Body mass index, or BMI, is currently the most widely used method for assessing whether someone is overweight or underweight. But now scientists think they've come up with a new and improved alternative: say hello to the relative fat mass index, or RFM.
The team of researchers behind RFM say it's more accurate than BMI, and it can also be worked out with just a tape measure – so you don't need a set of scales to calculate it, as you do with BMI.
In the case of RFM, it's the distance around your waist in relation to your height that counts, rather than your weight. The researchers say that gives a better idea of whether someone's body fat is at a healthy level or not.
"We wanted to identify a more reliable, simple and inexpensive method to assess body fat percentage without using sophisticated equipment," says lead researcher Orison Woolcott, from the Cedars-Sinai Medical Center in California.
"Our results confirmed the value of our new formula in a large number of subjects. Relative fat mass is a better measure of body fatness than many indices currently used in medicine and science, including the BMI."
To work out your BMI, you divide your weight in kilograms by your height in metres, then divide the answer by your height again. This figure can then be compared against a chart of healthy weights for each height: a normal BMI is between 18.5 and 25.
Experts have always admitted it isn't perfect – it doesn't take into account gender differences, it doesn't always accurately reflect muscle mass, and results can be skewed for children and the elderly.
That said, BMI has been a useful shorthand for tracking problems like obesity over time and across populations. It can flag up potential health issues related to someone's weight, even if it's only a rough guide.
Now, perhaps, BMI's time has come. To get your new RFM measurement, you measure your height and waist circumference, then plug the figures into this formula:
MEN: 64 – (20 x height/waist circumference) = RFM
WOMEN: 76 – (20 x height/waist circumference) = RFM
And the new calculations seem to stand up, too. Based on data from 3,456 adult patients in the US, the RFM measurements closely matched those taken by a high-tech DXA body scan, widely considered the gold standard for measuring body tissue, bone, muscle and fat.
In other words, RFM was almost as good a gauge of body fat as a specialist bit of medical equipment – and all it needs is a tape measure. The RFM calculations were more accurate than more than 300 other formulas the researchers tried.
For now though, more research across a greater number and a greater variety of people is required to make sure RFM is as accurate as its creators think it is. If that proves to be the case, it might be time to say goodbye to BMI.
"We still need to test the RFM in longitudinal studies with large populations to identify what ranges of body fat percentage are considered normal or abnormal in relation to serious obesity-related health problems," says Woolcott.
The research has been published in Scientific Reports.