Understanding BMI

Many people know their body mass index, or BMI, as a number, but far fewer understand just what that number means. Contrary to popular wisdom, the index does not measure excess body fat, nor is it a modern invention created in response to increasing average weights. Although it can be an effective tool for understanding overall health, it's just part of the whole picture.

What Is BMI?

A person's body mass index is an expression that relates weight to height. Sometime in the middle of the 19th century, a Belgian mathematician and statistician named Adolphe Quetelet noticed a strong linear relationship between people's heights and weights. To codify and simplify this relationship, he devised a table that recorded a new number derived by dividing a person's mass in kilograms by the square of their height in meters. He termed this figure the body mass index, and since then, it has been one indicator of overall fitness.

For example, someone who is 1.5 meters tall and weighs 77 kilograms has an index number of around 34. If you're used to working with pounds and inches, the relationship still works when the result is multiplied by 703. Plugging in the previous numbers or something close to them, someone who is 60 inches tall and weighs 177 pounds still has a final result of 34 when all the calculations are complete.

Quetelet had to do his own math to derive these figures, but now that you know how to calculate your BMI using the formula, you can plug them into a calculator and make finding your number easier. A calculator eliminates the math entirely. The Centers for Disease Control and Prevention's website has an excellent BMI calculator that works for English or metric measurements.

Interepreting the Numbers

Numbers without context mean little, so it's important to understand how to interpret that figure. Body mass index charts have diagonal lines that map out regions of the chart. Each region is associated with a standard of fitness. With height on one axis and weight on the other, it's easy to plot the index number on the chart and see which region it occupies.

An index number of between 18.5 and 24.9 is considered a normal weight. Between 25 and 29.9, a person's index number falls into the overweight category. A number above 30 percent is considered obese, and many charts further divide this into Class I, II and III obesity, each of which spans five index points. At the low end of the scale, an index number of less than 18.5 is considered underweight, and doctors consider people with a figure of under 15 to be severely underweight.

The Drawbacks of Body Mass Index

It's important to note that these figures do not measure the ratio of body fat to muscle. The index is notoriously inaccurate for people with high muscle mass. Many professional athletes fall into the overweight or obese category when they're manifestly in trim physical shape. Football players Tom Brady, Drew Brees and Calvin Johnson all count as overweight on the current scale. Quarterback Tony Romo is on the borderline of obesity with an index number of 29.8, and many professional bodybuilders are well on their way to Class II obesity with numbers higher than 35.

This inaccuracy when measuring muscular people shouldn't suggest that everyone with a high index number is a trained athlete. However it does point out that those with a naturally muscular build or who have trained with weights may get a slightly skewed picture of their health if this figure were the only one taken into consideration. Your doctor should look at the index in a wider context, comparing it against measures of body fat and waist-to-hip ratios, two other important indicators of general health.

Another problematic issue with the current charts is that they are not uniform throughout the world. What counts as a normal weight in one country may be another country's definition of overweight. The Japanese index, for instance, considers any number above 25 obese. In 1998, the numbers for North Americans were revised downward to match the World Health Organization's standards, rendering about 25 million people who had been of average size instantly overweight.

The Value of BMI

Although it has its flaws, the index can still be an important tool for better health. By itself, a high number may mean little; however, when taken in conjunction with other tests and assessments, it's another data point to examine when looking at your overall condition. It's also a good way to track progress during weight loss. You and your doctor should keep track of your body mass index, but keep it in perspective; it's a tool to help you get and stay healthy, not a judgment or an inevitability.