Body mass index, or BMI, is a metric used to assess body fatness. It is calculated as a ratio of your weight to your squared height. Although BMI does not directly calculate your body fat percentage, it does correlate with true measures of body fat percentage and is considered a fairly reliable alternative to determine obesity, based on research cited by the Centers for Disease Control and Prevention. You are considered obese if your BMI is 30.0 or greater, overweight if your BMI is between 25.0 and 29.9, normal weight if your BMI is between 18.5 and 24.9, and underweight if your BMI is lower than 18.5.
U.S. Standard Measurement System
Step 1
Weigh yourself in pounds at least three times on a reliable scale. Find the average of the three measurements by adding them and dividing by three. For example, if your first measurement reads 200 lbs., the second 202 lbs. and the third 201 lbs., your average weight equals 201 lbs.: (200 + 202 + 201) ÷ 3 = 201.
Step 2
Measure your height in inches three times. Find your average height using the same method as for the first step. For example, three measurements reading 72 inches, 71 inches and 73 inches average 72 inches.
Step 3
Calculate your BMI using the following formula: BMI = weight ÷ height squared × 703. For example, if you weigh 201 lbs. and are 72 inches tall, your BMI is 27.3 because 201 ÷ (72 × 2) × 703 = 27.3.
Metric System
Step 1
Weigh yourself in kilograms, or kg, at least three times. Find the average of the three measurements by adding them and dividing by three. For example, if your first measurement reads 100 kg, the second 102 kg and the third 101 kg, your average weight equals 101 kg.
Step 2
Measure your height in meters, or m, three times. Find your average height using the same method as for the first step. For example, three measurements reading 1.7 m, 1.72 m and 1.74 m average 1.72 m.
Step 3
Calculate your BMI using the following formula: BMI = weight ÷ height squared. For example, if you weigh 101 kg and are 1.72 m tall, your BMI is 34.1 because 101 ÷ (1.72 × 2) = 34.1.
Tips and Warnings
- Always use the same scale to weigh yourself when calculating your BMI. This will ensure accurate tracking of your BMI over time. When using the metric system, you may find it easier to measure height in centimeters, or cm, first and then convert cm to m using the following formula: cm ÷ 100 = m. For example, if you are 172 cm tall, you can convert this to 1.72 m: 172 ÷ 100 = 1.72.
- Exercise nutritionists McArdle, Katch and Katch warn that BMI can misclassify your weight, especially if you have a lot of muscle mass, in which case BMI may classify you as overweight or obese when, in fact, your body fat percentage is normal. If this applies to you, consult a health professional to measure your body fat percentage to determine whether your BMI classification is accurate. If you are overweight or obese based on your BMI, you have an increased risk for such conditions as high blood pressure, heart disease, diabetes and stroke. You should take steps to lose weight gradually until your BMI falls within the normal range. If you are underweight, you have an increased risk for diseases, including anemia and osteoporosis, and your immune system may not work properly, making your body’s defenses weak, according a Meghan Reese, a contributor to “Today’s Dietitian” magazine. You should work to gain weight until your BMI measures between 18.5 and 24.9.
Things You'll Need
- Scale
- Measuring tape
- Calculator
References
- Centers for Disease Control and Prevention: About BMI for Adults
- "Physiology of Sport and Exercise (Fourth Edition)"; Jack H. Wilmore, David L. Costill and W. Larry Kenney; 2008
- "Sports and Exercise Nutrition (Third Edition)"; William D. McArdle, Frank I. Katch and Victor L. Katch; 2009
- "Today's Dietitian": Underweight--A Heavy Concern
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