Body Mass Index (BMI)

What Is Body Mass Index (BMI)?

Body mass index (BMI) is a way to see if your weight is in a healthy range for your height. Healthcare providers currently use BMI to screen for potential health risks. They also use it to put you in a health category.

In most cases, a higher BMI means you have worsening health. This may include having higher or lower body fat. High body fat increases your risk for several health conditions. Low body fat may mean you’re not getting enough nutrients. Having the right amount of body fat helps your body function properly and stay healthy.

But your BMI alone doesn’t tell the whole story. It’s just a starting point that your provider may use as a reference to determine if there are steps you can take to improve your health.

How do I calculate my BMI?

You can calculate BMI yourself with these steps:

• Multiply your weight in pounds by 703.
• Divide that answer by your height in inches (there are 12 inches in 1 foot).
• Divide that answer by your height in inches again.

For example, a person who weighs 180 lbs. and is 5 feet and 5 inches tall (65 inches) would calculate their BMI as:

1. 180 x 703 = 126,540
2. 126,540 / 65 = 1,946.769
3. 1,946.769 / 65 = 29.95

Their BMI would be 29.9.

The BMI calculation is the same for adult men and women. The BMI calculator for children is different.

What is a normal BMI?

A healthy BMI range for an adult is 18.5 to 24.9. “Normal” is only a word that reflects where your BMI is on the chart. Your healthcare provider will help you understand your health and any changes you may need to make.

BMI and weight categories

BMI also helps providers categorize weight into categories. Your number corresponds to a category on the BMI chart:

• Underweight: Less than 18.5
• Optimum range: 18.5 to 24.9
• Overweight: 25 to 29.9

If your BMI is 30 or higher, you are affected by obesity. The range for obesity includes three additional categories:

• Class I obesity: 30 to 34.9
• Class II obesity: 35 to 39.9
• Class III obesity: More than 40

Screening for health risks using BMI

Each weight category has health risks associated with it.

In general, the higher your BMI, the higher your risk for:

• Heart disease
• High blood pressure
• Type 2 diabetes
• Certain cancers, including colon, breast and uterine

If you have a BMI of less than 18.5, you may be at a higher risk for:

• Malnutrition
• Anemia
• A weakened immune system
• Osteoporosis
• Infertility

It’s important to remember that you can have a health condition at any BMI. Things like genetics, sex and age play a role in developing many health conditions.

What are the limitations of BMI?

The standard BMI chart has limitations for several reasons. So, you shouldn’t put too much emphasis on your result. Healthcare providers use it because it’s a quick and inexpensive tool.

Some of the limitations are:

• It doesn’t differentiate between lean body mass and fat mass. Because of this, a person can have a high BMI (by being muscular), but still have low body fat.
• The same calculation is used for males and females, even though adult females may have more body fat than adult males.
• It doesn’t measure the location or distribution of body fat. This is an issue because excess belly fat tends to have more health risks than fat stored in other areas, like your thighs.
• It doesn’t account for factors like family history, ethnicity, sex and activity level.

You likely shouldn’t use the standard BMI chart to calculate body fat for:

• Athletes and bodybuilders
• Children and teenagers
• Anyone who’s pregnant
• Anyone over the age of 65
• People who have muscle atrophy