Ways to Calculate BMI

Body mass index, or BMI, is a measurement derived from your weight and height that estimates your body composition, or amount of body fat versus lean tissue, according to the Centers for Disease Control and Prevention. BMI is a fairly accurate indicator for most people, but the CDC warns that it tends to overestimate body fatness for very muscular individuals. BMI measurements are divided into four categories: underweight, normal weight, overweight and obese. You are considered underweight if your BMI is below 18.5, normal weight if your BMI falls between 18.5 and 24.9, overweight if your BMI falls between 25.0 and 29.9, and obese if your BMI is 30.0 or higher. There are several ways to calculate your BMI, depending on whether you are more familiar with the metric or standard measurement systems.

Metric System Formula

BMI is traditionally calculated using the metric measurement system, using kilograms to measure weight and meters to measure height, according to William D. McArdle, Frank I. Katch and Victor L. Katch, authors of the textbook "Sports and Exercise Nutrition." Use the following formula:

BMI = weight in kilograms / height in meters squared

If you only know your height in centimeters, then simply divide your height by 100 to determine your height in meters. For example, if you are 170cm tall, then divide 170 by 100, which equals 1.7m.

As an example, using this formula, if you weigh 75 kg and are 1.7 m tall, your BMI is 75 / 1.7^2, which equals 26.0 when rounded to the nearest tenth.

Standard System Formula

If you used to the standard measurement system, which measures weight in pounds and height in feet and inches, you can use a similar formula with a conversion factor to calculate your BMI:

BMI = (weight in pounds / height in inches squared) x 703

For example, if you weigh 175 lb. and are 5 feet, 6 inches, or 66 inches, tall your BMI equals [(175 / 66^2) x 703], or 28.2 when rounded to the nearest tenth.

Standard System Alternative Formula

The American Heart Association suggests an alternative formula to calculate your BMI using the standard measurement system:

BMI = (weight in pounds x 703) / height in inches / height in inches

Following the same example as above, if you weigh 175 pounds and are 66 inches tall, your BMI is [(175 x 703) / 66 / 66], which equals 28.2 when rounded to the nearest tenth.

Because both standard system formulas produce the same answer, you can use them interchangeably. Pick the formula you are most comfortable with mathematically.