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

Body mass index (BMI) 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.

What Is BMI?

BMI is a measurement that takes into account your age, sex, height, and weight to produce a calculation. This calculation is a measurement of your body size and can be used to determine how your body weight is related to your height.

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 fat, but it doesn't necessarily mean that a person is overweight or obese and it alone 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 older adults. There are other methods that are more accurate in estimating body fat.

How BMI Is Measured

Your BMI is calculated using your height, weight, age, and sex. 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.

Age and sex are important factors in calculating BMI. With age, we tend to lose muscle mass, and the older we are, the less muscle we maintain on our bodies. 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.

By designating between the sexes, this calculator also helps to account for the different ways men and women carry fat around their bodies and how they store body fat with age. Including both of these factors in the measurement can help in calculating a more precise BMI.

Imperial

• 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

Metric

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

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.

For adults, BMI results are interpreted as follows.

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:

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

Health Risks Related to Low BMI

While a high BMI may be an indicator for increased health risk, low BMI can also be indicative of health issues. People who are underweight according to the BMI scale can be predisposed to:

• Cardiovascular disease
• Depression
• Difficulty conceiving (in women)
• Dry skin
• Hair loss
• Irregular menstruation (in women)
• Nutrient deficiencies
• Osteoporosis
• Poor immune system

Benefits of a Normal BMI

Maintaining a normal BMI (18.5 to 24.9) comes with many benefits, including limiting your risk to all of the above-listed health concerns. Not only are you less likely to have high blood pressure, heart disease, or diabetes, but maintaining a normal BMI can also help with better sleep, improved circulation, and even better energy throughout the day.

Limitations

While BMI can be a tool doctors use to understand your health status better, it is not a solitary diagnostic tool and body fat is not the only predictor of health. When measuring your body fat composition, physicians also take into account your diet, lifestyle, level of physical activity, family history and genetics, as well as other health screenings.

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

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.

Other Body Fat Measurement Methods

There are several methods of assessing a body's percentage of fat and lean mass. These methods are referred to as body composition analysis. Some of the most common measurements include:

• 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.
• Dual-energy x-ray absorptiometry (DEXA): This is an x-ray scan that measures a person's bones (specifically their mineral density and bone loss) to calculate the possibilities of developing osteoporosis. However, these DEXA machines have limitations and the technology that is widely used is unable to measure the bones in people who weigh 300 pounds or more or who are taller than 6 feet.
• Skinfold thickness measurements: Many experts use this simple method to determine body composition.
• Underwater weighing: This procedure, also known as hydrodensitometry or hydrostatic weighing, is complex and complicated, so it is rarely used.

A Word From Verywell

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