese. At the same BMI, Asian individuals tend to carry more visceral fat and face higher metabolic risk than people of European descent.
Getting Accurate Measurements
Your BMI is only as good as the height and weight you plug in. For the most accurate reading, measure your height without shoes, standing straight against a wall with your head, shoulder blades, buttocks, and heels all touching the surface. Look straight ahead rather than up or down. Weigh yourself in light clothing, without shoes, belts, or heavy jackets. Even a half-inch error in height can shift your BMI by a full point.
How BMI Works for Children
The formula itself is identical for kids, but the result is interpreted completely differently. Because children’s body composition changes rapidly as they grow, a raw BMI number doesn’t map onto the same adult categories. Instead, a child’s BMI is plotted on sex-specific growth charts that compare them to other children of the same age. A child at the 85th to 94th percentile is considered overweight, and at the 95th percentile or above, the classification is obesity. The CDC provides a free online calculator specifically for children and teens aged 2 through 19.
What BMI Doesn’t Tell You
BMI measures weight relative to height. It cannot distinguish between muscle, bone, and fat. A muscular person can easily land in the “overweight” or even “obese” range while carrying very little body fat. This is one of the most widely recognized flaws of the metric.
The problem also runs in the other direction, especially in older adults. Starting as early as your 30s, you gradually lose skeletal muscle mass. By your 70s, you may have only half the muscle weight you had in your 20s. Because muscle is denser than fat, someone who has lost significant muscle and replaced it with fat tissue can register a “healthy” BMI while carrying an unhealthy amount of body fat. Johns Hopkins Medicine notes this makes BMI particularly misleading for older adults.
In 2023, the American Medical Association adopted a policy acknowledging these limitations. The AMA recommended that BMI not be used as a sole measure of health and should instead be paired with other assessments, including waist circumference, body composition analysis, and metabolic markers like blood sugar and cholesterol.
Waist-to-Height Ratio as a Complement
If you want a quick second check alongside BMI, your waist-to-height ratio offers a useful picture of where your body stores fat. Measure your waist at the narrowest point (usually around your navel) and divide that number by your height, using the same units for both. A ratio above 0.5 generally signals increased cardiovascular risk.
A large meta-analysis comparing the two metrics found that waist-to-height ratio outperformed BMI in predicting cardiovascular disease, cardiovascular death, and death from all causes. BMI did not prove superior to waist-to-height ratio in any of the health outcomes studied, across both sexes and across Asian and non-Asian populations. The reason is straightforward: fat stored around the midsection is more metabolically dangerous than fat stored in the hips or thighs, and BMI has no way to capture that distinction.
BMI remains a useful, free, no-equipment screening tool. It correlates well with health risks at the population level. But for any individual person, pairing it with a waist measurement gives you a more complete and more accurate picture of where you stand.