How to Calculate Your BMI (Body Mass Index)

Calculating your body mass index (BMI) involves using a simple formula with your weight (in pounds or kilograms) and height (in inches or meters). BMI is used to estimate your body fat and establish whether you are underweight, at your healthy weight, overweight, or have obesity.

BMI can also be used to assess your risk for certain health conditions, like heart disease. Even so, BMI can mean different things as you get older or if you are of high or low fitness level. Its flaws and limitations as a health metric are well-documented.

The article shows you how to calculate BMI and what it says about your health. It also describes the pros and cons of BMI scores and what they mean and don't mean.

How to Calculate BMI

You can calculate your BMI using simple math skills, either with a calculator or by hand. It can be done with the metric system (kilograms and meters) or the imperial system (pounds and inches).

The calculations are described in the following formulas:

Here are examples of how to apply those formulas:

Example 1: Calculating BMI With Kilograms

Example 2: Calculating BMI With Pounds

Online BMI calculators are also available online and as smartphone apps.

Other BMI Calculation Tools

The following handy reference charts can help you find your BMI based on your height and weight. There is also a reference chart for children 2 years and over.

BMI Chart for Adults

BMI Chart for Children

What Do BMI Scores Mean?

The optimum range for a healthy BMI is between 18.5 and 24.9. Anything above and below these values is technically considered underweight and overweight, respectively.

Body mass index scores are broken down into the following categories:

• Underweight: BMI less than 18.5
• Normal weight: BMI of 18.5 to 24.9
• Overweight: BMI of 25 to 29.9
• Obesity: BMI of 30 or higher

Obesity is sometimes broken down into additional categories:

• Class 1 (mild obesity): BMI 30 to 34.9
• Class 2 (moderate obesity): BMI 35 to 39.9
• Class 3 (severe obesity): BMI of 40 or higher
  

How BMI Affects Your Health

BMI scores in the overweight and obese categories can indicate that a person is at higher risk of diseases like:

• Heart disease
• High blood pressure
• Cancer
• Osteoarthritis
• Type 2 diabetes
• Gallstones
• Sleep apnea
• High cholesterol
• Liver diseases

Excess body fat has been linked to an increased risk of a number of health problems, but it's important to remember that BMI calculations alone can't diagnose health issues—just identify possible red flags.

Pros and Cons of BMI

BMI is a simple way to estimate body fat. It’s convenient, inexpensive, and can be performed anywhere. A BMI can be useful in identifying someone with certain health risks and, because of this, is used universally by healthcare providers around the world.

However, it also has significant limitations. As a tool, a BMI score has no real diagnostic value but is rather used to help calculate your risk of certain diseases (such as with a cardiac risk estimator).

Even with conditions like metabolic syndrome, a BMI plays no part in the diagnosis of the condition (although it can help set weight loss goals for someone who has been diagnosed).

BMI can also be a false alarm for a significant number of people due to the factors that BMI does not account for.

Here are five examples:

• Muscle mass: Because BMI cannot distinguish between fat and muscles, it can overestimate body fat in athletes or individuals with lots of muscle mass. It can also underestimate body fat in people with very little muscle mass.
• Sex: Females tend to have more body fat than males. Therefore, a woman in a “safe” category may be at higher risk of heart disease and other conditions than her male counterpart of the same BMI.
• Age: BMI may not accurately predict health risks for young and old people. For example, a BMI below 23 in someone over 75—which is in the "normal" range for adults—is associated with a higher risk of mortality (death), while being in the “overweight” range is not.
• Race/ethnicity: Body fat proportions can vary by race, meaning that BMI scores can mean very different things if you are Asian (who have greater proportional body fat than Whites at the same BMI) or White (who have greater proportional body fat than Blacks at the same BMI).
• Body shape: Evidence suggests that where body fat is located matters. Those who carry weight around the middle—what's called an “apple” body shape—have higher health risks than people with a "pear" shape (those who accumulate weight in the hips and thighs).
  

For this reason, some experts now argue that BMI interpretations need to be tailored for specific groups.

What Are the Alternatives to BMI?

Some have called for "retiring" BMI because of these limitations. While its disadvantages are widely accepted, BMI remains in use partly because of its simplicity and convenience.

Even so, there are other methods for measuring body fat and/or assessing health risks that can be used in place of (or in addition to) BMI, such as:

• Waist circumference: Fat carried around the middle of your body can increase your risk of type 2 diabetes and heart disease. The risk increases for women with waist measurements greater than 35 inches and for men with waist measurements greater than 40 inches.
• Waist-to-hip ratio: Dividing your waist circumference by your hip circumference can provide information about potential health risks as well. A measurement of 0.9 or more for women and 1.0 or more for men indicates a higher risk.
• Skin-fold measurements: Body fat percentages can be estimated through the use of calipers (a pliers-like tool that measures the thickness of the skin). Measurements are taken in multiple parts of the body. Body fat is then calculated based on a mathematical formula.
• Smart scale: Body fat can be estimated with certain "smart" bathroom scales. These scales measure body fat by sending a harmless electrical current through your body.

More sophisticated methods of measurement can also be found in specialist or research settings, including:

• Underwater weighing: Also called hydrostatic weighing or densitometry, it involves weighing yourself in air and then water. The two measurements are then compared to calculate body fat.
• Air displacement plethysmography (ADP): This uses air to estimate your body fat percentage based on the density of your body.
• Dual-energy X-ray absorptiometry (DEXA): This imaging technology is used used to evaluate body composition, including fat, muscle mass, and bone density.
  

Summary

Body mass index (BMI) is a calculation used to estimate your body fat. Based on the BMI score, you may be categorized as being underweight, of normal weight, overweight, or having obesity. The BMI score may also be used to estimate your risk of certain diseases, like heart disease.

BMI has a number of limitations. It does not take into account your age, race, sex, fitness level, or muscle mass. As such, the same BMI can mean very different things in different people.

Multiple factors need to be considered when determining what a "healthy" body weight is. BMI is only one value of many and one that cannot be used on its own to diagnose any disease.