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Equation for Adjusted BMI

By Janet Renee, MS, RD ; Updated July 18, 2017

The standard body mass index equation, or BMI, is a universally accepted calculation of body fat based on height and weight. Medical professionals have long relied on this formula to estimate your general health in relation to weight. Carrying excess weight is a strong risk factor for high blood pressure, elevated cholesterol, diabetes and certain cancers, and it increases the risk of premature death from all causes. However, Nick Trefethen, a mathematician and professor of numerical analysis from University of Oxford, observed a flaw in the standard BMI equation, which resulted in his proposal of an adjusted formula.

The Standard Body Mass Index

Developed in the 1840s to help define normal weight, the standard BMI is calculated using your weight in pounds divided by your height in inches squared; the exact formula is: BMI = 703 x weight / (height)^2. It's designed to assess a healthy weight range based on an individual's height and weight. For example, using this formula, a woman who is 5 foot, 5 inches tall and weighs 144 pounds has a BMI of 24, putting her at the upper end of the normal range. A BMI below 18.5 is considered underweight, while a BMI between 18.5 to 24.9 is categorized as normal. You're deemed overweight if your BMI falls between 25 to 29.9 and obese if it's above 30.

Observed Flaw in Standard BMI

Trefethen believes the standard BMI is less accurate for taller and shorter people, because the equation fails to take into account that taller people typically carry more natural weight. The standard BMI formula is based on people of average height, making the number used to multiply weight too large for shorter people and too small for tall people, which provides misleading results for these categories. In a nutshell, the standard formula deems taller people fatter than they are, while shorter people are led to believe they're thinner than they actually are.

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Adjusted BMI Equation

The BMI formula Trefethen created is designed so that it provides results identical to the old formula in people of average height, but it takes people of taller and shorter stature into account. The adjusted equation is: BMI = 1.3 x weight / (height)^2.5. Using the old formula, a 6-foot, 4-inch male weighing 215 pounds is deemed overweight, with a BMI of 26. Using the new formula, that same male is classified as having a healthy weight, with a BMI of 24.

BMI Is Not Perfect

Trefethen stresses that no single body mass index formula is perfect, but his adjusted formula may serve as a better tool for taller and shorter people. And other experts in the math community acknowledge that using BMI as a tool has drawbacks. Some members of the medical community stress that the BMI equation is only capable of guessing the amount of body fat and that you shouldn't expect precision, according to to a 2013 BBC News article. While some institutions use the adjusted formula, it's not universally accepted, and many health professionals, especially in the United States, continue to rely on the standard formula.

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