The current method of judging obesity is so complex it should be replaced by a simple tape measure, according to a leading specialist.
Dr Jude Oben, from the Obesity Action Campaign, said the body mass index, or BMI, is "tedious" to calculate and poorly understood by patients.
He told Sky News that straightforward measurement of waist size was a more accurate assessment of someone's fat - and far more user friendly.
"The fat inside your abdomen is a good indicator of your metabolic risk," he said.
"For example your risk of heart disease, liver disease, diabetes, post-menopausal breast cancer, colorectal cancer.
"So why not measure it simply? A tape measure does that."
He said studies show men should keep their waist - measured at the level of the belly-button - below 90 cm (35.5 inches). Women should keep it below 80cm (31.5 inches).
BMI is a complex calculation in which a patient's weight in kilogrammes is divided by the square of their height in metres.
If the result is greater than 25, patients are judged to be overweight.
"By that point most of them have switched off or left the room," said Dr Oben.
BMI takes no account of muscle mass. Athletes can register as being overweight even though they have minimal body fat.
Catherine Dias has had a weight problem for many years. But she still hasn't got to grips with BMI - and much prefers measuring her waist size.
"It's simple, it's easy. You know what you have to get down to," she said.
"To do it the other way it's a lot of calculations. I couldn't do that."