Physicians around the world rely on a simple tool to help screen patients for obesity: the body mass index (BMI) chart, which determines if someone is underweight, healthy weight or overweight.
What is BMI?
You can calculate your BMI by dividing your weight (in pounds) by height (in inches) squared, then multiply by 703. Or, find your height and weight on this BMI table. BMI screens people into one of four weight categories:
- Underweight: Less than 18.5
- Healthy: 18.5 to 24.9
- Overweight: 25 to 29.9
- Obese: 30 or higher
We can trace part of its widespread use back to its simplicity — all you need is a scale and a tape measure. Calculating body-fat percentage, on the other hand, requires hydrostatic weighing (also known as underwater weighing), an air displacement chamber (which uses volume and pressure to calculate the results) or scans to measure bone mineral density. Those methods just aren’t practical on a larger scale.
However, BMI critics point to its limitations, primarily because it is so simple. The measurement doesn’t account for gender, age or body fat percentage. Unsure about what your BMI means for your health? We tackle four common misconceptions below.
- I work out, so my BMI isn’t accurate. It's true that some professional athletes may register as overweight and even obese on the BMI scale. Why? These athletes, and even some serious amateurs, have low body fat percentages. Remember: Muscle is denser than fat. But the reality is that casual gym goers won't fall into this category and likely overestimate their lean muscle mass.
- The lower my BMI, the better. The healthy range of BMI exists for a reason. People who are underweight may have nutritional deficiencies, a weakened immune system, depression and, for women, an irregular menstrual cycle and trouble getting pregnant.
- I'm a woman; BMI doesn’t apply to me. Women, in general, carry more body fat than men do. However, like the question of muscle mass, the excess body fat likely makes little difference for people in the healthy range. And actually, women’s BMI categorization may be more accurate. Research has shown that BMI tends to underestimate obesity’s real prevalence, especially in white men.
- BMI is the end-all, be-all health measurement. In reality, your BMI is just one measurement a health professional can use to assess your wellness. To determine overweight and obesity, the waist circumference measurement is a complementary screening tool to BMI. That’s because abdominal fat is more dangerous to your health, increasing your risk of type 2 diabetes, heart disease and high blood pressure. The Centers for Disease Control outline these waist circumference measurements as at-risk:
- Men: More than 40 inches
- Non-pregnant women: More than 35 inches
Regardless, your waist circumference and BMI measurements are simply screening tools, not health diagnostics. If you’re worried about either measurement, schedule an appointment with your physician to discuss your concerns.
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