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What is BMI? - Definition, Formula & Calculation

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  • 0:00 What Is BMI?
  • 1:10 Calculating BMI: The Formula
  • 1:40 Special Considerations
  • 2:28 Example BMI Calculations
  • 4:18 Uses for BMI
  • 5:27 Lesson Summary
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Lesson Transcript
Instructor: Nadine James

Nadine has taught nursing for 12 years and has a PhD in Nursing research

In this lesson, the term body mass index (BMI) will be defined, and we'll explore the formula to calculate a person's BMI. Several different examples can assist you in learning this concept. Finally, a few examples of how to use BMI in the health care world will be described.

What Is BMI?

BMI (body mass index) is a calculated number representing a person's level of fat or obesity level. According to the Centers for Disease Control and Prevention (CDC), a BMI of 30 or above indicates obesity. BMI levels are broken down by weight range and are as follows:

BMI < 18.5 is underweight
BMI 18.5-24.9 is normal weight
BMI 25-29.9 is overweight
BMI 30-39.9 indicates obesity
BMI > 40 indicates morbid obesity

In children, age and gender are considered in tables that are produced for growth charts by the CDC. However, BMI is calculated with the same formula for both children and adults. In general, children are assessed as being underweight if they fall below the 5th percentile and overweight/obese if they fall above the 95th percentile.

Calculating BMI: The Formula

The formula to calculate BMI requires information about a person's height in meters and weight in kilograms. For weight in pounds and height in inches, there is a different formula, but due to rounding, it may not produce the exact same results. The BMI formulas are:

BMI = weight (in kilograms) / height (in meters) ^2

or

BMI = weight (in pounds) / height (in inches) ^2 * 703

Calculating BMI: Special Considerations

There are some concerns related to BMI that mostly have to do with the fact that fat and lean muscle tissue are not considered in the formula. For example, a man who is an athlete may have a higher BMI calculation than a man who never leaves his desk at the office and is overweight. Of course, you would not say that the athlete is overweight - but he is muscle bound, and muscle weighs more than fat.

Another situation that may cause a misleading BMI is the calculation for a tall and lean person. The extra height of the individual comes with extra weight. This might skew the BMI and make it appear high even if the person is relatively lean. Also, as a person ages they may lose height, so the BMI may be increased without any significant change in the person's weight.

Example BMI Calculations

If you have your weight in pounds, as most of us in the U.S. do, you'll need to convert the weight to kilograms first. The formula to convert pounds to kilograms is dividing the weight in pounds by 2.2. To convert inches to meters, you take the height in inches and multiply it by 0.0254.

Let's do a calculation. A man weighs 220 pounds and is 70 inches tall. We will perform the BMI calculation using both ways to show how to use the formulas.

English Units

BMI = weight (in pounds) / height (in inches) ^2 * 703

  • (220 / 70^2) * 703
  • (220 / 4900) * 703
  • 0.045 * 703
  • BMI = 31.63

Metric Units

BMI = weight (in kilograms) / height (in meters) ^2

  • 220 lbs / 2.2 = 100 kg
  • 70 in * 0.0254 = 1.78 m
  • 100 / 1.78^2
  • 100 / 3.17
  • BMI = 31.55

Do you see the difference in the results? Why are the BMI values not the same? The answer is that rounding after the decimal points causes slightly different end results. However, both BMI scores indicate that the man is overweight. Whenever possible, you should use the formula for the measurement you have the height and weight in, whether it's metric (kilograms and meters) or English (pounds and inches).

Uses for BMI

The BMI is useful information to have. BMI is an inexpensive way to indicate how fit a person is in relation to the amount of fat in their body. While fat is not directly measured in the formula, it still provides a good indicator.

Have you ever wondered why a thinner person's health insurance might be lower than a neighbor's who is overweight? It may be because many insurance companies use BMI to calculate the risk when providing insurance for an individual. Being overweight may cost your neighbor more for insurance because the insurance company predicts that he or she will have greater medical costs down the road.

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