DUEL-energy X-ray absorptiometry or DEXA is the gold-standard method of calculating fat percentages. It is very accurate but also expensive and not suitable for use in population-wide studies.

Another approach is the use of body-fat scales, where an undetectable electrical current is passed through your body. Since muscle contains more water, it conducts electricity better than fat. A greater electrical resistance will mean you have more body fat. But they are often inaccurate.

BMI is easy to calculate and easily scalable. For most of us, the higher our BMI, the more fat we tend to have. 

But BMI shouldn't be used by health care professionals as an accurate measure to inform treatment or advice. It has to be used with other information, such as fa...