BMI or body mass index is a measurement doctors use to estimate whether you have a healthy amount of “mass” relative to your height. It’s calculated using a mathematical formula that compares your body weight to your height. To get your BMI, you divide your weight in kilograms by the square of your height in meters. You don’t need to do the math yourself, there are calculators online where you can plug in your numbers and find out what your BMI is. How do you interpret the results? The definition for underweight, normal weight, overweight and obese based on BMI is as follows:
Underweight – BMI of 18.4 or lower
Normal weight – BMI of 18.5 to 24.9
Overweight – BMI of 25 to 29.9
Obese – BMI of 30 or more
Is BMI Really a Good Measure?
Unfortunately, BMI is not always a good indicator of whether or not you’re a healthy weight. The BMI calculation was developed as a formula to help doctors and insurance companies quickly determine whether a person falls into the overweight or obese category. But the formula has a major limitation. It doesn’t distinguish between fat and muscle tissue.
Surprisingly, if you measure the BMI of some muscle-bound sports stars who lift heavy weights and live on protein shakes, their BMI will often fall into the overweight range despite the fact they have a body fat percentage in the single digits. BMI measurements can overestimate the amount of fat tissue in athletes and people who work out intensely and have more muscle mass and underestimate it in people with a lower percentage of lean body mass.
BMI also doesn’t take into account the size of your frame. If you have large bones and a sizeable frame, you may be classified as overweight even if work out and have a low body fat percentage. Likewise, if you have a small frame, you may be carrying around a lot of excess body fat and still not fall into the overweight range.
According to a study published in Obstetrics and Gynecology, race and ethnicity should be accounted for when interpreting BMI. Different races and ethnic groups have variations in body composition that makes BMI a less accurate measurement of whether they’re overweight or obese. In fact, if you use current BMI measurements to screen for obesity, it misses almost half of younger women who meet the criteria by percentage of body fat. Obesity is defined as a body fat percentage greater than 35% in women and more than 25% in men.
What does this mean? If you’re athletic or unusually large or small-framed, your BMI may not be an accurate measurement of whether you’re overweight or healthy. Some experts believe waist circumference is a better measure of obesity and overall health risk than BMI. A waist size of greater than 35 inches for women and 45 inches for men is linked with a higher risk of heart disease and type 2 diabetes. Waist circumference can be used as an additional measure to identify people with high a high BMI who shouldn’t fall into the overweight category. For example, an athletic person who has a BMI that puts them in the overweight category but has a normal waist circumference shouldn’t be considered overweight.
The best measure of whether you’re a healthy weight is your body fat percentage. The most accurate way to measure that is using underwater weighing or a DXA scan, but these methods are expensive. You can buy a body fat scale that’s reasonably accurate for following your body fat if you use it properly. It’s a more accurate way than the BMI to find out whether you’re carrying around too much fat.
Obstetrics and Gynecology. 2010; 115. 982-88.
The Nutritionist. Robert Wildman, PhD, RD. 2002.