This is based on the fact that human bodies are geometrically different. Duh. For years, medical professionals have used the BMI index to classify ideal body fat. However, the controversial classification system has been widely criticised, largely because it doesn't take into account that human bodies are geometrically different, and a healthy BMI for one person may not necessarily be right for the next.

To calculate your BMI, you divide your weight in kilograms by the square of your height in metres.

For adults, a BMI under 18.5 is considered dangerously thin, 18.5-25 is the healthy weight range, 25-30 is overweight, and 30 or over is obese.

However, according to experts from University of South Australia, Tim Olds and Nathan Daniell, the key item missing from the BMI method is "shape". Speaking to the Mail Online, they said: "BMI doesn’t take body shape into account.

"Humans are not geometrically similar.

"As people get taller, their legs, and particularly their upper legs, get proportionally longer.

"Tall people look a little more like wolfhounds, shorter people like staffies.

"If someone 190cm tall had the same relative leg length as someone 150cm tall, they would shrink to 185cm."

Confused? They use fictional sisters Jenny and Susie to highlight their point:

"Take gym junkie Jenny.

"She’s 170cm tall, weighs 70kg and is ultra-lean with only 10% body fat. We can calculate that her body volume is 65 litres. 

"Now compare her to her sedentary sister Suzie, also 170 cm and 70kg, but 40% body fat.

"They’ve got the same BMI (24.2), but Suzie has a body volume of 69 litres, because fat is less dense than muscle and takes up more space."

"People today are fatter at the same BMI than people in those slimmer, bygone days — more Suzie, less Jenny."

A much better option, is the Surface-Based-Body-Shape-Index, which accounts for "body surface area, vertical trunk circumference (which is the measurement from the shoulder to the groin and back), height and waist circumference."

Although this means is pretty complex, and involves "running a tape from the groin over the shoulder to the bum crack", it is significantly more accurate than the current BMI test.

However, the system is not perfect but researchers are hard at work developing a new body shape index constructed from anthropometric determinants of body shape and body size.

Until then, it's good to know that the BMI test is not the be-all and end-all.