Body mass index, or BMI, and body composition are ways to determine whether you're at a healthy weight -- and, by extension, if you're facing a higher risk of obesity-related illnesses, like type 2 diabetes and heart disease. Both types of measurements have their benefits and drawbacks, and how you use them to measure your fitness progress depends on your starting point and goals.
BMI vs Body Composition
Body mass index provides insight into whether you're likely to be at a healthy body weight, based on your weight and height. You can easily enter your information in an online calculator to figure out your BMI, or calculate it by hand using the equation below:
BMI = [weight, in pounds / (height, in inches x height, in inches)] x 703.
Body composition doesn't directly look at your body weight. It's a measure of how much of your weight comes from lean mass, which includes muscle, bone, connective tissue and water, and how much comes from fat. If you weigh 150 pounds, and 120 pounds of that is lean mass, you have a body composition that's 20 percent body fat.
Pros and Cons of Measuring BMI
BMI is often used as a measure of obesity because it's easy to figure out; you don't need expensive equipment, just a scale and a tape measure get your weight and height. It's also a good measure of whether you're at a healthy weight for the average person.
However, BMI isn't the best measure for everyone. If you're "skinny fat" -- meaning you have a healthy body weight but a higher-than-healthy body fat -- you might be healthy according to BMI, but still have a higher risk of developing an obesity-related illness. BMI becomes less useful as you age. Most people lose muscle mass and gain fat as they get older, so you have a higher risk of being "skinny fat" at a healthy BMI the older you get. Conversely, BMI isn't always a good fit for healthy and muscular people; your BMI might be considered overweight or obese, even though you have low fat levels.
Pros and Cons Measuring Body Composition
Measuring body composition offers much more insight into your health and disease risk than BMI. Because it measures how much of your weight comes from lean mass and how much comes from fat, you'll get insight into your risk of obesity-related illness, no matter what your actual weight is on the scale.
For example, the "skinny fat" person at a healthy weight according to BMI would be considered over-fat according to their body composition, so they'd be alerted to their higher risk of obesity-related illness. Conversely, the muscular person who has an "obese" BMI will have a healthy body composition due to their high lean mass and low fat levels. Looking at body composition would show they're at a lower risk of obesity-related illness, even if they weigh more than the average person.
Body composition is more difficult to measure than BMI, though. It generally requires access to a trained professional, as well as expensive medical equipment, to get an accurate look at your body composition. Methods of measuring body composition at home -- like body fat scales -- aren't generally very precise, and they can give inaccurate readings.
Goals to Aim for
A healthy BMI ranges from 18.5 to 24.9 -- that's 125 to 168 pounds for a person who's 5 feet, 9 inches tall, for example -- according to the Centers for Disease Control and Prevention. Any lower than that, and you're considered underweight. If your BMI is 24 to 29.9, you're considered overweight, and a BMI over 30 is considered obese.
Ideal body composition goals depend on your gender, as women need more body fat than men for childbearing. In general, a body composition that's over 32 percent fat is considered obese for women; for men, the cutoff is 25 percent body fat.
Which metric you should choose to measure your progress depends on your current weight and fitness goals. If you're currently significantly overweight or obese from carrying excess weight, start tracking your progress by measuring your BMI. As you get closer to your goal weight, you can shift your focus to body composition to measure fat loss, since you might notice more subtle changes on the scale as you approach a healthy weight.
If you're already at a healthy weight, consider measuring your body composition to ensure you're not carrying too much fat. Gaining muscle and losing weight might not have a significant effect on your BMI, but it can profoundly affect your body composition, as well as your disease risk.