Your body mass index, or BMI, represents an estimation of your body fat. BMI scales weigh you, just like any other scale, and ask you to input your height. The scale then performs a calculation that computes your BMI. BMI scales save you the extra step of consulting an online calculator or figuring the measure yourself. The scales help you determine if you're at a healthy weight for your height but aren't the only tool you can use.
What is BMI?
Your BMI is equal to your weight in kilograms divided by your height in meters squared. In American measurements, a conversion factor is used, so the equation reads:
BMI = weight in pounds / (height in inches x height in inches) x 703.
A BMI scale has this formula embedded in its programming. After you've input your height and step on the scale to get your weight, it does the math for you and reveals your BMI.
The resulting number provides insight into your health based on your weight. A BMI below 18.5 indicates that you're underweight. A BMI of 18.5 to 24.9 means your weight poses no health risk. A BMI of 25 to 29.9 suggests you are overweight, and a result of 30 or greater indicates that you are obese.
Pros and Cons of a BMI Scale
A BMI scale makes it easy and convenient to determine where you fall on the index. Knowing your BMI helps you monitor your risk of disease related to your weight, but the measurement isn't diagnostic. A high BMI indicates that you'd benefit from additional health screenings from your doctor, such as cholesterol level checks and blood pressure monitoring. Figuring your BMI on your own is rather simple, though, so the extra cost of a scale with this feature may be unnecessary.
A BMI scale cannot determine if your weight is mostly fat or muscle. Extremely athletic people may have an abundance of muscle, so their weight seems heavy for their height and the scale registers a high BMI in the overweight range. Muscle is a denser tissue than body fat, which explains the higher BMI, but this abundance of muscle doesn't present the same health risks as does an excess of fat.
A normal BMI doesn't clear you of health risks either. Some people can have a normal weight and fall into the healthy BMI range, but have too much body fat. If your body composition is 20 percent fat or greater as a man or 30 percent or greater as a woman, you may be at the same risk of diseases, such as cardiovascular disease and type 2 diabetes, as someone who is overtly overweight. Sedentary people and older adults are at risk of being overly fat, but of a normal weight and BMI -- a condition called normal weight obesity.
Other Ways to Measure Body Fat
A body fat scale is an alternative to a BMI scale. These scales employ bio-electrical impedance to more directly measure body fat. When you stand on the metal plates on these scales, a mild electrical current is sent through one leg, over your pelvis and down the other leg. Fat is a poorer conductor of electricity than lean mass, including muscle, organs and bones. The scale measures the resistance the electrical current faces when passing through your body, plugs that into an equation and reveals a body fat percentage.
The scales do provide a direct measure of fat -- which isn't available via BMI -- but they aren't always accurate. When six different body fat scales were evaluated by Consumer Reports in 2016, they revealed inaccuracies between 21 and 34 percent. To get the most accurate readings, use a clinical grade scale -- usually found in fitness centers and hospitals -- be adequately hydrated, urinate before stepping on the scale and be sure your feet are clean and dry.
The most accurate ways to determine your body fat are other direct measures such as skinfold caliper tests, underwater weighing, an air displacement technique or bone density X-ray scans. These methods are usually available only in health centers and clinical settings and must be performed by a trained professional. Because they require specialized equipment, they do come with a higher price tag than figuring your BMI.
Healthy Changes Are More Important Than BMI
If you do invest in a BMI scale to help you manage your weight, recognize its limitations. If you start out with a high BMI, you might seek to lower it with healthier food choices and more exercise. These changes do contribute to better health, even if you don't see a tremendous change in your BMI.
Increased physical activity, for example, can greatly improve your heart health regardless of your weight or BMI. Exercise can also cause you to gain muscle as you lose fat, which may not drastically change your weight or your BMI, but still represents an improvement in your health. Choosing healthy foods such as whole grains, fruits, vegetables and lean meats -- even if you don't change your calorie intake -- can improve other markers of your health, such as blood pressure and blood cholesterol, without notably changing your BMI.
If you're working to improve your health, remember that BMI is just one marker -- and a rough one at that. Keep eating well and exercising. Use the way you feel and the improvements in your fitness as confirmation that you're doing good things for your body.