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You’ve likely heard BMI mentioned before and may even know your own, but what does BMI – or Body Mass Index – really mean and how can it help you better understand your overall health? Over recent years, there has been a lot of debate about whether or not it’s actually a useful measure of health. So who’s right and who’s wrong?

Let’s start with the basics.

What is BMI?

“Body Mass Index” or BMI for short is an estimate of body fat based on a person’s height and weight. The measurement is used as a tool to help people understand if they are at a healthy or unhealthy body weight. A high BMI indicates that the body has too much fat, whereas a low one indicates the body has too little fat. The higher an individual’s BMI, the greater the chance there is for developing certain health conditions like heart disease, diabetes, and high blood pressure.

However, it’s important to note that BMI is only an approximation and is not a precise measure of body fat. To better understand BMI, let’s take a closer look at how you can calculate your BMI, what your BMI means for your health, and some of the limitations of BMI.

Black chalkboard saying 'BMI' next to fruit and measuring tape

How do you calculate BMI?

Anyone can calculate their BMI using a simple formula. All you need to know is your weight and height. Depending on if you’re using the metric system or not, the formula will differ slightly.

When using the metric system, the formula is calculated with weight in kilograms and height in meters. Simply divide your weight by your height squared.

BMI = weight (kg) / [height (m)]²

You can also use this metric system adult calculator.

If you’re calculating with pounds and inches, the formula is calculated by dividing your weight (in pounds) by your height (in inches) squared and multiplying that by 703.

BMI = weight (lb) / [height (in)]² x 703

You can also calculate your BMI using pounds and inches with this adult calculator.

What is a healthy range?

BMI calculations are interpreted based on standard weight status categories. These categories are: underweight, healthy weight, overweight, and obese. You can use your BMI to understand which category you fall into. However, interpretation varies slightly depending on whether you’re an adult (in this case 20 years or older) or a child or teen.

Body Mass Index for adults

Adult BMI weight status categories are the same for both men and women—no matter what your body type or age.

For adults 20 years of age and older, the standard weight categories are as follows:

BMIWeight Status
Below 18.5Underweight
18.5 – 24.9Healthy weight
25.0 – 29.9Overweight
30.0 and aboveObesity

A score over 30 can further be broken down into the following obesity classes:

BMIObesity Class
30 – 34.9Class I obesity (mild)
35 – 39.9Class II obesity (moderate)
40.0 and aboveClass III obesity (severe)
Small boy being measured by his mother against a door frame

BMI for children and teens

For children and teens, BMI is calculated the same way it is for adults but interpreted differently. Unlike for adults, gender and age are taken into account. This is because body fat at these ages changes and the amount of body fat differs for boys and girls. For these reasons, BMI for children and teens in the U.S. is interpreted using BMI-for-age growth charts with percentiles.

You can find age growth charts on the U.S. CDC website. The weight status categories assigned to the different percentile ranges are shown below:

Weight StatusPercentile Range
UnderweightLess than the 5th percentile
Healthy weight5th percentile to less than the 85th percentile
Overweight85th to less than the 95th percentile
ObesityEqual to or greater than the 95th percentile

How low or high BMI may impact your health

A BMI that falls below or above the healthy weight category may indicate that you are at an increased risk for certain health problems. This applies to both those who are underweight and overweight as well as obese.

For those with a low Body Mass Index (underweight), potential health risks include:

  • Osteoporosis
  • Poor immune system
  • Iron deficiency
  • Heart problems

For those with a high Body Mass Index (overweight or obese), potential health risks include:

  • High blood pressure
  • Heart disease
  • High cholesterol
  • Type 2 diabetes
  • Osteoarthritis
  • Stroke
  • Gallbladder disease
  • Certain cancers (endometrial, breast, colon, kidney, gallbladder, and liver)
  • Sleep apnea

However, it’s important to note that BMI is not the be-all and end-all measure of one’s health. In other words, just because you have a BMI that classifies you as overweight, doesn’t mean you are guaranteed to develop any of the conditions listed above. Instead, it indicates that you may be at a higher risk. And that’s only assuming it’s accurate for your body.

Is BMI a good measurement of health?

So, when it comes down to it, is it really a good way to measure your health? Although it’s widely used tool for estimating body fat, BMI has its limitations.

It’s is a simple, quick, and inexpensive way to measure an individual’s health. That’s why it’s commonly used by healthcare providers to do so. However, it’s not necessarily an accurate representation of a person’s overall health. There are many limitations in terms of accuracy and application.

When measuring body fat, BMI fails to take into account several different factors that may impact a person’s results, including:

  • Fat vs. muscle: Muscle tends to weigh more than fat so BMI may overestimate body fat, especially in athletes or people with muscular bodies. It may also underestimate body fat in people who have lost muscle mass, particularly older adults.
  • Age: On average, older adults have more body fat than younger adults, BMI may underestimate body fat in older adults.
  • Gender: In general, women tend to have more body fat than men but Body Mass Index does not take into account gender differences.
  • Race: Body fat distribution tends to vary for different races and ethnic groups, and BMI was initially based on Caucasian males – so it often isn’t an accurate indicator of health for people of color.
  • Body fat distribution: BMI doesn’t tell us how body fat is distributed. Body fat may be concentrated in certain parts of the body. Individuals with more body fat around the abdomen and organs are at a higher risk of health problems.

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So should we rely on it?

When it comes down to it, BMI can be a useful tool for estimating body fat. However, it is not a one-size-fits-all solution. Knowing your BMI can be a helpful guideline in understanding whether you may be at an unhealthy or healthy weight. But it’s definitely not a measurement without limitations, so should really only be used as a very rough guideline.

In most cases, it’s best to consult with your doctor to get a clearer understanding of your overall health. Accurately measuring your body composition through a smart watch or scale can also help paint a better picture of whether your body fat percentage is in the healthy range.