Ways to Calculate BMI

When you go for a checkup, your doctor may measure your height and weight to determine your BMI -- or body mass index. This gives him an idea of your body's level of fatness and whether you're at a healthy size. If your BMI is high, you may be at risk for conditions that come with being overweight, such as type 2 diabetes and cardiovascular complications. Your BMI helps your doctor decide if additional health screenings need to be performed. BMI isn't a perfect measure, but it's noninvasive and easy to calculate.

A calculator makes your BMI easy to figure out. (Image: daizuoxin/iStock/Getty Images)

Online BMI Calculators

An online BMI calculator is the easiest and quickest method for computation. A number of U.S.-based health organizations offer one on their websites, including the National Heart, Lung and Blood Institute and the Centers for Disease Control and Prevention. The National Health Service of Britain also has an online calculator, which allows you to use metric or imperial measurements.

A BMI between 18.5 and 24.9 is considered normal and indicates that you're at minimal risk of chronic disease related to your weight. A BMI of less than 18.5 puts you in the category of underweight, meaning you might improve your health by gaining a few pounds. Overweight is represented by a BMI of 25 to 29.9, while obese is 30 or higher. These high BMIs indicate the need for further evaluation of your health.

Mathematical Equation for BMI

The equation for BMI uses metric measurements and is figured by dividing your weight in kilograms by your height in meters squared. Metric heights are often represented by centimeters; to determine your height in meters, divide height in centimeters by 100. BMI = weight in kilograms / height in meters x height in meters.

To figure your BMI using the imperial measurements of pounds and inches, a conversion factor of 703 is used in the equation. The equation for BMI then becomes weight in pounds divided by height in inches squared, the total of which is then multiplied by 703 to get your final BMI. Here's the equation: BMI = weight in pounds / (height in inches x height in inches) x 703.

Determining BMI for Children and Teens

Figure the BMI for children and teens using the same equation as the one used for adults, but the interpretation of the results accounts for age and gender. When kids are growing up, their body fat levels change dramatically and vary according to gender.

A child's or teen's BMI is compared to growth charts to see if they fall into a healthy percentile. Those who are in the 5th percentile or lower are considered underweight, those in the 5th to 84th percentile are considered normal and those in the 85th to 94th percentile are overweight. Child obesity is indicated when BMI falls in the 95th percentile or greater.

A child with a BMI putting him in the overweight or obese category may be at risk for the same chronic health conditions as heavy adults.

Consider BMI as a Screening Tool

BMI can be a decent way to gauge a person's fatness, but it has limitations, which is why BMI is used only as a screening tool, and isn't diagnostic. Two people with the same BMI can have different levels of fatness. For example, if a man and woman have the same BMI, the woman will tend to have more body fat than lean tissue, such as muscle, bones, organs and connective tissue. Older people also tend to have more body fat than younger adults, even when their BMIs are the same. Higher BMIs in the overweight and obese ranges tend to be more accurate than lower BMI values.

BMI also misses some people who have an unhealthy amount of body fat, even though their weight is normal. These cases of "normal weight obesity" usually occur in sedentary adults and older adults who have lost considerable muscle mass. If body fat levels are 20 percent or greater for a man or 30 percent or greater for a woman, a person may be vulnerable to some of the same health problems -- such as type 2 diabetes -- that affect people who are overtly overweight or obese. Doctors sometimes miss these cases because their BMIs register as normal, confirming that BMI should be only one of several screening tools.

Athletic individuals may also find their results misinterpreted by the BMI calculations. Their abundance of muscle mass makes them seem too heavy for their size, but in reality they have a small percentage of fat tissue. Too much fat, not too much muscle, is dangerous to your health.

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