What Is Body Mass Index (BMI) and What Does It Measure?

BMI, which stands for body mass index, is a calculation that is sometimes used in healthcare settings as an indirect method to determine a person's body fat category. This BMI calculator can help you learn what this measurement means and how it relates to your health and fitness.

BMI is not a diagnostic tool nor is it a measurement of body fat percentage. A high BMI may be an indicator of high body fatness, but it doesn't necessarily mean that a person is overweight or obese and it is not a direct indicator of health.

In some populations, BMI has been found to be a fairly reliable indicator of body fat measures. But the calculation is less effective in other groups, such as bodybuilders and the elderly. There are other methods that are more accurate in estimating body fat.

How BMI Is Measured

Your BMI is calculated using your height and weight. It can be a starting point for understanding the way your body fat may impact your overall health. You can use the number along with other health measurements to begin a conversation with your healthcare provider about ways to reduce your risk for disease and improve your overall wellness.

Metric BMI

• Formula: weight (kg) / [height (m)]²
• Example: weight = 68 kg, height = 165 cm (1.65 m)
• BMI calculation: 68 / (1.65)² = 24.98

English BMI

• Formula: weight (lb) / [height (in)]² x 703
• Example: weight = 150 lbs, height = 5’5” (65")
• BMI calculation: [150 / (65)²] x 703 = 24.96

Note that BMI is interpreted differently in children. Growth charts and percentiles are used. If children are at or above the 95th percentile of children their age, they are considered obese.

Other Body Fat Measurement Methods

There are several methods of assessing the percent of fat and lean mass of an individual. These methods are referred to as body composition analysis. Some of the most common measurements include:

• Underwater weighing: This procedure, also called hydrodensitometry, is complex and complicated, so it is rarely used.
• Skinfold thickness measurements: Many experts use this simple method to determine body composition.
• Bioelectrical impedance: This common method of assessing body fat percentage determines total body weight, the percent and amount of body fat, muscle mass, water, and even bone mass. While readings can be affected by hydration level and other factors, they provide fairly accurate results over time. Some body fat scales for home use employ this method of measurement.

Limitations to BMI

Even though there is a fairly strong correlation between BMI and body fat measurement, there are some limitations based on an individual's gender, age, and athletic ability.

Women tend to have more body fat than men, and older people tend to have more body fat than younger people. Highly trained athletes often have a high BMI due to higher levels of muscle mass, which increases their body weight measurement.

Body Composition, Body Fat, and BMI

Athletes who have higher levels of muscle mass need to be leery of the BMI calculation. Because the BMI number cannot distinguish the different components that make up total body weight, an athlete is better served by using a direct measurement of body composition and body fat.

Health Risks Related to High BMI

The reason BMI is used for screening the health of the general population is due to the correlation between being overweight or obese and having certain health problems. People who are overweight or obese have an increased risk for:

• Hypertension
• Sleep apnea and respiratory problems
• Type 2 diabetes
• Coronary heart disease
• Stroke
• Osteoarthritis
• Some cancers

It is important to understand, however, that body fatness is not the only predictor of health. Researchers are trying to understand the "obesity paradox." Specifically, experts have found that despite the potential adverse impact of overweight and obesity, evidence demonstrates that being overweight or mildly obese is associated with improved survival.

Fitness is also important. Researchers have found that being fit negates the adverse effects of body fatness, as well as other traditional cardiovascular risk factors, including overweight/obesity, metabolic syndrome, type 2 diabetes mellitus, and hypertension.

A Word From Verywell

While body mass index can be a useful tool for some people, it is just one number that should be balanced out with other evidence. Talk to your healthcare provider about the best way to understand your BMI as part of a comprehensive plan for good health and longevity.