To figure out your BMI, you divide your weight by your height squared. If you’re using pounds and inches, you multiply the result by 703. If you’re using kilograms and meters, the formula works as is. The whole calculation takes about 30 seconds with a calculator, and free online tools from the CDC can do it instantly.
The BMI Formula
BMI stands for body mass index, and there are two versions of the formula depending on which units you use.
Using pounds and inches: Divide your weight in pounds by your height in inches squared, then multiply by 703. So if you weigh 160 pounds and stand 5’7″ (67 inches), the math is: 160 ÷ (67 × 67) × 703 = 25.1.
Using kilograms and meters: Divide your weight in kilograms by your height in meters squared. A person who weighs 72 kg and is 1.70 m tall would calculate: 72 ÷ (1.70 × 1.70) = 24.9.
Both formulas give the same result. The 703 multiplier in the imperial version simply converts pounds and inches to match the metric scale.
What Your Number Means
The CDC uses these BMI ranges for adults 20 and older:
- Underweight: below 18.5
- Healthy weight: 18.5 to 24.9
- Overweight: 25 to 29.9
- Obesity Class 1: 30 to 34.9
- Obesity Class 2: 35 to 39.9
- Obesity Class 3 (severe): 40 or higher
These categories were designed for population-level screening. They give you a rough sense of where you fall, but they don’t tell the full story for any single person.
Getting Accurate Measurements
Your BMI is only as good as the numbers you put into it. Small errors in height or weight can shift your result by a full point or more.
For height, stand barefoot on a hard floor (not carpet) against a flat wall. Keep your feet together, stand straight, and look directly ahead. Have someone place a flat, rigid object like a hardcover book on top of your head at a right angle to the wall, then mark that spot and measure down to the floor. Record to the nearest eighth of an inch or tenth of a centimeter.
For weight, use a digital scale on a hard surface. Spring-loaded bathroom scales are less reliable. Remove shoes and heavy clothing, stand with both feet centered on the scale, and record the number to the nearest decimal. Weighing yourself in the morning before eating tends to give the most consistent readings, since your weight fluctuates throughout the day.
Why BMI Doesn’t Work for Everyone
BMI treats all weight the same. It cannot distinguish between fat, muscle, and bone mass. A muscular person and a sedentary person of identical height and weight will get the same BMI, even though their health profiles are very different. This is the biggest limitation of the number.
The standard thresholds also don’t account for differences across ethnic backgrounds. Research on Asian populations uses lower cut points: overweight starts at 23 (instead of 25) and obesity at 27.5 (instead of 30). At the same BMI, people of Asian descent tend to carry more visceral fat and face higher metabolic risk. If you’re of Asian heritage, a BMI of 24 may carry more significance than the standard categories suggest.
Age and sex also matter. Older adults naturally lose muscle and bone density, which can mask increased body fat behind a “normal” BMI. Women typically carry a higher percentage of body fat than men at any given BMI. In 2023, the American Medical Association adopted a policy stating that BMI should not be used as a standalone diagnostic tool, recommending it be paired with other measures like waist circumference, body composition, and metabolic markers.
BMI for Children and Teens
The formula itself is the same for kids, but the interpretation is completely different. Because children are still growing, a raw BMI number doesn’t mean much on its own. Instead, a child’s BMI is plotted against CDC growth charts that compare them to other kids of the same age and sex. The result is expressed as a percentile. A 10-year-old boy at the 75th percentile has a higher BMI than 75% of boys his age. The categories (underweight, healthy weight, overweight, obese) are based on these percentile ranges rather than fixed numbers. The CDC’s online calculator for children handles this automatically.
A Better Measure to Use Alongside BMI
If you want a more complete picture, measure your waist-to-height ratio. Divide your waist circumference by your height, both in the same units. A ratio above 0.5 signals increased risk.
Research from the University of Pittsburgh found that waist-to-height ratio outperforms BMI at predicting heart disease risk. The reason is straightforward: BMI doesn’t account for where fat sits on your body. Fat stored around your organs (visceral fat) is far more dangerous than fat stored just under the skin, and waist measurement captures that difference. In the Pittsburgh study, after adjusting for factors like age, smoking, exercise, and blood pressure, only waist-to-height ratio held up as an independent predictor of cardiovascular disease. BMI did not.
People with a BMI under 30 but a waist-to-height ratio over 0.5 may still face elevated risk of coronary artery calcification, a key marker of heart disease. That means you could fall in the “healthy” or “overweight” BMI range and still carry meaningful cardiovascular risk that BMI alone would miss. Measuring your waist takes 10 seconds with a tape measure at the level of your belly button, and the added information is worth it.