How to Calculate your Body Mass Index (BMI)

Your Body Mass Index (BMI) is a number computed based on your weight and height. Invented in the early 1800's, the BMI is a mathematical tool used by doctors to assess the health risks of a particular person of interest. Today, you can use the information to determine where you stand within the commonly accepted range of healthy and unhealthy weights. Typically the higher your BMI is, the higher your risk is for being diagnosed with some of the various obesity related diseases plaguing the world. These diseases include, but are not limited to, diabetes, hypertension, and heart disease.

The formula to determine your body mass index is pretty simple. Your BMI is equal to your mass divided by the square of your height. Of course the units of which you measure your height and weight does matter. If you are measuring your height in meters and your mass in kilograms, the formula is simply:

If you are measuring your height in inches and your weight in pounds, the formula becomes:

If you are weary of using formulas you can use this neat online calculator or the chart below instead.

The BMI has been divided into 4 categories (some countries use 6 categories) based on numerous studies of risk factors for obesity related diseases.

Note that what is considered normal, underweight, or obese may differ from country to country. The above table is for the USA. When determining the BMI for children (younger than 18) this table should not be used. Typically a doctor will compare the BMI number with those of other children in the same age group. A child may be considered underweight or obese if they rank below the 5th percentile or above the 95th percentile.

Many of you out there would probably agree that the number computed from these formulas does not adequately represent everyone in every case. For example, athletes could be considered obese based on the BMI even though they may actually have a really low body fat percentage. The BMI also has problems when it comes to assessing the health risks of the elderly because these individuals have typically lost considerable amounts of muscle mass. Caution should be used when the BMI calculation is applied in either of these cases. In addition to this, the BMI does not account for the many different body shapes and bone structures that people in this world have. Because of these limitations, the BMI should always be used in conjunction with other methods of health risk assessment such as the waist and neck circumference or body fat percentage.