Whether you’re on a mission to lose, gain, or maintain your weight, you’ve undoubtedly taken steps to assess how healthy your current weight is. Learn what body mass index is and discover how a BMI calculator is a key starting point for measuring your health.

What Is Body Mass Index?

Body mass index reflects the body fat that you carry and can also assess your general health status. This index compares your height to your weight in order to determine your body fat ratio. BMI spans 18.5 to 30, but some people may have ratios that are higher or lower than the standard index.

Your BMI is more than a mere number, though. Every measurement includes an assessment based on where it falls within the index. For example, those with a BMI between 18.5 and 24.9 are considered to be within a normal weight range. Those with a BMI under 18.5 are considered underweight, which may suggest too little body fat, while those with a BMI between 25 and 29.9 are considered overweight, which could mean too much body fat. If your BMI is above 30, you’re considered obese, which may mean that you carry too much weight for your height.

How Does a BMI Calculator Work?

A BMI calculator offers a quick and straightforward approach to determine your body mass index. Since most calculator tools allow you to provide either metric or imperial measurements, you won’t even have to convert your numbers. Simply enter your height and your weight, choose the correct unit of measurement and await your results. Once you have your BMI in hand, you can check to see where it falls within the index and determine what it suggests about your body fat.

If you’d rather calculate your BMI by hand, simply take your weight, divide it by your height squared, and multiply the total by 703. You can also use a BMI table to determine your body mass index manually. Note that the standard BMI table has some limitations, though. It’s designed for adult use only and includes a relatively small range of measurements. If you’re much shorter than average or if you’re considered underweight, you may not be able to use the BMI table.

Can Your BMI Measure How Fat or Thin You Are?

BMI can serve as a screening tool, but it isn’t a diagnostic tool. This means it won’t diagnose you as “fat” or “thin,” and you shouldn’t consider it the ultimate measure of your body composition or how your weight affects your overall health. In fact, the 1972 study that popularized BMI cautioned against using the index to assess how fat or thin a person is. Instead, the study intended BMI primarily for research and data collection purposes. It wasn’t until 1985 that the National Institutes of Health (NIH) began to use BMI to screen for obesity.

It’s important to note that while BMI is designed to assess your body fat, its assessments aren’t always accurate. It typically works best for people who have relatively average measurements. For instance, if you’re an athlete who has a muscular build, BMI might overestimate your body fat, mistakenly assuming that some of your muscle weight is fat. Along the same lines, BMI often underestimates body fat in people who have lost a large amount of muscle or those who have less muscle than average.

It’s also essential to remember that there’s only one BMI, so it doesn’t account for factors like your age, gender, or ethnicity. This presents issues with accuracy, as women typically carry more body fat than men do, and younger people average less body fat than older people. In addition, white people tend to carry more body fat than African-Americans but less body fat than Asian-Americans. That means you may want to take your BMI with a grain of salt.

Does Your BMI Determine How Healthy You Are?

Today, many medical professionals use your BMI as a general measure of how healthy you are. Since an overly high amount of body fat can correlate with health problems, the NIH considers BMI a tool for identifying higher levels of risk for serious conditions like heart disease, Type 2 diabetes, high blood pressure, and even some kinds of cancer.

Although BMI can assess some health factors and risks, it isn’t always a smart measurement of your overall health. It should never serve as a stand-alone assessment of your health, either. In fact, a 2017 study shows that assessing BMI alone without accounting for other weight-related measurements causes medical professionals to miss up to half the cases of potentially harmful body fat.

Instead, the NIH recommends using BMI, your waist circumference, and your existing risk factors to assess your health in a more comprehensive manner. If your body fat sits around your waist instead of at your hips, you could have a higher risk of developing conditions like heart disease. In addition, existing risk factors like low physical activity level, high blood pressure, high blood glucose, and a smoking habit could further increase your risk of developing heart disease.

What Other Tools Should You Use to Assess Your Health?

While BMI is still the most common tool for getting a snapshot of your health, one of the reasons it’s so popular is that it’s fast and inexpensive. To gain a more accurate picture of how your weight affects your health, some medical professionals advocate using more technical tools, such as a bioelectrical impedance analysis. This noninvasive method is already common in hospitals and can offer a more complete picture of your body composition.

Other medical professionals advocate focusing on your waist-to-height ratio or your hip-to-waist ratio instead of your BMI. These measurements quickly identify one of the most dangerous types of fat, which means they can offer even more insight into your health than your BMI can.

While BMI isn’t a perfect measure of your health, it can still be a good starting point. When you calculate yours, be sure to view it in the context of other measures of your health, such as your hip-to-waist ratio and your risk factors for serious conditions.

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