Most of us now understand that the our weight alone doesn't tell the full story about our health or fitness. Neither, frankly, does body mass index (BMI) or body fat percentage, but these are additional pieces that can help us put the puzzle together. Unfortunately, there is a lot of confusion about what these numbers are and what they mean, so here is an attempt to clear them up.
First off, BMI and body fat percentage are different things! BMI is a set of numbers used to estimate the body composition of average, non-active people based on their height, weight and gender (as compared to a large data set of people of the same height and weight). You can get an estimation of your BMI here. A BMI of 18.5 or lower is considered underweight, over 18.5 to under 25 is considered "normal," 25 and over is considered overweight and a BMI of 30 or more is considered obese. The problem with BMI is that it is based on averages and not on your individual makeup. A very muscular athlete, for example, would probably be classified as overweight according to BMI. Muscle weighs more than fat, after all. If you begin an exercise program and begin developing muscle and losing fat, the scale might not budge, and neither will your BMI since it is based on your weight and height. Frustrating!! What will change, besides the image in the mirror, the fit of your clothes, and the way you feel, is your body fat percentage.
Your body fat percentage is how much of your body is made up of fat, including the essential fat that you need to survive and function. It can be measured in several ways, with the most accurate being hydrostatic (underwater) weighing, which is done in a medical facility and somewhat impractical for repeated measurements. Other methods include circumference measurements (using a tape measure), skinfold measurements with a caliper, and bioelectrical impedance, in which a small electrical current is sent through the body - muscle conducts electricity better than fat since it holds more water (but this method varies depending on the hydration level of the subject). The easiest method is probably bioelectrical impedence and a variety of scales are now available for home use that include this function. Whatever method you choose, note that you should measure under similar conditions each time. Drink plenty of water before measuring, measure before exercise and measure at the same time of day if possible. Try not to measure every day since daily fluctuations are not as meaningful as weekly or monthly patterns of gain or loss.
According to the American Council on Exercise, the following ranges of body fat percentage have been classified under these categories. Women have higher fat percentage needs for reproductive and hormonal functions.
Description: Women Men
Essential fat 10–12% 2–4%
Athletes 14–20% 6–13%
Fitness 21–24% 14–17%
Acceptable 25–31% 18–25%
Overweight 32-41% 26-37%
Obese 42%+ 38%+
Keep in mind that these numbers are not the only numbers out there - there are other professional opinions. Also, if you are classified as overweight or obese, this is not the end of the world nor something to browbeat yourself about - it is just a number, another tool to use in assessing and improving your health. If it makes you crazy, set it aside and focus on being active and eating well. This is, after all, the point!