What is BMI and Why Does it Matter?

    
Emmie Satrazemis, RD, CSSD
Emmie Satrazemis, RD, CSSD

Your body mass index is a common health screening tool used worldwide by medical professionals to asses population weight status and risk for disease. But what exactly is BMI, how is it calculated and how accurate is it really? Here’s everything you need to know about this long-used health metric and what you can do to get yours into a healthy range. 

BMI Calculator

Learn your body mass index in minutes using this simple BMI calculator

What Does BMI Mean?

Body Mass Index (BMI) is a measurement of your body size based on your height and weight ratio. 

It is a basic formula that has been in use since the mid-19th century - using your current height and weight to provide a single numerical value to estimate your body fat level and associated health risks. It is also one of the few, affordable methods available to measure obesity in individuals and large groups. 

BMI and Obesity

While body mass index is not a direct measurement of body fat percentage, it is frequently used as a benchmark for a healthy weight range. Some argue that this is not an accurate method since many athletes and individuals with high amounts of lean muscle mass would fall into a higher BMI range, making it appear they were overweight or obese when in actuality they are heavy in weight but lean in body composition. 

Additionally, studies suggest that a high BMI is only somewhat correlated with higher body fat percentages found in body composition tests like skinfold thickness measurements, bioelectrical impedance, underwater weighing, and dual energy x-ray absorptiometry (DXA scans) (1,2,3,4). 

However, this doesn’t mean that this measurement is useless or should not be included in individual health assessments. Your height and weight ratio still provide meaningful data.

BMI and Health Risks

Where BMI provides a much stronger association to health and is likely a better unit of measurement is in population studies to assess the overall fitness of large groups. Research has often found that a higher body mass index is linked to metabolic disease and poor health problems that are directly tied to increased body fat percentage (5,6,7). 

Because obesity is linked to so many poor health outcomes, it is no surprise that a high body mass index has also been associated with numerous health concerns including high blood pressure, high cholesterol, type 2 diabetes, heart disease, stroke, gallbladder disease, poor mental health, sleep apnea, and osteoarthritis (8,9,10,11). 

It is also crucial to note that because BMI is increasingly used in chronic disease studies, it also plays a major role in many clinical treatment guidelines used by the medical community. 

Who Uses BMI?

Calculating BMI is easy and affordable making it a desirable tool for many large organizations like the Centers for Disease Control (CDC) and many healthcare professionals. Its also understood worldwide. 

While it is not a perfect assessment, it is a quick and efficient method that can play a valuable role in overall disease control and prevention. 

It is also a great screening tool for medical professionals to evaluate their patients’ weight status and risk for obesity. Of course, further assessment of physical activity level, nutrition, family history, and biometrics is typically needed to establish any potential health risks of an individual.   

Bottom line: BMI is a potentially valuable measurement used to determine your current health and fitness status. 

What’s a Normal BMI?

For most people, when your body weight is proportionate to your height, your BMI would fall into the healthy or normal range of 18.5 to 24.9 for adults. Anything below this range would indicate that you are underweight and anything over would imply that you are potentially overweight or obese. 

Keep in mind, this measurement is just an estimate and does not take into account your muscle mass and fitness level. You can get a much better assessment of your body fat percentage and weight status by undergoing a body composition test. 

Once you know your body mass index score, you can easily determine where you fall using the following BMI ranges or BMI Chart. 

BMI

Weight Category

>18.5

Underweight

18.5 – 24.9

Normal or Healthy Weight

25.0 – 29.9

Overweight

30.0<

Obese

BMI for Kids vs Adults

Because BMI is a measure of your height and weight proportions, children and teens are assessed differently than fully grown adults. A BMI percentile is used in kids to compare their height and weight to children of the same age and sex - giving healthcare providers a good indicator of how the child is growing. 

Both a child’s height and weight can be a sign of excess calories or poor nutrition intake. Thus, anyone under the age of eighteen should have their body mass index measured on an appropriate growth chart.

What is My BMI?

You can learn your body mass index using this simple formula or by providing your height, weight, and age in an online BMI calculator. 

Use one of the following two formulas: 

  • weight (kg) / [height (m)]2
  • weight (lb) / [height (in)]2 x 703

Just remember, BMI is only one way to measure your health status and is not a perfect measurement of your body fat percentage. Speak with your doctor or a registered dietitian if you are concerned about your results and want to take steps to lose weight or improve your measurement. 

How to Lower Your BMI

It is fairly impossible to change your height, so the only way to improve your body mass index score is by losing weight. This can be achieved by sticking to a calorie-controlled diet, increasing your physical activity level, and improving the nutrition of your everyday food choices. 

Start by figuring out exactly how many calories you need a day and then log your daily food intake to ensure you are sticking to this goal. This can take some practice, but the longer you stick with and try to stay consistent, the better results you will see. You can also increase your daily movements to burn more calories and speed up the process a bit. 

Start with small changes you can build on and keep at it. Over time you’ll find these small goals become habits and living a healthy lifestyle begins to feel easier and more attainable.

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