Really this should be re-phrased as "How much fat should I have?".

The correct answer is .... "it depends".

Different volumes, different surface areas

Different volumes, different surface areas

It depends on the area over which the fat is required to be spread.

The larger the area, the thinner a given amount of fat is going to be spread, and vice versa. So, in order to estimate the total volume of fat a particular individual should carry to result in a thickness of x mm, we need to adjust for body surface area (BSA). 

We really want to know the BSA of the body minus the fat layer, because this represents the underlying skeletal and muscular frame over which the fat is required to be spread. This equates to the BSA of the fat-free mass (FFM). DEXA body fat scans calculate FFM very accurately, so we can apply standard mathematical formulae to calculate BSA.

We then allocate an average fat thickness for each category (overweight, healthy, lean, athlete). For example, the “healthy” category represents a fat thickness of 8.5 mm (top end) down to 6.5 mm (bottom end). The bottom end of lean reflects an average fat layer of 4.5mm, whilst the bottom of athlete equates to an average fat thickness of 2.5 mm. The permitted thickness for females is increased by 50% for all categories.

The next step is to calculate how much fat (in kg) equates to an average thickness of x mm given that particular individuals' BSA. 

This is a way of accurately classifying an individual that reflects the actual (average) thickness of their fat layer, and this is really what is most important. It’s more important than even the absolute amount of fat because it’s the combination of fat mass and area it has to cover that will determine how lean we say someone is.

BMI obviously has major flaws in reflecting the true state of "fatness" of an individual. The more lean mass you have, the worse you do. 

Similarly, using population based weight charts or even % fat categories do not really help an individual get a very specific target to work towards. We want a very accurate amount of fat to aim for that will correspond to the visual goal of looking lean.

So, the fat thickness for each category is constant for all, but the actual amount of fat in kg for each category will vary amongst individuals due to differences in each persons BSA.

Any goal becomes easier to achieve if it is realistic, objective and measurable. "Knowing your Numbers" takes the guesswork out of the fat loss process. Most clients say DEXA body fat scans help them take a longer term perspective and stops them being sidetracked by short-term weight fluctuations.

BMI classifies this 30 year old man as "obese" when he has only 6 kg of fat!

BMI classifies this 30 year old man as "obese" when he has only 6 kg of fat!