Defining and Calculating Body Mass Index
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When you’re trying to get or stay fit, your body mass index can provide you with a snapshot of your overall health and how your level of fitness — or maybe your lack of fitness — is affecting your body. You can calculate your BMI by multiplying your weight in pounds by 703 and then dividing by the square of your height in inches. So, if you are 65 inches (5 feet 5 inches) tall and weigh 225 pounds, your BMI calculation would be 225 pounds times 703 divided by 65 squared (4,225), which is 37.4.
When calculated correctly, BMI gives you an idea of the overall fat composition of your body, and body fat levels are a good indicator of overall physical health. If you don’t want to do the calculation manually, it’s easy to find BMI calculators online, including on the CDC's website. Multiple versions of the calculator are available for different age groups, Spanish speakers and metric calculations. The National Institute of Health and the National Health Service in the UK also offer reliable BMI calculators on their sites.
Importance of Body Mass Index
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Overall, BMI calculations are important because they tell you much more about risk factors to your health than simple weight measurements. Doctors routinely use BMI information along with cholesterol levels, glucose readings, blood counts and other diagnostic tests to look for early indicators of a host of diseases.
Harvard Health notes that although BMI doesn't tell everything about a person's health, it provides a "useful starting point that quantifies what health issues will become more likely when a person is overweight or obese." Simple BMI calculations that can be done from home minimize the need for more complex — and more expensive — testing tools to measure fat and lean tissue.
Body Mass Index Ranges
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Acceptable BMI ranges vary according to age and gender as well as by individual characteristics in some cases. Your doctor will consider your full medical profile when evaluating your BMI. However, in general, a person would be considered underweight if their BMI measured lower than 18.5, overweight if it measured from 25 to 29.9 and obese if it measured higher than 30. The typical healthy weight BMI range for most people is 18.5 to 24.9.
Calculations for children and older adults are different than those for young and middle-aged adults, as they naturally have different normal percentages of body fat. Furthermore, data for females is interpreted differently, because they typically have higher percentages of body fat. Additionally, other groups, such as athletes, may have much larger reserves of muscle that can make the results of BMI calculations harder to interpret.
If you’re planning to use your BMI to guide diet and lifestyle changes, be sure to look for calculators that accommodate these types of special variables. Once you calculate your BMI, talk to your doctor about any questions or concerns you may have to ensure you correctly interpret what the figures mean in the context of your body type, health history and lifestyle.
Problems Potentially Indicated by a High or Low BMI
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According to the CDC, being overweight increases your risk of stroke and cardiovascular issues and could make you more susceptible to various forms of cancer, diabetes, gallbladder disease, and mood and mental disorders, such as depression and anxiety. Obesity has also been linked to osteoarthritis, sleep apnea, breathing problems, inflammation and a generally lower quality of life.
In addition, being obese with a higher percentage of fat than muscle — potentially due to a diet high in fat — can lead to high blood pressure and high cholesterol, both of which are linked to heart disease. As a result, people with high BMIs should watch for signs of an elevated resting heart rate with higher heartbeats per minute. The American Heart Association provides detailed information about healthy heart rate and pulse rate.