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.

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 your body size and can be used to determine how your body weight is associated with 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 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 Is BMI 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:

BMI results
BMI Measurement Weight
Below 18.5 Underweight
18.5 - 24.9 Normal weight
25.0 - 29.9 Overweight
30.0 and above Obese

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

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:

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

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 fatness is not the only predictor of health. When measuring your body fat composition, physicians also take into account your diet, your lifestyle, the physical activity you perform, your 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 body fatness, as well as other traditional cardiovascular risk factors, including overweight/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.

The BMI calculation is used to screen the general population for health risks related to having too much body fat. This tool does not work well for most athletes who are curious about their body composition.

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.
  • Dual Energy X-ray Absorptiometry (DEXA): This is an X-ray scan that measures person's bones and 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 are taller than six feet.

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.

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Article Sources
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  1. Centers for Disease Control and Prevention. About adult BMI. Updated April 10, 2020.

  2. Centers for Disease Control and Prevention. About adult BMI. Updated April 10, 2020.

  3. National Institute of Diabetes and Digestive and Kidney Diseases. Health risks of overweight and obesity. Updated February 2018.

  4. Lavie CJ, McAuley PA, Church TS, Milani RV, Blair SN. Obesity and cardiovascular diseases: implications regarding fitness, fatness, and severity in the obesity paradox. J Am Coll Cardiol. 2014;63(14):1345-54. doi:10.1016/j.jacc.2014.01.022