We've all done it: Calculate our body-mass index (BMI) by using one of the many online calculators that takes into account our height and weight. The BMI – also known as the Quetelet index – is a handy way to gauge the amount of muscle, fat and bone in an individual frame. It's the most widely used diagnostic indicator to identify a person's optimal weight depending on his height.

But now all those HTML jockeys who coded those calculators may have to go back to the drawing board. New research indicates that your waist-to-height ratio is a more accurate indicator of your BMI than a weight-to-height comparison.

The focus of the research was on improving the way obesity is measured and classified by looking at the whole-body fat percentage and and visceral adipose tissue (VAT) – the fat stored around the abdominal region where most of internal organs lie – of a group of 81 adults. The scientists wanted to discover the most precise means of predicting this measurement in a clinical environment and set clear-cut thresholds for obesity.

The researchers gathered accurate whole-body and abdominal fat data using a total body dual energy X-ray absorptiometry (DXA) scanner. This is a means of measuring body composition and fat content. They then calculated five predictors of whole-body fat and VAT, which could be easily calculated in a doctor's office, fitness center or at home, comparing the results to those of the DXA scan and determining which simple predictor of obesity is the most accurate.

The five predictors tested were:

• BMI,
• waist circumference (WC),
• waist-to-hip ratio (WHR),
• waist-to-height ratio (WhtR), and
• waist-to-height ratio0.5 (WHtR0.5).

In the final analysis, the best predictor of both whole-body fat percentage and VAT in both men and women was waist-to-height ratio. The WHtR cut-points align broadly to current guidelines that adults and children should keep their waist circumference to less than half their height. 

Lead author Dr. Michelle Swainson, Senior Lecturer in Exercise Physiology in the Carnegie School of Sport at Leeds Beckett explained: “The conventional measurement of obesity used by GPs is BMI. Although there are benefits to this method, there is concern that a lot of people are being classified as obese by BMI when they are not or are being missed by this classification when they need to be referred for help. This is most definitely the case when people have a ’normal’ BMI but high abdominal fat that is often dismissed. Whole-body fat percentage, and specifically VAT mass, are associated with health conditions including insulin resistance, type 2 diabetes and cardiovascular disease, but are not fully accounted for through BMI evaluation.

“Carrying fat around the abdominal area has been shown to be an independent predictor of all-cause mortality in men and women. Put simply, it is more important, especially for cardio-metabolic conditions, that your belt notch goes down than the reading on the scales.”

The research has been published in the journal PLOS ONE.