Evidence Based Fact Checked

What is BMI (Body Mass Index)? How to Calculate your BMI?

BMI stands for body mass index. BMI measurement is essentially a tool to help doctors and patients assess patients’ height-to-weight ratios.

BMI doesn’t measure your fats directly, and it also doesn’t depend on your age, sex, ethnicity, and muscle mass in adults.

Like any tool, it is not perfect. It is helpful, however, especially to diagnose obesity or morbid obesity.

BMI chart and BMI calculator

It’s simple to check your own BMI. Use an online BMI calculator. For adults who are age 20 and older can use the BMI calculator developed by the CDC.

Enter your weight and height, and the calculator will provide your BMI result along with a range to help you determine into which category you fall.

Likewise, you can also use BMI charts provided by the National Heart, Lung, and Blood Institute (NHLBI).

The BMI chart looks for height (in inches) and the corresponding body weight (in pounds). Now you have to locate the category (normal weight, overweight, and obesity) in which your BMI falls.

Manually, you can calculate BMI by dividing a person’s weight by squaring their height.

BMI = m/h

m = body weight (in kilograms)

h = height (in meters)

Understanding the BMI results (in adults)

BMI has different ranges, and the below table helps you locate your weight status.

Weight status
Below 18.5
Healthy weight
30.0 and above


If your BMI is less than 18.5, you are considered underweight. Being underweight can be just as stressful on your body as being overweight.

Some health concerns for underweight people are as follows:

  • The weakening of the immune system
  • Higher risk of osteoporosis
  • Infertility

Being seriously underweight can lead to problems with your heart and other organs.

Talk to your doctor about healthy ways to add weight, including increasing your caloric intake with healthy foods and strength training to develop lean muscle.


When your BMI is between 18.5 and 24.9, your weight is considered normal for your height.

It is possible to have a normal BMI and have trouble being overweight, including high cholesterol and type 2 diabetes.

Eat a healthy diet and exercise regularly to stay within this range. Have regular checkups with your doctor to make sure your cholesterol and insulin levels are good.


If your BMI is 25 to 29.9, you are considered overweight.

This category can be tricky and is the most difficult to interpret. One of the flaws of BMI measurement is that it only measures total weight.

It doesn’t take into account your skeletal structure or your muscle mass. Since muscle is denser than fat, if you are fairly muscular, you will weigh more than an average person of your height, even though you may look leaner.

Professional athletes often have BMIs in the overweight category, although they have hardly any fat on their bodies.

To help determine if you are truly overweight, your doctor will use a caliper to take a body fat reading. If you do, in fact, fall into this category, it’s time to consider a lifestyle change.

Even a moderate amount of extra weight increases your risk for several health conditions, including stroke, heart attack, and diabetes.


If your BMI is over 30, you are medically obese.

As your BMI rises, your health risks are considerably greater.

According to a study co-authored by the American Cancer Society and the National Cancer Institute, one in three Americans over the age of 20 are obese. Approximately 17 percent of women and 11 percent of men are severely obese, with BMIs over 40.

One of the most frightening results of the study is that people with BMIs of 30 to 35 have a 44 percent increase in their risk of death than people with BMIs in the normal range.

People with BMIs between 35 and 40 increase their risk of death by 88 percent. Those with BMIs over 40 have an astonishing 250 percent increased risk of death. According to the study, these numbers remained static, regardless of other risk factors, including alcohol use and activity level.

Why is the increased risk of death so dramatic? A higher-than-30 BMI drastically increases your chances of developing the following serious health conditions:

  • High blood pressure
  • Stroke
  • Heart disease
  • Diabetes
  • Difficulty breathing
  • Certain types of cancer

While many people are aware of these risks, few know that a BMI in the obese range means tremendous kidney strain.

One study found that nearly 35 percent of kidney disease could be prevented if obesity were not a factor.

The same study found that overweight people had a 40 percent increased risk of kidney disease than those with normal BMIs, and obesity meant an 83 percent higher risk.

If you are overweight or obese, work with your doctor to create a weight loss plan focused on a healthy diet and exercise plan. Reducing your BMI will reduce your risk of disease and early death.

BMI calculation in Children and Teens

The BMI calculation is children and teens are taken differently. The BMI calculation is not dependent on gender for adults, but it works differently for children and teens.

The children and teens both develop fat at different ages and a different rates.

So, age and gender are taken into consideration while calculating the BMI of children and adolescence.

Firstly, the doctors will calculate BMI for both children and teens in the same way they calculate for adults. After that, they compare BMI numbers with a person’s age and gender.

BMI Calculator for Children and Teens

The Centers for Disease Control and Prevention (CDC) has developed a BMI calculator for children and teens, which provides BMI and the corresponding BMI-for-age-percentile.

After calculation, you need to compare with the age of the boys and girls.

You can use the chart below for girls and boys.

What does the result mean?

If the percentile is below 5th, then it is underweight, and if it is above the 5th percentile and below 85th percentile, then it is a healthy weight.

If the percentile is between 85th and 95th, then it is Overweight, and if the percentile is equal and above 95th, then it is obesity.

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DISCLAIMER: This article is for educational purposes only, always check with your medical doctor before stopping any prescription medications or when implementing any dietary and lifestyle changes.

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