What it is: A mathematical measurement that adjusts your weight for your height. Your number will tell you if you fall in the underweight, normal, overweight, or obese categories.
Does it work? Well, sometimes. A 2012 study in the International Journal of Obesity found that 29% of people who were “normal” weight actually had a body fat percentage in the obese range. So it may miss people who look thin but actually carry a lot of body fat. Another issue: if you’re especially muscular and fit (hello, Serena Williams), you’ll naturally weigh more since muscle tissue is denser than fat. That can throw your numbers into the overweight category.
What to do: You can find a BMI calculator here. While the average non-athlete can trust that they won’t get a false “overweight” result, given the limitations, you should still interpret your results with caution.
What it is: For women, the ideal waist measurement is less than 35 inches; for men, less than 40.
Does it work? Yes, according to a large consensus statement released from a group of organizations like the American Diabetes Association. It’s a marker of excess abdominal fat, which puts you at risk for heart disease, diabetes, and stroke. William Yancy, MD, director of the Duke University Diet and Fitness Center, uses waist circumference for patients who are overweight, but not obese.
What to do: Place the tape measure around your middle, just above your hipbones, advises the CDC. It should be snug but not too tight. Breathe out and take the measurement.
What it is: Beloved by Tumblr pages and a hot trend in 2013, fans of this method claim that if you can stand with your legs together and see a gap between your thighs, you’re slim.
Does it work? No. “It’s all based on your bone structure and musculature, not if you’re thin or not,” says Charlie Seltzer, MD, a doctor specializing in weight loss in Philadelphia and a diplomat of the American Board of Obesity Medicine. He points to a very lean and healthy figure skater he’s worked with who has muscular thighs and tight hips, but no thigh gap.
What to do: There’s no reason to stand in the mirror and try to stand in such a way you can spot a gap, and critics point out that it can be a knock to your body-image and even trigger eating disorders.