The body mass index, or BMI, estimates body fat and health risk based on your weight and height. While a chart gives you the flexibility of identifying a healthy weight range based on your height, a BMI calculator gives you a quick assessment of your current status. BMI calculators interpret weight categories for adults age 20 and over.
Variables
The formula programmed into BMI calculators is designed to align with body mass index charts and can help you determine if you are underweight, normal weight, overweight or obese. Although BMI gives you a generally reliable estimation of your weight category, some variables in body composition are not calculated into the BMI formula. For example, BMI calculators don't make adjustments based on gender, age and fitness level that may affect your ratio of muscle to fat.
Metric Formula
The original BMI formula was designed for metric inputs. Metric BMI calculators ask for you to input your height in centimeters and your weight in kilograms. The calculator converts centimeters to meters by dividing by 100 and then squares the height in meters. The next step is to divide the weight in kilograms by the squared height to determine BMI. An individual with a height of 170 centimeters who weighs 70 kilograms would have a BMI of 24.2.
Pounds and Inches
A conversion factor of 703 is used to calculate BMI using pounds and inches. Some calculators ask for height in feet and inches and convert feet to inches by multiplying by 12. The formula squares your height in inches, then divides your weight in pounds by your squared height in inches. To compute your BMI, this number is multiplied by 703, according to the Centers for Disease Control and Prevention.
Considerations
Body mass calculators may be combined with other measurement tools to help you estimate calorie needs. For example, the Baylor College of Medicine provides a BMI and calorie calculator that asks for your gender, age and activity level, in addition to your height and weight. The added variables are not incorporated into the BMI assessment, but are used to help you understand how many calories you need daily in order to maintain your current weight and BMI. If you need to lose weight, this information can help you set a calorie and exercise target that will enable you to reach a healthier weight.
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