BMI vs. Body Fat

Body fat percentage and BMI are both effective ways to evaluate your fitness level.

Body fat percentage and BMI are both effective ways to evaluate your fitness level.

It's important to have an accurate way of measuring your fitness progress. Without a reliable system of evaluating your achievements, it can be difficult stay motivated. Both body mass index (BMI) and body fat percentage give you reliable ways to track the gains your making toward improving your health. Each method has distinct pros and cons that make it more useful in certain situations. Choosing the system that works best for you will allow you to set and reach your fitness goals.

BMI Accuracy

The BMI formula was created in the 1840s and was designed to be easily calculated in your head. The equation produces a number, your BMI, which is then compared to a chart that rates your height-to-weight ratio from underweight to obese. Because of its simplicity, many people have criticized BMI for being inaccurate, especially for tall or muscular individuals. Despite this, the BMI method remains popular because of the inherent accessibility. This is especially true with the advent of smartphones, which allow you to have a BMI calculator in your pocket, and many websites which provide calculators.

New Formula

Oxford University mathematician Nick Trefethen set out to revise the BMI formula to correct these problems. The new formula is more complicated, though, and may give many people numbers they don't necessarily like. According to Trefethen, the original formula has made shorter people think that their weight is healthier than it actually is. The new formula is 5,734 times the weight in pounds, divided by height in inches to the power of 2.5. But you can simplify the process by using online calculators for the revised method as well.

Body Fat Percentage

There are several methods for measuring body fat percentage, including bioelectrical impedance and the skinfold method. Other systems include air displacement plethysmography and hydrostatic weighing, all of which require specialized equipment and a trained staff, making them inaccessible to most people. Although they are commercially available, both the bioelectrical impedance and skinfold methods require tools that cost $3 to $300.

Which Works Best for You?

Both BMI and body fat percentage have their usefulness, despite their weaknesses. The BMI is an easy way to quickly decide whether your weight is appropriate for your height without buying any special tools. For athletes or very muscular people, though, a small investment in a body fat measuring tool will provide a more in-depth analysis of your fitness.


About the Author

Jonathan Thompson is a personal trainer certified through the American Council on Exercise and has extensive experience working with clients as well as teaching. Thompson holds specializations in longevity nutrition and muscle management for runners. He began writing in 2004.

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