There is really no simple answer to this question, since so many variables affect different people. However, the method commonly used to determine if weight loss is required is by calculating an individual’s Body Mass Index, or BMI.

The traditional method of computation for BMI is the individual’s weight in kilograms divided by the square of his height in metres. For non-metric folk like us Brits there is a method using imperial measurements, as follows: (weight in pounds x 703) divided by (height in inches) squared. So, if we assume a person of 5’9″ height (or 69 inches) and 12 stones (or 168 pounds) in weight, their BMI is (168 x 703 = 118104) divided by (69 x 69 =4761), or 24.8. And that’s all there is to it! Piece of cake! (no thanks, I’m on a diet!).

So, having calculated your BMI, how should you interpret it? Well, it can place you into one of several categories, as follows:

So our mythical individual with a BMI of 24.8 is just within the range defined as normal.

Of course, just because someone has a BMI of over 25 does not necessarily mean that that person should lose weight. For one thing, it should always be remembered that muscle is heavier than fat, so a person with a well-muscled body is likely to have a BMI that is too high. Also, BMI also does not account for body frame size; an individual may have a small frame and be carrying too much excess fat, but according to their BMI they might be classed as “healthy”. Conversely, a large framed individual may be quite healthy with a fairly low body fat percentage, but be classified as “overweight” by BMI. BMI also takes no account of height loss through ageing, so that an older person will see their BMI increase as the years go by even if their body fat does not increase.

BMI, then, is a blunt instrument, but it’s the only easily calculated indicator that we have. In any event, anyone with a BMI of 30 or over (Obese) should try to lose weight, and for anyone with a BMI of over 40 (severely obese) weight loss is essential!