# What Is BMI (Body Mass Index)?

Body mass index (BMI) is a number derived from a person’s weight and height that is often used in an attempt to quantify how much tissue a person has, and thus categorize whether a person is underweight, in the healthy/normal range, overweight, or obese. Although, there are many criticisms of BMI being used for this purpose (some of which are valid – see below), its simplicity and easy of calculation (plus the fact it needs only easily obtainable data to calculate), has made it into a popular and widespread measure.

BMI is calculated by dividing the person’s weight (measured in kilograms) by the square of their height (measured in meters), and thus has units of kg per meter squared. In situations where a person’s weight and/or height is measured in other units (such as weight in pounds, or height in inches), a conversion table or chart can be used. Alternatively, there are many online BMI calculators, many of them free, which can calculate BMI regardless of whether metric or English units are used.

Generally accepted BMI ranges are as follows:

• Under 18.5 – underweight
• 18.5 to 25 – normal
• 25 to 30 – overweight
• Over 30 – obese

That said, these ranges do vary depending on the specific population being studied, and have also been debated and adjusted over time. For example, about 23 is the recommended normal upper limit for Southeast Asian body types, and prior to 1998, the United States used a upper limit of 27.8 as the top of the normal range.

Additionally, regardless of the exact ranges used, BMI is a very imperfect measure of obesity:

• The results are not generally considered valid for children or pregnant women
• BMI takes no account of frame size.
• BMI takes improper account of height: it can mislead shorter people into thinking they are thinner than they actually are, and taller people into thinking they are fatter than they actually are.
• BMI uses total tissue mass, and takes no account of whether that mass is fat or muscle. An athlete with a large amount of muscle mass (who may appear overweight on the BMI scale) is in reality in a very different situation from a less active overweight person with a large amount of fat.

So, given its limitations does that mean BMI is useless? Not at all, for many people it can give a useful idea of where they stand, even if it is not completely accurate. Of course, as said, it’s not applicable in all circumstances – so if you feel that BMI does not provide you with useful information, you might want to look into alternate measures of body fat.

For more information about weight loss and BMI, including a free BMI calculator please visit Guide2WeightLoss.com

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