BMI calculators aren’t accurate, but our body fat calculator is

Coming for your middle.
Coming for your middle.
Image: Unsplash/Jennifer Burk
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The Body Mass Index (BMI) has been relied on as the tool to measure relative fat levels and weight status for decades. It first came into popular use in the early 1970s, when the famous American physiologist Ancel Keys gave the formula, devised in the 1840s, a new name. Among government health agencies and doctor offices everywhere, it became and remains king.

But the metric, with an underlying formula of weight divided by height squared, is as criticized as it is ubiquitous. And it isn’t only public health advocates who have questioned its effectiveness. A professor of numerical analysis at Oxford University once called it a “bizarre measurement” in a letter to The Economist (paywall). It was invented before calculators, when a formula had to be very simple, he wrote. “As a consequence of this ill-founded definition, millions of short people think they are thinner than they are, and millions of tall people think they are fatter.”

Even the measurement’s inventor, a Belgian mathematician named Adolphe Quetelet, explicitly said it was too blunt to be used to calculate fat levels for individuals.

What today’s BMI critics find most troubling is that the measurement doesn’t fully take into account abdominal fat, also known as visceral adipose tissue (VAT), or just visceral fat, which gathers around the internal organs as people gain excess weight and is more dangerous than regular subcutaneous fat because it behaves differently in the body. As such, abdominal fat is linked to a higher risk of cardiovascular disease, Type 2 diabetes, and breast cancer in women. It’s also been connected to an increased risk of dementia, and, overall, a shorter lifespan.

The waist-height ratio is a better tool for estimating visceral fat

Now, in a paper just published in PLOS ONE, physiologists at Leeds Beckett University in the United Kingdom have confirmed that there’s a better way to predict body and abdominal fat: the waist-height ratio (WHtR), determined by dividing your waist by your height.

They reached this conclusion after measuring actual body fat and composition in 81 people using a highly precise total body scanner, then comparing the results with those of various anthropometrics commonly used to predict body fat and obesity, including BMI, WHtR, waist-to-height ratio0.5 , waist circumference, and waist-to-hip ratio.

Relying on the BMI alone, only one in seven of the participants would have been classified as obese in terms of their whole body fat, whereas according to the WHtR, the more accurate figure of one in two, or half the participants would meet that threshold. WHtR was also a closer proxy for measuring abdominal fat in both men and women. (The waist-to-hip ratio was the weakest formula of the bunch.)

The study, led by Michelle Swainson, a professor of exercise physiology, found the cutoff for predicting whole body obesity was 0.53 in men and 0.54 in women using the WHtR ratio; while a ratio of 0.59 and above indicated abdominal obesity in both genders.

That number could be even lower. In separate research, Margaret Ashwell, a senior visiting fellow at City University London, has discovered through several studies— including a meta-analysis of 44 studies in adults and 13 in children—that the risk of developing diabetes and cardiovascular disease begins to rise with a WHtR above .50. Her research also found that the WHtR measurement is more precise at measuring life expectancy. It also beats the BMI and waist circumference matrix some doctors rely on.

Use a tape measure, a piece of string, or our calculator

As a screening tool, both Ashwell and Swainson note, the WHtR metric has several other selling points: It’s easier to calculate than BMI, and it works for any race, age, or gender. You don’t even need to have a measuring tape handy to use it. With a piece of string, a person can measure their height, then fold the string in half and check to see whether it fits around their waist.

Even easier, punch your numbers into our calculator.

Ashwell has proposed that governments adopt a simple public health message: “Keep your waist to less than half your height.” That means someone who is 5 foot 5 (65 inches; 167.64 centimeters) should maintain a waistline smaller than 33 inches or 84 centimeters. A person who is six feet tall (72 inches; 182 centimeters) should keep his or her waist below a trim 36 inches or 91 centimeters.

Focus on health risks, not weight loss

According to Ashwell’s  Shape Chart, a WHtR between .40 and .50, which typically creates a pear-shaped body, is ideal, but a person should consider shedding some weight when his or her WHtR ratio is between .50 and .60 (somewhere between a pear and an apple). She calls the body shape below the .40 ratio a chili pepper, and suggests you “take care,” at that level, adding, “You will not need to decrease your waistline.”

At a ratio of .60 and above, which typically produces an apple-shaped silhouette, she asks: ”Why not talk to your GP and take action?”