Calculating BMI

Calcaulating BMI or body mass index, is a measure used to estimate health risks taking into account weight and height. The body mass index calculation was developed by the National Institutes of Health in 1998 as a way to help the general public become aware of obesity problems and to encourage healthier living.

How to Calculate Body Mass Index
Obesity is becoming an American epidemic with statistics showing that 60% to 65% of Americans over the age of 20 are overweight or obese. Your body mass index is guideline that estimates health risks based on the relationship between your weight and height.

Do an Internet search for body mass index calculators and you'll find several you can use to calculate your body mass index for you, but the math to find your BMI is simple:

(Weight in pounds / (Height in inches x Height in inches) x 703 = BMI

For a person who weighs 135 pounds and stands 5' 5" tall, the formula would look like:

(135 / (65 x65) x 703 = 22.5

The resulting number is your body mass index. In the example above, that person who weighs 135 pounds and is 5'5" tall has a BMI of 22.5. This number acts as a guide to determine if the amount of weight you're carrying on your body frame is healthy or unhealthy.

It's important to remember that body mass index is an overall point of reference that may point to any health risks associated with your weight. Determining your actual health risks requires working with a physician to assess your personal and family medical histories, stress levels, and current diet, exercise and other lifestyle habits.

What's In a Number
According to the Center for Disease Control, the weight categories as they correspond to BMI ranges for adults are:

  • Underweight: below 18.5
  • Normal: 18.5-24.9
  • Overweight: 25-29.9
  • Obese: 30 and above

Risks Associated with a High BMI
Again, keep in mind that your BMI is a starting point for determining any health issues you're at risk for that relate to your individual weight.

However, the general health risks for people who are overweight or obese include heart and lung disease, high blood pressure, high cholesterol (notably an increase in bad, LDL-cholesterol and a decrease in good, HDL-cholesterol), kidney disease, osteoarthritis and sleep apnea.

For people with a BMI of 30 of higher, it's estimated that the risk of developing high blood pressure, heart disease and Type II diabetes are 50% to 150% greater than for people with a BMI in the normal range of 18.5-24.9.

BMI versus Percentage of Body Fat
The body mass index is based on your total-bones, muscle and fat-and isn't a measurement of what percent of your body weight is made up fat. Some people may turn up what's considered to be a high BMI, but not have a high percent of body fat.

Highly trained athletes, for example, have a higher percentage of muscle than the average person. Muscle weigh more than fat, so increased muscularity adds weight to their body frame. Similarly, people who fall into the underweight portion of the BMI scale appear thin, but may also have an unhealthy percentage of body fat or their low BMI may be the result of lost muscle mass due to aging or long-term recovery from illness.

Waist circumference is another measurement which is currently being used along with BMI to estimate the health risks related to obesity. Research has found that weight that's carried around the waist versus the hips or distributed evenly on the body indicates a higher risk for weight-related illnesses.

If you're interested in determining your body fat percentage, ask your doctor or a trainer at your gym to perform a caliper test or weigh you on an in-water scale. Some bathroom scales measure body fat percentage with tiny electrical currents, but the measurements aren't as accurate as other methods.

Does the BMI Formula Work for Everyone?
The formula used to calculate BMI is used for anyone over the age of 20 and the corresponding weight ranges apply as well-regardless of gender. The BMI doesn't allow for women's bodies containing a naturally higher percentage of fat than men's.

The BMI does, however, make allowances for children. The formula for calculating BMI is the same, but they have separate weight ranges from adults that break out ranges by age and gender to account for the differences in girls' and boy's bodies and growth patterns.

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