What is Body Mass Index (BMI)?

Body mass index (BMI) is an estimate of body fat based on your weight and height. This calculation helps determine if you are underweight, normal weight, overweight or obese.

BMI can be used to assess your risk of certain health problems. For example, people with a higher body weight are thought to be at greater risk for high blood pressure and high cholesterol.

However, computing has its limitations. That is, BMI does not take into account age, specified gender, ethnicity, or muscle mass.

This article discusses BMI, how it is calculated, its advantages and disadvantages, and alternative methods of assessing potential health risks.

How BMI is Calculated

BMI is calculated by dividing your weight (kg) by your height (meters) squared.

  • Formula: Weight (kg) / [height (m)]2

BMI can also be calculated by dividing your weight in pounds by the square of your height in inches and multiplying the answer by 703.

  • Formula: Weight (lbs) / [height (in)]2×703

You may not have all these numbers ready. Online BMI calculators can do the job for you.

BMI score

Body mass index scores fall into the following categories:

  • Underweight: less than 18.5
  • Normal: 18.5 to 24.9
  • Overweight: 25 to 29.9
  • Obesity: 30 years and older

Obesity is sometimes divided into other categories:

  • Grade 1: BMI 30 to <35
  • Grade 2: BMI 35 to <40
  • Level 3 (severely obese): BMI of 40 or higher

Healthy weight and BMI ranges for older adults

Child Body Mass Index Chart

Adult BMI Chart

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BMI is calculated by dividing weight by height. It’s a way for healthcare providers to assess the health risks associated with weight — whether it’s too little or too much.

Body mass index and health

Excessive body fat is associated with an increased risk of many health problems. BMI scores in the overweight and obesity categories can indicate that a person is at higher risk for certain diseases, such as:

  • heart disease
  • hypertension
  • cancer
  • Osteoarthritis
  • type 2 diabetes
  • Gallstones
  • sleep apnea
  • high cholesterol
  • liver problems

BMI Pros and Cons

On the plus side, BMI is an easy way to estimate body fat. It’s convenient, inexpensive, and can be used routinely. So it’s an easy way to identify people who may have health risks that are worth investigating. It is also commonly used in research by medical professionals around the world.

However, it also has significant limitations. First, it can’t be used to diagnose health problems — just to identify possible red flags.

This could also be a false positive for a large number of people since BMI does not take into account the following factors.

muscle mass

Muscle and bone are denser than fat. Since the only metrics used to calculate BMI are weight and height, it may overestimate body fat in athletes or people with large muscle mass.

It may also underestimate body fat in people with very little muscle mass.

Race

BMI doesn’t take into account a person’s race, which affects weight-related health risks and body composition, Body mass composed of fat and lean muscle tissue.

For example, on average, Asians and South Asians have higher percentages of body fat than Europeans at the same weight, while blacks have less body fat and more muscle at the same weight.

For this reason, some experts now advocate tailoring cutoffs for specific groups.

specify gender

Those born women tend to have more body fat than men. BMI does not take this difference into account. Therefore, a woman may be registered in a “safe” BMI category, but the level of body fat may still represent a health risk.

age

A normal BMI range may not accurately predict health risks in young and old age. For example, in older adults, a BMI below 23 (in the so-called normal range for adults) was associated with higher mortality, but not in the “overweight” range.

how fat is distributed

Evidence suggests that the location of body fat also matters. Those whose weight is concentrated in the middle — the so-called “apple” body type — have higher health risks than those with a “pear” body type (that is, those who accumulate weight on their hips and thighs).

BMI and people of color

Alternatives to BMI

Because of these issues, some have called for a retirement BMI. Although its shortcomings are widely accepted, BMI is still in use, in part because of its ease of use.

When needed or deemed necessary, other methods of measuring body fat or assessing your health risk can be used in place of or in addition to BMI.

These include:

  • Waist circumference: Fat in the middle of the body increases the risk of type 2 diabetes and heart disease. Women with waist circumferences greater than 35 inches and men with waist circumferences greater than 40 inches were at increased risk. Many experts now recommend that waist circumference is a better measure for all people, but may be especially helpful in assessing health risks for people of color.
  • Waist-to-hip ratio: Dividing waist circumference by hip circumference can provide information about potential health risks. A measurement of 0.9 or higher in women and 1.0 or higher in men indicates a higher risk.
  • Skinfold measurement: Body fat percentage can be estimated by using a caliper, a tool that measures the thickness of the skin. Take measurements at multiple locations on the body and plug in formulas.
  • Smart scales: Some “smart” bathroom scales can be used to estimate body fat. These scales measure body fat by sending a harmless electrical current to your body. Note that the technology used by smart scales requires a constant level of hydration in the body.try weighing
    Avoid inaccurate measurements at the same time of day when you know you’ll be hydrating consistently.

More sophisticated methods of measuring body fat and composition also exist. However, most are not available in standard medical offices. Instead, they are used in specialist or research settings.

Examples include:

  • Underwater Weighing: With this method, also known as hydrostatic weighing or densitometry, you weigh in air and then in water. These two measurements are then compared to calculate body fat.
  • Air Displacement Plethysmography (ADP): ADP uses air to estimate your body fat percentage based on your body density.
  • Dual Energy X-ray Absorption (DEXA) and MRI: These sophisticated imaging techniques are used to examine body composition, including fat, muscle mass, and bone density.

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BMI is not the only way to assess body fat or risk associated with weight. Health risks can also be estimated by looking at waist circumference and its relationship to hip circumference. Body fat can be calculated with a skinfold test, a smart scale, or several other methods.

Normal body mass index and physical fitness

You may have a normal BMI and still have higher health risks.

For example, maybe your BMI is in the normal range, but you’re not physically active, have a poor diet, have a family history or smoke — any of which can affect your risk for a range of health problems.

Older people may have a normal BMI but have serious health problems such as hardening of the arteries (atherosclerosis).

And, as discussed, your BMI may be in the healthy range, but your race, age, gender, or other factors make that measurement less accurate in your case.

Consider what BMI is — an estimate, not a measurement, it’s just information that can help paint a picture of your overall health. Your healthcare provider can help you better understand how your BMI fits.

generalize

BMI estimates your body fat, which affects your risk of disease. Calculating BMI is quick and free BMI calculators are available online.

However, BMI has many limitations. This measure doesn’t take into account your age, race, gender or fitness level.

Additionally, there are a number of factors to consider when determining a healthy weight for you. BMI can provide a small piece of information, but it shouldn’t be the only resource to use when deciding how much weight you should maintain to stay healthy.

Frequently Asked Questions

  • Is BMI an accurate way to measure body fat?

    BMI is an estimate of body fat, but it is not an accurate way to determine a specific body fat percentage.

  • Is someone’s gender a factor in calculating BMI?

    Adult BMI measurements are not based on someone’s gender.

  • Is there a better measurement to replace BMI?

    Assessing body composition (the amount of body fat versus the amount of lean muscle tissue) is a more accurate way to determine overall health risk.

  • How do I calculate my ideal healthy weight?

    Health is not just a number on a scale. Talk to your healthcare provider to determine your recommended weight range, taking into account other factors that affect your overall health.

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