Weight-based ideals are ever-evolving and rarely connected to actual health, so it can be difficult to separate unrealistic beauty standards from what’s actually good for you. But anyone who’s ever had to take a gym class knows that at least there’s a doctor-approved, universal formula that can tell you if you’re actually overweight, right? Well, no.

Doctors, government health organizations and insurance companies have encouraged the use of body mass index, or BMI, to measure overweight and obesity for decades. The formula (your weight in kilograms divided by your height, squared) gives you a number that purportedly tells you how much fat you have. If you score above 25 (overweight) or 30 (obese), it might scare you into losing weight. But it hasn’t actually seemed to have that effect. According to the Centers for Disease Control and Prevention, the average American woman over the age of 20 in 2010 was 5’3″ and weighed 166.2 lbs. That makes her BMI 29.4. The average man was just shy of 5’8 and 195.5 lbs, which puts him at about 29.7. So the average American is not only overweight, but almost obese.

That kind of macro analysis is why the BMI was created—the mathematician who invented the formula used it to to compare large populations (pdf), and noted that it shouldn’t be used for individuals. However, as Mother Jones notes, BMI entered the mainstream after World War II, thanks to insurance companies who wanted a way to quantify fat. Their theory was policyholders who weighed the most seemed to die younger. Their promotion of the tool made converts out of researchers, and eventually even clinicians.

Americans labeled obese paid 22% more in health insurance premiums in 2013 than people in the normal range, even though research has suggested that people in the overweight range might live longer than their “normal” counterparts, and that it’s actually most dangerous to be underweight.

The trouble is, BMI is clearly not a perfect indicator of health; it’s just the easiest one. It doesn’t differentiate between muscle and fat content, so an athlete like professional US women’s basketball player Ebony Hoffman could be classified as overweight. Hoffman is 6’2″ and weighs 215 lbs., which puts her at a 27.6 BMI.

AP Photo/Jessica Hill That’s Ebony Hoffman on the right.

Athlete or average citizen, the BMI isn’t the universal tool it’s been promoted to be. It doesn’t take into account age, gender and race, which has led to potentially misidentifying populations of African American people as overweight.

And focusing on your BMI score may be detrimental to your health. In a study from the US Agricultural Service, people who received group education about body acceptance and intuitive eating had better cholesterol and blood pressure after two years than they did at the beginning of the trial. The other group’s health didn’t change even though they kept a food diary, learned how to limit calories and fat intake and monitored their weight. Neither group weighed less after two years, but the overall wellness group was more active and practiced better eating control.

If you want to know more definitively if you’re overweight and at a higher health risk because of that, measure your waist size—high abdominal fat is a more specific predictor of health problems. Either way, if you really want to get healthy fast it’s probably best to disregard the BMI and go to the gym instead. For common diseases like diabetes, exercise—not body mass—is the best predictor of how long, and how, you’ll live.