So What's the Deal With Calculating Body Fat, and Does That Number Actually Matter?

Body mass index (BMI) is a measure of body fat based on your weight in relation to your height. Through a calculation or by use of a BMI chart, medical professionals can determine whether a patient is overweight, underweight, obese, or at the ideal weight for their height.

According to Dr. Mikhail "Doctor Mike" Varshavski DO, a board-certified family medicine physician at Overlook Medical Center in Summit, NJ, who spoke to POPSUGAR in an email, BMI is calculated by using the formula BMI = kg/m2 or using a BMI chart. With no differences for men or women in the formula, only height and weight is taken into account, not age, sex, ethnicity, or muscle mass. A healthy BMI is considered 18.5 to 24.9, according to the chart.

BMI calculations, said Dr. Varshavski, are a good general measure of overall health, but there are some issues with it as well, he said, that medical professionals need to be aware of, including taking muscle mass into account. He recalled a personal example for POPSUGAR in which his BMI had labeled him as overweight even though he was, in fact, muscular.

"During medical school, I was around 220 pounds muscular with little body fat, but based on my height, the BMI formula labeled me overweight," he said. "Muscle composition is not taken into account when calculating the BMI; therefore someone who is very muscular will have a falsely elevated reading."

According to Eva Tseng, MD MPH and assistant professor, division of general internal medicine at Johns Hopkins University, studies have shown that BMI correlates well to other direct measures of body fat so we use it as a surrogate measure of body fat, but the measurement has more limitations than just muscle mass when being evaluated for current and future health risks.

"BMI has been used as a tool to evaluate population health trends over time. However, there are some limitations to BMI: it does not directly measure excess body fat, and and it does adjust for factors like age, sex, ethnicity, and muscle mass. It also does not tell us the distribution of body fat in a person. We know that people with more central or abdominal adiposity have a higher risk of premature cardiovascular disease, diabetes, and death compared to people with a similar BMI but less abdominal adiposity. Older people tend to have more body fat than younger people although they may have the same BMI. In general, women have more total body fat than men with the same BMI. Asians and Asian Americans have increased health risks at a lower BMI, so the current cutoffs may not accurately reflect health risks in this ethnic/racial group," she said.

Dr. Varshavski added that BMI combined with other factors and a bit of common sense is the best way to evaluate the overall health of a patient, but that BMI does help to play an import role. "BMI is a solid tool to gauge the risk of illness. Also, while women and men carry different risks for the same BMI number, it is still useful for acting as a marker of when to start focusing on weight loss or weight gain," he said.

And while BMI calculations may put you on the good side of healthy currently, it's not just about what you weigh in relation to your height. Dr. Varshavski said that taking care of one's health in combination with maintaining a healthy BMI are both equally as important for future health.

"It's important to remember that the BMI is not a representative of your current health but solely the risks posed to your future health. You can have a normal BMI and be very unhealthy," said Dr. Varshavski. "In general, the higher your BMI, the higher the risk of developing a range of diseases that are associated with being overweight or obese, common ailments being diabetes, arthritis, high blood pressure, liver disease, and sleep apnea, among others."