BMI is a reasonable guide to a healthy weight, but to get a more accurate and personal picture of the ideal weight for you, you’ll need to dig deeper. We know that fat tissue is an essential part of our body makeup and serves several important roles, but how do you know when you’ve got too much? Ask a health professional or personal trainer, and they will likely point you to BMI (body mass index)—a number score that measures your weight while taking into account your height.
Your BMI will tell you whether you are significantly overweight or underweight. It’s also a useful marker for governments to “guesstimate” how much of their population is carrying too much fat or is undernourished. While you can use BMI to roughly calculate whether you are over- or underweight, beware—when it comes to using your score to figure out how much of the excess weight you are carrying is fat, BMI is decidedly flaky. It doesn’t take account of muscle, which is heavier than fat, and neither does it adjust for height properly—if you’re tall, your BMI will be overinflated; if you’re shorter than average, your BMI will be unduly low. You can easily have a BMI outside the recommended range and be in peak fitness.
The best way to find out how much fat you’re carrying under your skin and wrapped around your organs (the latter being the most harmful to health because it disrupts the body’s insulin control) is to have a body scan. However, since few will be able to access a scanner, “smart scales” are a relatively inexpensive alternative, which give reasonably reliable results. When you place your hands and feet on the panels, the scales use small electrical currents to estimate the proportions of fat, water, and muscle in your body.
A newer, more accurate formula to calculate body fat than BMI is relative fat mass (RFM), which is a ratio taken from your height and waist measurements. It’s easy to do—all you need is a tape measure—and there are plenty of online calculators to crunch the numbers for you.
BMI is calculated as your weight in pounds divided by the square of your height in inches multiplied by 703. Doctors usually say that for an average adult, a BMI between 18.5 and 24.9 indicates a healthy proportion of body fat.
BMI is belgian—who knew?
In reality, the formula is a 200-year-old math hack, originally based on the sizes and weights of sedentary Belgians—and badly in need of an update.
In the 1800s, a statistics-loving Belgian astronomer, Adolphe Quetelet, wanted a neat way to figure out how many Belgian people were obese—and it turned into a bit of an obsession for him. In an era without calculators or computers, he devised a simple formula to find what he was after: a rough and ready “fatness” score that accounted for people’s height.
Life insurance companies soon cottoned on to the idea that they could turn the data into tables showing the “average weight”—and charge higher premiums for anyone who was “overweight” or “obese.” A hundred years later, Quetelet’s formula was rebranded as “body mass index” by Ancel Keys—the famous American nutrition scientist.