Your body mass index, or BMI, helps you understand whether you're carrying too much fat. It represents a relationship between your weight and height that's figured with a specific calculation. BMI scales weigh you and then have you plug in your height to do the computation for you. Knowing your BMI can be helpful and demonstrate positive trends if you're trying to lose weight, but it's not an infallible measure. The best scales provide consistent results so you can track changes in your fat over time.
What is BMI?
Figure your BMI by dividing your weight in kilograms by your height in meters squared: BMI = weight in kilograms / (height in meters x height in meters).
The calculation using American measurements includes a conversion factor: BMI = weight in pounds / (height in inches x height in inches) x 703.
Many reputable health organizations, such as the National Heart, Lung and Blood Institute, offer online calculators in which you can input your height and weight, so you can easily figure your BMI.
A BMI below 18.5 indicates that you're underweight, and one between 18.5 and 24.9 is considered normal. If your BMI is 25 to 29.9, it's likely you're overweight, and 30 or more classifies you as obese.
At-Home Scales That Estimate Body Fat
Scale manufacturers understand that knowing just your weight isn't necessarily a measure of your health. A bathroom scale doesn't tell you the components of that weight -- how much is body fat and how much is muscle.
Some scales -- called bioelectrical impedance scales -- measure your body fat by sending a light electrical current through your body to estimate how much lean tissue and fat tissue you have. These scales can be wildly inaccurate, however, as the results depend on your hydration level, the quality of the instrument, your body shape, and whether you've just eaten or worked out. This type of scale measures your body fat, but it doesn't calculate your BMI.
A BMI scale, which doesn't necessarily directly measure your body fat, is often more accurate as it simply measures your weight and you input your height for it to compute your BMI. But this BMI calculation is only as accurate as the measure of your weight. The price of a model doesn't always mean it's the most precise. Consumer Reports reported in 2016 that some models of expensive scales can be off by 6 to 10 pounds in either direction.
Choosing the Best BMI Scale
The best scales are consistent, not necessarily embellished with bells and whistles like body fat measures and Bluetooth. Test models in the store before you buy. Step on a scale multiple times in a row; the measure should be relatively consistent and not shift wildly if you move gently side to side. If the scale is consistent, you can track your weight trends.
Even the most accurate and consistent BMI scales will reflect the natural weight fluctuations you experience on any given day. Hormones, water retention and even constipation can make your weight vary 3 to 5 pounds from day to day. Your weight is also different at specific times of the day. If you weigh yourself in the morning with no clothes, you'll likely be lighter than if you weigh yourself after a heavy dinner and you're wearing shoes and jeans. For the most part, these weight fluctuations shouldn't change your BMI, however.
For the most accurate measure of your weight and BMI, rely on your doctor's scale, which should be calibrated regularly. Most doctor's offices calculate your BMI for you, but if yours doesn't, just use your weight and height from your office visit and plug them into a BMI calculator.
Limitations of BMI
A scale that measures your BMI can be handy in tracking this measure of your health, but remember that BMI isn't perfect. It's used as a screening tool, not as a diagnostic, because it can be misleading for some people. For example, if you're extremely fit and muscular, your BMI might register as high due to your abundance of muscle -- not because you're overly fat. Muscle is a denser tissue than fat and can make your body heavier. Having a lot of muscle isn't a health risk, though.
BMI also fails to take into account body shape and people with normal weight obesity. If your BMI registers as normal -- between the 18.5 and 24.9 range -- but you carry too much fat, especially in your midsection, you're at the same risk of chronic disease from your weight as someone who has an overtly unhealthy BMI. A waist circumference larger than 40 inches on a man or 35 inches on a woman indicates an unhealthy level of abdominal fat. If you're sedentary or older, visit your doctor regularly to monitor your health -- don't rely only on a BMI scale.
Overall Health and a BMI Scale
Even if you make progress in becoming healthier, your BMI scale won't always register measurable results. For example, you may make a shift in your eating plan to consume more fresh fruits, vegetables, whole grains, nuts, low-fat dairy and lean protein but not actually cut calories, which is key in losing pounds. Your weight and BMI won't change, but you'll benefit from reducing your intake of sugar, saturated fats and added sodium.
Also, if you add exercise and become more physically fit, you're not guaranteed to see a change in your BMI, but you improve your heart health, develop respiratory strength and decrease risk factors for chronic disease. A BMI scale also can't tell if your body composition positively shifts due to exercise. If you lose fat, but put on muscle, your weight and BMI stay the same -- but you've got an overall healthier physique.